Type Driven Development

Monday, 10 August 2015 12:44:00 UTC

A strong type system can not only prevent errors, but also guide you and provide feedback in your design process.

Have you ever worked in a statically typed language (e.g. C# or Java), only to wish that you'd be allowed to focus on what you're doing, instead of having to declare types of arguments and create new classes all the time?

Have you, on the other hand, ever worked in a dynamic language (e.g. Javascript) and wished you could get static type checking to prevent a myriad of small, but preventable errors?

You can have the best of both worlds, and more! F#'s type system is strong, but non-obtrusive. It enables you to focus on the behaviour of the code you're writing, while still being statically typed.

Not only can it prevent syntax and usage errors, but it can even provide guidance on how to proceed with a given problem.

This is best explained with an example.

Example problem: simulate a continuously running process as a series of discrete processes

A couple of years ago I had a very particular problem: I needed to simulate a continuously running task using a sequence of discrete processes.

In short, I needed to start a process (in reality a simple .exe file) that would act as a Polling Consumer, but with the twist that it would keep track of time. It would need to run for one minute, and then shut down so that an overall scheduler could start the process again. This was to guarantee that the process would be running on at most one server in a farm.

The Polling Consumer should pull a message off a queue and hand it to some dispatcher, which would then make sure that the message was handled by an appropriate handler. This takes time, so makes the whole process more complex.

If the Polling Consumer estimates that receiving and handling a message takes 200 milliseconds, and it's 100 milliseconds from shutting down, it shouldn't poll for a message. It would take too long, and it wouldn't be able to shut down in time.

My problem was that I didn't know how long it took to handle a message, so the Polling Consumer would have to measure that as well, constantly updating its estimates of how long it takes to handle a message.

This wasn't a harder problem that I could originally solve it in a highly coupled imperative fashion. I wasn't happy with that implementation, though, so I'm happy that there's a much more incremental approach to the problem.

A Finite State Machine

The first breakthrough came when I realised that I could model the problem as a finite state machine:

Polling Consumer state machine transition diagram

The implicit start state is always the Ready state, because when the Polling Consumer starts, it has no message, but plenty of time, so therefore ready for a new message.

From the Ready state, the Polling Consumer may decide to poll for a message; if one is received, the new state is the Received state.

From the Received state, the only legal transition is back into the Ready state by handling the message.

From the Ready state, it may also turn out that there's currently no message in the queue, in which case the new state is the No message state.

In the No message state, the Polling Consumer may decide to idle for perhaps five seconds before transitioning back into the Ready state.

When in the Ready or No message states, the Polling Consumer may also realise that time is running out, and so decide to quit, in which case the (final) state is the End state.

This means that the Polling Consumer needs to measure what it does, and those measurements influence what it decides to do.

Separation of concerns

It sounds like we have a time concern (do I dare say dimension?), and a concern related to doing something. Given multiple concerns, we should separate them

The first thing I did was to introduce a Timed<'a> generic record type. This represents the result of performing a computation, as well as the times the computation started and stopped. I changed it slightly from the linked article, so here's the updated code for that:

type Timed<'a> =
        Started : DateTimeOffset
        Stopped : DateTimeOffset
        Result : 'a 
    member this.Duration = this.Stopped - this.Started
module Untimed =
    let map f x =
        { Started = x.Started; Stopped = x.Stopped; Result = f x.Result }
    let withResult newResult x = map (fun _ -> newResult) x
module Timed =
    let capture clock x =
        let now = clock()
        { Started = now; Stopped = now; Result = x }
    let map clock f x =
        let result = f x.Result
        let stopped = clock ()
        { Started = x.Started; Stopped = stopped; Result = result }
    let timeOn clock f x = x |> capture clock |> map clock f

This enables me to capture timing information about any operation, including the transitions between various states in a finite state machine.

The way I modelled the finite state machine for the Polling Consumer explicitly models all states as being instantaneous, while transitioning between states takes time. This means that we can model all transitions as functions that return Timed<'something>.

State data types

The first step is to model the data associated with each state in the finite state machine. This is an iterative process, but here I'll just show you the final result. If you're interested in seeing this design process in more details, you can watch my Pluralsight course Type-Driven Development with F#.

// Auxiliary types
type MessageHandler = unit -> Timed<unit>
// State data
type ReadyData = Timed<TimeSpan list>
type ReceivedMessageData = Timed<TimeSpan list * MessageHandler>
type NoMessageData = Timed<TimeSpan list>

All of these types are simply type aliases, so in fact they aren't strictly necessary. It's just helpful to give things a name from time to time.

The auxiliary MessageHandler type is a function that takes nothing (unit) as input, and returns 'timed nothing' as output. You may wonder how that handles a message. The intent is that any MessageHandler function is going to close over a real message: when a client calls it, it handles the message it closes over, and returns the time it took. The reason for this design is that the Polling Consumer doesn't need to 'see' the message; it only needs to know that it was handled, and how long it took. This design keeps the Polling Consumer decoupled from any particular message types.

There are three state data types: one for each state.

Hey! What about the End state?

It turns out that the End state doesn't need any associated data, because once the End state is reached, no more decisions should be made.

As promised, the three other states all contain a Timed<'something>. The timing information tells us when the transition into the given state started and stopped.

The data for the Ready and No message states are the same: Timed<TimeSpan list>, but notice that the TimeSpan list also appears in the Received state. This list of durations contains the statistics that the Polling Consumer measures. Every time it handles a message, it needs to measure how long it took. All such measurements are collected in this TimeSpan list, which must be passed around in all states so that the data isn't lost.

The data for the Received message state is different, because it also contains a MessageHandler. Every time the Polling Consumer receives a message, this message must be composed into a MessageHandler, and the MessageHandler passed as the second element of the state's tuple of data.

With all four states defined, we can now define a discriminated union that models a snapshot of the state machine:

type PollingConsumer =
| ReadyState of ReadyData
| ReceivedMessageState of ReceivedMessageData
| NoMessageState of NoMessageData
| StoppedState

That is, a PollingConsumer value represents a state of the Polling Consumer state machine.

Already at this stage, it should be apparent that F#'s type system is a great thinking tool, because it enables you to define type aliases declaratively, with only a single line of code here and there. Still, the advantage of the type system becomes much more apparent once you start to use these types.


With data types defined for each state, the next step of implementing a finite state machine is to define a function for each state. These functions are called transitions, and they should take a concrete state as input, and return a new PollingConsumer value as output. Since there are four concrete states in this example, there must be four transitions. Each should have the type 'concreteData -> PollingConsumer, e.g. ReadyData -> PollingConsumer, ReceivedMessageData -> PollingConsumer, etc.

As you can see, we're already getting guidance from the type system, because we now know the types of the four transitions we must implement.

Let's begin with the simplest one. When the Polling Consumer is Stopped, it should stay Stopped. That's easy. The transition should have the type unit -> PollingConsumer, because there's no data associated with the StoppedState.

Your first attempt might look like this:

let transitionFromStopped () : PollingConsumer = ??

This obviously doesn't compile because of the question marks (which aren't proper F# syntax). Knowing that you must return a PollingConsumer value, which one (of ReadyState, ReceivedMessageState, NoMessageState, or StoppedState) should you return?

Once the Polling Consumer is in the Stopped state, it should stay in the Stopped state, so the answer is easy: this transition should always return the Stopped state:

let transitionFromStopped () : PollingConsumer = StoppedState

It's as easy as that.

Since the (unit) input into the function doesn't do anything, we can remove it, effectively turning this particular function into a value:

let transitionFromStopped : PollingConsumer = StoppedState

This is a degenerate case, and not something that always happens.

Another transition

Let's do another transition!

Another good example is the transition out of the No message state. This transition must have the type NoMessageData -> PollingConsumer, so you can start typing:

let transitionFromNoMessage (nm : NoMessageData) : PollingConsumer =

This function takes NoMessageData as input, and returns a PollingConsumer value as output. Now you only need to figure out how to implement it. What should it do?

If you look at the state transition diagram, you can see that from the No message state, the Polling Consumer should either decide to quit, or transition back to the Ready state after idling. That's the high-level behaviour we're aiming for, so let's try to put it into code:

let transitionFromNoMessage (nm : NoMessageData) : PollingConsumer =
    if shouldIdle nm
    then idle () |> ReadyState
    else StoppedState

This doesn't compile, because neither shouldIdle nor idle are defined, but this is the overall behaviour we're aiming for.

Let's keep it high-level, so we'll simply promote the undefined values to arguments:

let transitionFromNoMessage shouldIdle idle (nm : NoMessageData) : PollingConsumer =
    if shouldIdle nm
    then idle () |> ReadyState
    else StoppedState

This compiles! Notice how type inference enables you to easily introduce new arguments without breaking your flow. In statically typed languages like C# or Java, you'd have to stop and declare the type of the arguments. If the types you need for those arguments doesn't exist, you'd have to go and create them first. That'd often mean creating new interfaces or classes in new files. That breaks your flow when you're actually trying to figure out how to model the high-level behaviour of a system.

Often, figuring out how to model the behaviour of a system is an exploratory process, so perhaps you don't get it right in the first attempt. Again, with languages like C# or Java, you'd have to waste a lot of time fiddling with the argument declarations, and perhaps the types you'd have to define in order to declare those arguments.

In F#, you can stay focused on the high-level behaviour, and take advantage of type inference to subsequently contemplate if all looks good.

In the transitionFromNoMessage function, the shouldIdle argument is inferred to be of the type NoMessageData -> bool. That seems reasonable. It's a function that determines whether or not to idle, based on a NoMessageData value. Recall that NoMessageData is an alias for Timed<TimeSpan list>, and that all transition functions take time and return Timed<'something> in order to capture the time spent in transition. This means that the time data in NoMessageData contains information about when the transition into the No message state started and stopped. That should be plenty of information necessary to make the decision on whether there's time to idle or not. In a future article, you'll see how to implement a shouldIdle function.

What about the idle argument, then? As the transitionFromNoMessage function is currently written, this argument is inferred to be of the type unit -> ReadyData. Recall that ReadyData is an alias for Timed<TimeSpan list>; what we're really looking at here, is a function of the type unit -> Timed<TimeSpan list>. In other words, a function that produces a Timed<TimeSpan list> out of thin air! That doesn't sound right. Which TimeSpan list should such a function return? Recall that this list contains the statistics for all the previously handled messages. How can a function produce these statistics given nothing (unit) as input?

This seems extra strange, because the statistics are already there, contained in the nm argument, which is a NoMessageData (that is: a Timed<TimeSpan list>) value. The inferred signature of idle suggests that the statistics contained in nm are being ignored.

Notice how the type inference gives us an opportunity to contemplate the current implementation. In this case, just by looking at inferred types, we realise that something is wrong.

Instead, let's change the transitionFromNoMessage function:

let transitionFromNoMessage shouldIdle idle (nm : NoMessageData) =
    if shouldIdle nm
    then idle () |> Untimed.withResult nm.Result |> ReadyState
    else StoppedState

This seems more reasonable: the function idles, but then takes the timing information from idling, but replaces it with the statistics from nm. The inferred type of idle is now unit -> Timed<'a>. That seems more reasonable. It's any function that returns Timed<'a>, where the timing information indicates when idling started and stopped.

This still doesn't look like a pure function, because it relies on the side effect that time passes, but it turns out to be good enough for this purpose.

Higher-order functions

Perhaps one thing is bothering you: I said that the transition out of the No message state should have the type NoMessageData -> PollingConsumer, but the final version of transitionFromNoMessage has the type (NoMessageData -> bool) -> (unit -> Timed<'a>) -> NoMessageData -> PollingConsumer!

The transitionFromNoMessage function has turned out to be a higher-order function, because it takes other functions as arguments.

Even though it doesn't exactly have the desired type, it can be partially applied. Imagine that you have two functions named shouldIdle' and idle', with the appropriate types, you can use them to partially apply the transitionFromNoMessage function:

let transitionFromNoMessage' = transitionFromNoMessage shouldIdle' idle'

The transitionFromNoMessage' function has the type NoMessageData -> PollingConsumer - exactly what we need!

All transitions

In this article, you've seen two of the four transitions necessary for defining the behaviour of the Polling Consumer. In total, all four are required:

  • ReadyData -> PollingConsumer
  • ReceivedMessageData -> PollingConsumer
  • NoMessageData -> PollingConsumer
  • (StoppedData) -> PollingConsumer
In this list, I put StoppedData in parentheses, because this type doesn't actually exist; instead of the fourth function, we have the degenerate transitionFromStopped value.

In this article, I will leave it as an exercise to you to implement ReadyData -> PollingConsumer and ReceivedMessageData -> PollingConsumer. If you want to see full implementations of these, as well as a more detailed discussion of this general topic, please watch my Type-Driven Development with F# Pluralsight course.

Imagine that we now have all four transitions. This makes it easy to implement the overall state machine.

State machine

Here's one way to execute the state machine:

let rec run trans state =
    let nextState = trans state
    match nextState with
    | StoppedState -> StoppedState
    | _ -> run trans nextState

This run function has the inferred type (PollingConsumer -> PollingConsumer) -> PollingConsumer -> PollingConsumer. It takes a trans function that turns one PollingConsumer into another, as well as an initial PollingConsumer value. It then proceeds to recursively call the trans function and itself, until it reaches a StoppedState value.

How can we implement a PollingConsumer -> PollingConsumer function?

That's easy, because we have all four transition functions, so we can use them:

let transition shouldPoll poll shouldIdle idle state =
    match state with
    | ReadyState r -> transitionFromReady shouldPoll poll r
    | ReceivedMessageState rm -> transitionFromReceived rm
    | NoMessageState nm -> transitionFromNoMessage shouldIdle idle nm
    | StoppedState -> transitionFromStopped

The transition function has the type (ReadyData -> bool) -> (unit -> Timed) -> (NoMessageData -> bool) -> (unit -> Timed<'a>) -> PollingConsumer -> PollingConsumer. That looks positively horrendous, but it's not so bad; you can partially apply it in order to get a function with the desired type PollingConsumer -> PollingConsumer.

Implicit to-do list

Even though we now have the run and transition functions, we only have the high-level behaviour in place. We still have a lot of implementation details left.

This is, in my opinion, one of the benefits of this approach to using the low-friction type system: First, you can focus on the desired behaviour of the system. Then, you address various implementation concerns. It's outside-in development.

Another advantage is that at this point, it's quite clear what to do next.

The transitionFromNoMessage function clearly states that it needs the functions shouldIdle and idle as arguments. You can't call the function without these arguments, so it's clear that you must supply them.

Not only did the type system allow us to introduce these function arguments with low friction, but it also tells us the types they should have.

In my Pluralsight course you can see how the transition out of the Ready state also turns out to be a higher-order function that takes two other functions as arguments.

In all, that's four functions we still need to implement before we can use the state machine. It's not going to be possible to partially apply the transition function before these four functions are available.

The type system thereby tells us that we still need to implement these four functions:

  • ReadyData -> bool
  • unit -> Timed<MessageHandler option>
  • NoMessageData -> bool
  • unit -> Timed<'a>
It's almost as though the type system implicitly provides a to-do list. There's no reason to keep such a list on a piece of paper on the side. As long as we still have work to do, we're not going to be able to compile a composition of the run function. Once we can compile a composition of the run function, there are no implementation details left.


Although this turned out to be quite a lengthy article, it only provides a sketch of the technique. You can see more code details, and a more in-depth discussion of the approach, in my Type-Driven Development with F# Pluralsight course.

The F# type system can be used in ways that C# and Java's type systems cant:

  • Dependencies can be introduced just-in-time as function arguments.
  • You can contemplate the inferred types to evaluate the soundness of the design.
  • The type system implicitly keeps a to-do list for you.
In future articles, I'll demonstrate how to implement some of the missing functions, as well as how to compose the Polling Consumer.

Idiomatic or idiosyncratic?

Monday, 03 August 2015 12:39:00 UTC

A strange programming construct may just be a friendly construct you haven't yet met.

Take a look at this fragment of JavaScript, and reflect on how you feel about it.

var nerdCapsToKebabCase = function (text) {
    var result = "";
    for (var i = 0; i < text.length; i++) {
        var c = text.charAt(i);
        if (i > 0 && c == c.toUpperCase()) {
            result = result + "-";
        result = result + c.toLowerCase();
    return result;

Do you understand what it does? Does it feel appropriate for the language? Does it communicate its purpose?

Most likely, you answered yes to all three questions - at least if you know what NerdCaps and kebab-case means.

Would your father, or your 12-year old daughter, or your English teacher, also answer yes to all of these questions? Probably not, assuming they don't know programming.

Everything is easy once you know how to do it; everything is hard if you don't.

Beauty is in the eye of the beholder

Writing software is hard. Writing maintainable software - code that you and your team can keep working on year after year - is extraordinarily hard. The most prominent sources of problems seem to be accidental complexity and coupling.

During my career, I've advised thousands of people on how to write maintainable code: how to reduce coupling, how to simplify, how to express code that a human can easily reason about.

The most common reaction to my suggestions?

"But, that's even more unreadable than the code we already had!"
What that really means is usually: "I don't understand this, and I feel very uncomfortable about that."

That's perfectly natural, but it doesn't mean that my suggestions are really unreadable. It just means that you may have to work a little before it becomes readable, but once that happens, you will understand why it's not only readable, but also better.

Let's take a step back and examine what it means when source code is readable.

Imperative code for beginners

Imagine that you're a beginner programmer, and that you have to implement the nerd caps to kebab case converter from the introduction.

This converter should take as input a string in NerdCaps, and convert it to kebab-case. Here's some sample data:

Input Expected output
Foo foo
Bar bar
FooBar foo-bar
barBaz bar-baz
barBazQuux bar-baz-quux
garplyGorgeFoo garply-gorge-foo

Since (in this imaginary example) you're a beginner, you should choose to use a beginner's language, so perhaps you select Visual Basic .NET. That seems appropriate, as BASIC was originally an acronym for Beginner's All-purpose Symbolic Instruction Code.

Perhaps you write the function like this:

Function Convert(text As StringAs String
    Dim result = ""
    For index = 0 To text.Length - 1
        Dim c = text(index)
        If index > 0 And Char.IsUpper(c) Then
            result = result + "-"
        End If
        result = result + c.ToString().ToLower()
    Return result
End Function

Is this appropriate Visual Basic code? Yes, it is.

Is it readable? That completely depends on who's doing the reading. You may find it readable, but does your father, or your 12-year old daughter?

I asked my 12-year old daughter if she understood, and she didn't. However, she understands this:

Funktionen omformer en stump tekst ved at først at dele den op i stumper hver gang der optræder et stort bogstav. Derefter sætter den bindestreg mellem hver tekststump, og laver hele teksten om til små bogstaver - altså nu med bindestreger mellem hver tekststump.

Did you understand that? If you understand Danish (or another Scandinavian language), you probably did; otherwise, you probably didn't.

Readability is evaluated based on what you already know. However, what you know isn't constant.

Imperative code for seasoned programmers

Once you've gained some experience, you may find Visual Basic's syntax too verbose. For instance, once you understand the idea about scope, you may find End If, End Function etc. to be noisy rather than helpful.

Perhaps you're ready to replace those keywords with curly braces. Here's the same function in C#:

public string Convert(string text)
    var result = "";
    for (int i = 0; i < text.Length; i++)
        var c = text[i];
        if (i > 0 && Char.IsUpper(c))
            result = result + "-";
        result = result + c.ToString().ToLower();
    return result;

Is this proper C# code? Yes. Is it readable? Again, it really depends on what you already know, but most programmers familiar with the C language family would probably find it fairly approachable. Notice how close it is to the original JavaScript example.

Imperative code for experts

Once you've gained lots of experience, you will have learned that although the curly braces formally delimit scopes, in order to make the code readable, you have to make sure to indent each scope appropriately. That seems to violate the DRY principle. Why not let the indentation make the code readable, as well as indicate the scope?

Various languages have so-called significant whitespace. The most widely known may be Python, but since we've already looked at two .NET languages, why not look at a third?

Here's the above function in F#:

let nerdCapsToKebabCase (text : string) = 
    let mutable result = ""
    for i = 0 to text.Length - 1 do
        let c = text.Chars i
        if (i > 0 && Char.IsUpper c) then
            result <- result + "-"
        result <- result + c.ToString().ToLower()

Is this appropriate F# code?

Not really. While it compiles and works, it goes against the grain of the language. F# is a Functional First language, but the above implementation is as imperative as all the previous examples.

Functional refactoring

A more Functional approach in F# could be this:

let nerdCapsToKebabCase (text : string) = 
    let addSkewer index c =
        let s = c.ToString().ToLower()
        match index, Char.IsUpper c with
        | 0, _ -> s
        | _, true -> "-" + s
        | _, false -> s
    text |> Seq.mapi addSkewer |> String.concat ""

This implementation uses a private, inner function called addSkewer to convert each character to a string, based on the value of the character, as well as the position it appears. This conversion is applied to each character, and the resulting sequence of strings is finally concatenated and returned.

Is this good F#? Yes, I would say that it is. There are lots of different ways you could have approached this problem. This is only one of them, but I find it quite passable.

Is it readable? Again, if you don't know F#, you probably don't think that it is. However, the F# programmers I asked found it readable.

This solution has the advantage that it doesn't rely on mutation. Since the conversion problem in this article is a bit of a toy problem, the use of imperative code with heavy use of mutation probably isn't a problem, but as the size of your procedures grow, mutation makes it harder to understand what happens in the code.

Another improvement over the imperative version is that this implementation uses a higher level of abstraction. Instead of stating how to arrive at the desired result, it states what to do. This means that it becomes easier to change the composition of it in order to change other characteristics. As an example, this implementation is what is known as embarrassingly parallel, although it probably wouldn't pay to parallelise this implementation (it depends on how big you expect the input to be).

Back-porting the functional approach

While F# is a multi-paradigmatic language with an emphasis on Functional Programming, C# and Visual Basic are multi-paradigmatic languages with emphasis on Object-Oriented Programming. However, they still have some Functional capabilities, so you can back-port the F# approach. Here's one way to do it in C# using LINQ:

public string Convert(string text)
    return String.Concat(text.Select(AddSkewer));
private static string AddSkewer(char c, int index)
    var s = c.ToString().ToLower();
    if (index == 0)
        return s;
    if (Char.IsUpper(c))
        return "-" + s;
    return s;

The LINQ Select method is equivalent to F#'s mapi function, and the AddSkewer function must rely on individual conditionals instead of pattern matching, but apart from that, it's structurally the same implementation.

You can do the same in Visual Basic:

Function Convert(text As StringAs String
    Return String.Concat(text.Select(AddressOf AddSkewer))
End Function
Private Function AddSkewer(c As Char, index As Integer)
    Dim s = c.ToString().ToLower()
    If index = 0 Then
        Return s
    End If
    If Char.IsUpper(c) Then
        Return "-" + s
    End If
    Return s
End Function

Are these back-ported implementations examples of appropriate C# or Visual Basic code? This is where the issue becomes more controversial.

Idiomatic or idiosyncratic

Apart from reactions such as "that's unreadable," one of the most common reactions I get to such suggestions is:

"That's not idiomatic C#" (or Visual Basic).

Perhaps, but could it become idiomatic?

Think about what an idiom is. In language, it just means a figure of speech, like "jumping the shark". Once upon a time, no-one said "jump the shark". Then Jon Hein came up with it, other people adopted it, and it became an idiom.

It's a bit like that with idiomatic C#, idiomatic Visual Basic, idiomatic Ruby, idiomatic JavaScript, etc. Idioms are adopted because they're deemed beneficial. It doesn't mean that the set of idioms for any language is finite or complete.

That something isn't idiomatic C# may only mean that it isn't idiomatic yet.

Or perhaps it is idiomatic, but it's just an idiom you haven't seen yet. We all have idiosyncrasies, but we should try to see past them. If a language construct is strange, it may be a friendly construct you just haven't met.

Constant learning

One of my friends once told me that he'd been giving a week-long programming course to a group of professional software developers, and as the week was winding down, one of the participants asked my friend how he knew so much.

My friend answered that he constantly studied and practised programming, mostly in his spare time.

The course participant incredulously replied: "You mean that there's more to learn?"

As my friend told me, he really wanted to reply: “If that's a problem for you, then you've picked the wrong profession." However, he found something more bland, but polite, to answer.

There's no way around it: if you want to be (and stay) a programmer, you'll have to keep learning - not only new languages and technologies, but also new ways to use the subjects you already think you know.

If you ask me about advice about a particular problem, I will often suggest something that seems foreign to you. That doesn't make it bad, or unreadable. Give it a chance even if it makes you uncomfortable right now. Being uncomfortable often means that you're learning.

Luckily, in these days of internet, finding materials that will help you learn is easier and more accessible than ever before in history. There are tutorials, blog posts, books, and video services like Pluralsight.


Craig Main

I would have done this...

Regex.Replace(s, "(?<=.)[A-Z]", _ => "-" + _.Value).ToLower();

2015-08-04 06:01 UTC

Craig, fair enough :)

2015-08-04 06:37 UTC

In addition to Craig's proposal (which I immediately though of too hehe), think of this one:

        const string MATCH_UPPERCASE_LETTERS = "(?<=.)[A-Z]";
        // ...
        Regex.Replace(s, MATCH_UPPERCASE_LETTERS, letterFound => "-" + letterFound.Value).ToLower();

It probably does not follow the whole conventions of the language, but the great thing about it is that it can be easily understood without knowledge of the language. This brings it to another complete level of maintainability.

Of course, the meta-information that you convey through the code is subject to another complete analysis and discussion, since there are way too many options on how to approach that, and while most of us will lean on "readable code", it all comes down to what the code itself is telling you about itself.

Finally, as a side note, something that I found quite interesting is that most of the introductory guides to Ruby as a language will talk about how Ruby was design to read like english.

        door.close unless door.is_closed?

(based on Why's Poignant Guide to Ruby.)

While I found that pretty appealing when learning Ruby, some other constructs will take it far away from how English works, pretty much what happened to your example with F#. Not that they cannot be made to read easier, but that extra effort is not always performed.

My conclusion is: readability is not about the code or the algorithm (which definitely can help), but about the information that the code itself gives you about what it does. Prefer to use pre-built language functions rather than implementing them yourself.

2015-08-04 11:42 UTC

In under than 30 seconds of seeing the code blocks, I can't follow any of the process. But I know what the operation is doing thanks to the operation name, nerdCapsToKebabCase. Surely that the terms nerdCaps and KebabCase is uncommon, but I only need some minutes to know what are those using internet.

So for me, no matter how good you write your code, you can't make it commonly readable by using the code itself. Won't the readability can be enhanced by using comments / documentation? Ex: /* convert thisKindOfPhrase to this-kind-of-phrase */. I've also used unit tests to improve the readability to some extent too.

2015-08-05 04:35 UTC

Type Driven Development with F# Pluralsight course

Friday, 17 July 2015 16:42:00 UTC

The F# type system can do much more than C# or Java's type system. Learn how to use it as a design feedback technique in my new Pluralsight course.

Not only can the F# type system help you catch basic errors in your code, but it can also help you by providing rapid feedback, and make suggestions on how to proceed. All you have to do is learn to listen.

My new Pluralsight course, Type-Driven Development with F#, first walks you through three modules of exploration of a real problem, showing various ways you can take advantage of the type system in an Outside-in fashion. As you can tell, it involves some celebration related to Finite State Machines:

Course screenshot, man and woman cheering over Finite State Machine

It then shows you how to stabilize the prototype using Property-based Testing with FsCheck.

REST implies Content Negotiation

Monday, 22 June 2015 09:10:00 UTC

If you're building REST APIs, you will eventually have to deal with Content Negotiation.

Some REST services support both JSON and XML, in which case it's evident that Content Negotiation is required. These days, though, more and more services forego XML, and serve only JSON. Does that mean that Content Negotiation is no longer relevant?

If you're building level 3 REST APIs this isn't true. You'll still need Content Negotiation in order to be able to evolve your service without breaking existing clients.

In a level 3 API, the distinguishing feature is that the service exposes Hypermedia Controls. In practice, it means that representations also contain links, like in this example:

    "users": [
            "displayName""Jane Doe",
            "links": [
                { "rel""user""href""/users/1234" },
                { "rel""friends""href""/users/1234/friends" }
            "displayName""John Doe",
            "links": [
                { "rel""user""href""/users/5678" },
                { "rel""friends""href""/users/5678/friends" }

This representation gives you a list of users. Notice that each user has an array of links. One type of link, identified by the relationship type "user", will take you to the user resource. Another link, identified by the relationship type "friends", will take you to that particular user's friends: another array of users.

When you follow the "user" link for the first user, you'll get this representation:

    "displayName""Jane Doe",
    "address": {
        "street""Any Boulevard 42",
        "zip""12345 Anywhere"

This user representation is richer, because you also get the user's address. (In a real API, this representation should also have links, but I omitted them here in order to make the example more concise.)

Breaking changes

As long as you have no breaking changes, all is good. Things change, though, and now you discover that a user can have more than one address:

    "displayName""Jane Doe",
    "addresses": [
            "street""Any Boulevard 42",
            "zip""12345 Anywhere"
            "street""Some Avenue 1337",
            "zip""67890 Nowhere"

This is a breaking change. If the service returns this representation to a client that expects only a single "address" property, that client will break.

How can you introduce this new version of a user representation without breaking old clients? You'll need to keep both the old and the new version around, and return the old version to the old clients, and the new version to new clients.

Versioning attempt: in the URL

There are various suggested ways to version REST APIs. Some people suggest including the version in the URL, so that you'd be able to access the new, breaking representation at "/users/v2/1234", while the original representation is still available at "/users/1234". There are several problems with this suggestion; the worst is that it doesn't work well with Hypermedia Controls (links, if you recall).

Consider the list of users in the first example, above. Each user has some links, and one of the links is a "user" link, which will take the client to the original representation. This can never change: if you change the "user" links to point to the new representation, you will break old clients. You also can't remove the "user" links, for the same reason.

The only way out of this conundrum is to add another link for the new clients:

    "users": [
            "displayName""Jane Doe",
            "links": [
                { "rel""user""href""/users/1234" },
                { "rel""userV2""href""/users/v2/1234" },
                { "rel""friends""href""/users/1234/friends" }
            "displayName""John Doe",
            "links": [
                { "rel""user""href""/users/5678" },
                { "rel""userV2""href""/users/v2/5678" },
                { "rel""friends""href""/users/5678/friends" }

In this way, old clients can still follow "user" links, and updated clients can follow "userV2" links.

It may work, but isn't nice. Every time you introduce a new version, you'll have to add new links in all the places where the old link appears. Imagine the 'link bloat' when you have more than two versions of a resource:

    "users": [
            "displayName""Jane Doe",
            "links": [
                { "rel""user""href""/users/1234" },
                { "rel""userV2""href""/users/v2/1234" },
                { "rel""userV3""href""/users/v3/1234" },
                { "rel""userV4""href""/users/v4/1234" },
                { "rel""userV5""href""/users/v5/1234" },
                { "rel""friends""href""/users/1234/friends" },
                { "rel""friendsV2""href""/users/1234/V2/friends" }
            "displayName""John Doe",
            "links": [
                { "rel""user""href""/users/5678" },
                { "rel""userV2""href""/users/v2/5678" },
                { "rel""userV3""href""/users/v3/5678" },
                { "rel""userV4""href""/users/v4/5678" },
                { "rel""userV5""href""/users/v5/5678" },
                { "rel""friends""href""/users/5678/friends" },
                { "rel""friendsV2""href""/users/5678/V2/friends" }

Remember: we're talking about Level 3 REST APIs here. Clients must follow the links provided; clients are not expected to assemble URLs from templates, as will often be the case in Level 1 and 2.

Versioning through Content Negotiation

As is often the case, the root cause of the above problem is coupling. The attempted solution of putting the version in the URL couples the identity (the URL) of the resource to its representation. That was never the intent of REST.

Instead, you should use Content Negotiation to version representations. If clients don't request a specific version, the service should return the original representation.

New clients that understand the new representation can explicitly request it in the Accept header:

GET /users/1234 HTTP/1.1
Accept: application/vnd.ploeh.user+json; version=2

Jim Liddell has a more detailed description of this approach to versioning.

It enables you to keep the user list stable, without link bloat. It'll simply remain as it was. This means less work for you as API developer, and fewer bytes over the wire.


The most common criticisms against using Content Negotiation for versioning is that it makes the API less approachable. The argument goes that with version information in the URL, you can still test and explore the API using your browser. Once you add the requirement that HTTP headers should be used, you can no longer test and explore the API with your browser.

Unless the API in question is an anonymously accessible, read-only API, I think that this argument misses the mark.

Both Level 2 and 3 REST APIs utilise HTTP verbs. Unless the API is a read-only API, a client must use POST, PUT, and DELETE as well as GET. This is not possible from a browser.

Today, many APIs are secured with modern authentication and authorisation formats like OAuth, where the client has to provide a security token in the Authorization header. This is not possible from a browser.

Most APIs will already be impossible to test from the browser; it's irrelevant that versioning via the Accept header also prevents you from using the browser to explore and test the API. Get friendly with curl or Fiddler instead.


If you want to build a true REST API, you should seriously consider using Content Negotiation for versioning. In this way, you prevent link bloat, and effectively decouple versioning from the identity of each resource.

If you've said REST, you must also say Content Negotiation. Any technology you use to build or consume REST services must be able to play that game.


For that example, it's only a breaking change because you also removed the old "address" property. It might have been easier to leave that in and populate it with the first address from the collection. Older clients could continue to use that, and newer ones could use the new "addresses" field. Obviously at some point you will need to deal with versioning, but it can be better to avoid it until absolutely necessary.

2015-06-30 13:37 UTC
Nik Petroff

I've been catching up on REST, and discovered the 5 levels of media type just a few minutes before reading your post. This keeps it approachable, you can still use your browser to explore, and the higher levels progressively enhance the API for smarter clients.

An example can be found here where Ali Kheyrollahi exposes Greg Young's CQRS sample though a REST inferface.

2015-07-01 11:37 UTC

Graham, thank you for writing. You are completely right: if it's possible to evolve the API without introducing a new version, that's always preferable. Your suggestion for doing that with my particular example demonstrates how it could be done.

As you also correctly point out, sooner or later, one will have to deal with a truly breaking change. My purpose of writing this article was to explain how the need to do that implies that content negotiation is required. This is the reason I made up an example of a breaking change; unfortunately, that example didn't convince you that versioning was necessary ;)

2015-07-09 12:03 UTC

I think I would consider going one step further, and mandate a version in the Accept header right from the beginning.

i.e., Accept: application/json would return a 406 Not Acceptable HTTP response. You have to use Accept: application/vnd.ploeh.user+json; version=1

Forces clients to be upgrade friendly to begin with, and easier to retire old versions when they are not being called any more.

It strikes me though, that having to know these MIME types for versioning is a bit like needing to know the right incantation to open the magic door. We don't want clients hard-coding the URL to a particular API call as part of HATEOAS, but still need to know the magic MIME type to get an appropriate version of the data.

2015-07-16 4:31 UTC

Evan, thank you for writing. As I also wrote in a comment to Jim Liddel's article, I would consider the context before making any hard rules. If you control both API and all clients, requiring specific Accept headers may make managing the entire ecosystem a little easier.

On the other hand, if you make an API available to arbitrary clients, you should follow Postel's law in order to make it easy for client developers to use your API.

Ultimately, even if you can somehow solve the problem of not having to know specific MIME types, you'll still need to know how to interpret the representations served by the API. For example, as a client developer, you still need to know that a user representation will have displayName and address properties. As far as I can see, you can push that problem around, but if it can be solved, I've yet to learn how.

2015-07-25 17:05 UTC
Ben Brown

Mark, I've been doing some preliminary research on API versioning for a personal project and I like this approach. Thanks for the post and accompanying links!

I read a few of Jim Liddel's posts and can see that Nancy has this sort of content negotiation baked in. Being more familiar with Web Api, I've been looking for an existing way of doing this using that framework instead. Web Api's support for attribute routing seems promising, but as of yet, I haven't seen any examples of this in action using Web Api 2. The flexibility and simplicity of Nancy is really attractive, and I may end up going that route anyway, but I'm hesitant to pick a framework based solely on something like this. Have you come across anything similar for Web API 2?

2015-10-10 15:40 UTC

Ben, thank you for writing. Web API has supported Content Negotiation since back when it was still a WCF Web API CTP. Out of the box it supports XML and JSON, but you can also make it support Content Negotiation-based versioning as described in this article.

2015-10-13 08:13 UTC


Friday, 08 May 2015 05:40:00 UTC

stringf is an F# function that invokes any ToString(string) method.

F# comes with the built-in string function, which is essentially an adapter over Object.ToString. That often comes in handy, because it lets you compose functions without having to resort to lambda expressions all the time.

Instead of writing this (to produce the string "A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z"):

['A' .. 'Z'] |> List.map (fun x -> x.ToString()) |> String.concat ", "

You can write this:

['A' .. 'Z'] |> List.map string |> String.concat ", "

That's nice, but some .NET types provide an overloaded ToString function taking a format string as argument:

and so on.

While these methods can be useful, it would be nice to be able to use them as functions, like the string function. The problem, it seems, is that this ToString overload is part of no interface or base class.

Statically typed duck typing

That's not a problem for F#, because it enables us to do statically typed duck typing!

In this case, you can define a stringf function:

let inline stringf format (x : ^a) = 
    (^a : (member ToString : string -> string) (x, format))

You can use the function like this:

> DateTimeOffset.Now |> stringf "o";;
val it : string = "2015-05-07T14:24:09.8422893+02:00"
> DateTimeOffset.Now |> stringf "T";;
val it : string = "14:24:18"
> DateTime.Now |> stringf "t";;
val it : string = "14:24"
> TimeSpan.FromDays 42. |> stringf "c";;
val it : string = "42.00:00:00"
> 0.42m |> stringf "p0";;
val it : string = "42 %"

Perhaps it would be better to define more domain-specific functions like percent, roundTrippable, time, etc., but it's interesting that such a function as stringf is possible.

Functional design is intrinsically testable

Thursday, 07 May 2015 06:13:00 UTC

TDD with Functional Programming doesn't lead to test-induced damage. Here's why.

Over the years, there's been much criticism of Test-Driven Development (TDD). Perhaps David Heinemeier Hansson best condensed this criticism by claiming that TDD leads to test-induced design damage. This isn't a criticism you can just brush away; it hits a sore point.

Personally, I don't believe that TDD has to lead to test-induced damage (not even in Object-Oriented Programming), but I'm the first to admit that it's not a design methodology.

In this article, though, you're going to learn about the fundamental reason that TDD with Functional Programming doesn't lead to test-induced damage.

In Functional Programming, the ideal function is a Pure function. A Pure function is a function that always returns the same value given the same input, and has no side-effects.


The first characteristic of a Pure function means that an ideal function can't depend on any implicit knowledge about the external world. Only the input into the function can influence the evaluation of the function.

This is what Jessica Kerr calls Isolation. A function has the property of Isolation when the only information it has about the external word is passed into it via arguments.

You can think about Isolation as the dual of Encapsulation.

In Object-Oriented Programming, Encapsulation is a very important concept. It means that while an object contains state, the external world doesn't know about that state, unless the object explicitly makes it available.

In Functional Programming, a function is Isolated when it knows nothing about the state of the external world, unless it's explicitly made available to it.

A Pure function, the ideal of Functional Programming, is Isolated.

Unit testing

Why is this interesting?

It's interesting if you start to think about what unit testing means. There are tons of conflicting definitions of what exactly constitutes a unit test, but most experts seem to be able to agree on this broad definition:

A unit test is an automated test that tests a unit in isolation from its dependencies.
Notice the use of the word Isolation in that definition. In order to unit test, you'll have to be able to isolate the unit from its dependencies. This is the requirement that tends to lead to Test-Induced Damage in Object-Oriented Programming. While there's nothing about Encapsulation that explicitly states that it's forbidden to isolate an object from its dependencies, it offers no help on the matter either. Programmers are on their own, because this concern isn't ingrained into Object-Oriented Programming.

Venn diagram showing that while there's an intersection between Encapsulation and Isolation, it's only here that Object-Oriented Programming is also testable.

You can do TDD with Object-Oriented Programming, and as long as you stay within the intersection of Encapsulation and Isolation, you may be able to stay clear of test-induced damage. However, that zone of testability isn't particularly big, so it's easy to stray. You have to be very careful and know what you're doing. Not surprisingly, many books and articles have been written about TDD, including quite a few on this blog.

The best of both worlds

In Functional Programming, on the other hand, Isolation is the ideal. An ideal function is already isolated from its dependencies, so no more design work is required to make it testable.

Stacked Venn diagram that show that an ideal function is a subset of isolated functions, which is again a subset of testable functions.

Ideal Functional design is not only ideal, but also perfectly testable, so there's no conflict. This is the underlying reason that TDD doesn't lead to test-induced damage with Functional Programming.


Isolation is an important quality of Functional Programming. An ideal function is Isolated, and that means that it's intrinsically testable. You don't have to tweak any design principles in order to make a function testable - in fact, if a function isn't testable, it's a sign that it's poorly designed. Thus, TDD doesn't lead to Test-Induced Damage in Functional Programming.

If you want to learn more about this, as well as see lots of code examples, you can watch my Test-Driven Development with F# Pluralsight course.

Test-Driven Development with F# Pluralsight course

Wednesday, 06 May 2015 07:21:00 UTC

Test-Driven Development and Functional Programming is a match made in heaven. Learn how and why in my new Pluralsight course.

A common criticism against Test-Driven Development (TDD) is that it leads to Test-Induced Damage. However, it doesn't have to be that way, and it turns out that with Functional Programming (FP), the design ideals of FP coincide with TDD.

Course screenshot

In my new Pluralsight course on Test-Driven Development with F#, you'll learn how the intersection between TDD and F# presents opportunities for better design and better testability.

Introduction to Property based Testing with F# Pluralsight course

Friday, 17 April 2015 06:23:00 UTC

My latest Pluralsight course is an introduction to Property-Based Testing with F#.

Soon after the release of my Unit Testing with F# Pluralsight course, it gives me great pleasure to announce my new course, which is an Introduction to Property-based Testing with F#.

In this course, you'll get an introduction to the concept of Property-Based Testing, and see a comprehensive example that demonstrates how to incrementally implement a feature that otherwise would have been hard to address in an iterative fashion.

C# will eventually get all F# features, right?

Wednesday, 15 April 2015 08:32:00 UTC

C# will never get the important features that F# has. Here's why.

The relationship between C# and F# is interesting, no matter if you look at it from the C# or the F# perspective:

There's nothing wrong with this. F# is a great language, so obviously it makes sense to look there for inspiration. Some features also go the other way, such as F# Query Expressions, which were inspired by LINQ.

It's not some hidden cabal that I'm trying to expose, either. Mads Torgersen has been quite open about this relationship.

Why care about F#, then?

A common reaction to all of this seems to be that if C# eventually gets all the best F# features, there's no reason to care about F#. Just stick with C#, and get those features in the future.

The most obvious answer to such statements is that F# already has those features, while you'll have to wait for a long time to get them in C#. While C# 6 gets a few features, they are hardly killer features. So perhaps you'll get the good F# features in C#, but they could be years away, and some features might be postponed to later versions again.

In my experience, that argument mostly falls on deaf ears. Many programmers are content to wait, perhaps because they feel that the language choice is out of their hands anyway.

What F# features could C# get?

Often, when F# enthusiasts attempt to sell the language to other programmers, they have a long list of language features that F# has, and that (e.g.) C# doesn't have. However, in the future, C# could hypothetically have those features too:

  • Records. C# could have those as well, and they're being considered for C# 7. Implementation-wise, F# records compile to immutable classes anyway.
  • Discriminated Unions. Nothing in principle prevents C# from getting those. After all, F# Discriminated Unions compile to a class hierarchy.
  • Pattern matching Also being considered for C# 7.
  • No nulls. It's a common myth that F# doesn't have nulls. It does. It's even a keyword. It's true that F# doesn't allow its Functional data types (records, unions, tuples, etc.) to have null values, but it's only a compiler trick. At run-time, these types can have null values too, and you can provide null values via Reflection. C# could get such a compiler trick as well.
  • Immutability. F#'s immutability 'feature' is similar to how it deals with nulls. Lots of F# can be mutable (everything that interacts with C# code), but the special Functional data types can't. Again, it's mostly in how these specific data types are implemented under the hood that provides this feature, and C# could get that as well.
  • Options. These are really just specialised Discriminated Unions, so C# could get those too.
  • Object Expressions. Java has had those for years, so there's no reason C# couldn't get them as well.
  • Partial Function Application. You can already do this in C# today, but the syntax for it is really awkward. Thus, there's no technical reason C# can't have that, but the C# language designers would have to come up with a better syntax.
  • Scripting. F# is great for scripting, but as the success of scriptcs has shown, nothing prevents C# from being a scripting language either.
  • REPL. A REPL is a really nice tool, but scriptcs already comes with a REPL, again demonstrating that C# could have that too.
This list in no way implies that C# will get any or all of these features. While I'm an MVP, I have no inside insight; I'm only speculating. My point is that I see no fundamental reason C# couldn't eventually get those features.

What F# features can C# never get?

There are a few F# features that many people point to as their favourite, that C# is unlikely to get. A couple of them are:

  • Type Providers. Someone that I completely trust on this issue told me quite authoritatively that "C# will never get Type Providers", and then laughed quietly. While I don't know enough about the technical details of Type Providers to be able to evaluate that statement, I trust this person completely on this issue.
  • Units of Measure. Here, I simply have to confess ignorance. While I haven't seen talk about units of measure for C#, I have no idea whether it's doable or not.
These are some loved features of F# that look unlikely to be ported to C#, but there's one quality of F# that I'm absolutely convinced will never make it to C#, and this is one of the killer features of F#: it's what you can't do in the language.

In a recent article, I explained how less is more when it comes to language features. Many languages come with redundant features (often for historical reasons), but the fewer redundant features a language has, the better.

The F# compiler doesn't allow circular dependencies. You can't use a type or a function before you've defined it. This may seem like a restriction, but is perhaps the most important quality of F#. Cyclic dependencies are closely correlated with coupling, and coupling is the deadliest maintainability killer of code bases.

In C# and most other languages, you can define dependency cycles, and the compiler makes it easy for you. In F#, the compiler makes it impossible.

Studies show that F# projects have fewer and smaller cycles, and that there are types of cycles (motifs) you don't see at all in F# code bases.

The F# compiler protects you from making cycles. C# will never be able to do that, because it would be a massive breaking change: if the C# compiler was changed to protect you as well, most existing C# code wouldn't be able to compile.

Microsoft has never been in the habit of introducing breaking changes, so I'm quite convinced that this will never happen.


C# could, theoretically, get a lot of the features that F# has, but not the 'feature' that really matters: protection against coupling. Since coupling is one of the most common reasons for code rot, this is one of the most compelling reasons to switch to F# today.

F# is a great language, not only because of the features it has, but even more so because if the undesirable traits it doesn't have.


I would say C# won't get non-nullable reference types either, even in the form of a compiler trick. It would either introduce too much of breaking changes or be very limited and thus not especially usefull.

2015-04-18 15:36 UTC

Vladimir, thank you for writing. You're probably correct. Many years ago, I overheard Anders Hejlsberg say that it wouldn't be possible to introduce non-nullable reference types into the .NET platform without massive breaking changes. I can't say I ever understood the reasoning behind this (nor was it ever explained to me), but when Anders Hejlsberg tells you that, you sort of have to accept it :)

FWIW, there's a bit of discussion about non-nullable reference types in the C# Design Meeting Notes for Jan 21, 2015, but I have to admit that I didn't follow the link to Eric Lippert's blog :$

2015-04-18 18:57 UTC

Less is more: language features

Monday, 13 April 2015 08:16:00 UTC

Many languages have redundant features; progress in language design includes removing those features.

(This post is also available in Japanese.)

There are many programming languages, and new ones are being introduced all the time. Are these languages better than previous languages? Obviously, that's impossible to answer, since there's no clear measurement of what constitutes a 'better' programming language.

Still, if you look at a historical trend, it looks as though one way to make a better language is to identify a redundant language feature, and design a new language that doesn't have that feature.

"perfection is attained not when there is nothing more to add, but when there is nothing more to remove." - Antoine de Saint Exupéry

In this article, you'll see various examples of language features that have already proven to be redundant, and other features where we are seeing strong indications that they are redundant.

Limitless ways to shoot yourself in the foot.

When the first computers were created, programs had to be written in machine code or assembly language. In machine code, you can express everything the CPU can do, because the machine code is written it terms of a CPU's instruction set. While it's possible to write correct programs in machine code, you can also write a lot of incorrect programs, including programs that crash horribly, or perhaps even destroy the machine on which they are running.

You're most likely used to writing code in a higher-level language, but even so, if you share my experience with this, you'll agree that it takes a lot of trial and error to get things right. For every correct program, there are many incorrect variations. The set of incorrect programs is much bigger than the set of correct programs.

With machine code, you have limitless ways to create incorrect programs. Yes: you can express everything the CPU can execute, but most of it will be incorrect. Thus, the set of valid programs is a small subset of the set of all possible instructions.

The set of all valid programs, inside the much larger set of all possible instructions.

Early computer programmers quickly discovered that writing in machine code was extremely error-prone, and the code was also as unreadable as it could be. One way to address this problem was to introduce assembly language, but it didn't solve the underlying problems: most assembly languages were just 'human-readable' one-to-one mappings over machine code, so you still have unlimited ways to express incorrect programs.

High-level languages

After having suffered with machine code and assembly language for some time, programmers figured out how to express programs in high-level languages. The first programming languages aren't in much use today, but a good example of a 'low-level' high-level language still in use today is C.

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of all possible instructions in a high-level language.

In C, you can express almost all valid programs. There are still tons of ways to write incorrect programs that will crash the program (or the computer), but since the language is an abstraction over machine code, there are instructions you can't express. Most of these are invalid instruction sequences anyway, so it's for the better.

Notice what happened: moving from machine code to a high-level programming language removes a feature of the language. In machine code, you can express anything; in a high-level language, there are things you can't express. You're okay with that, because the options that were taken away from you were bad for you.


In 1968, Edsger Dijkstra published his (in)famous paper Go To Statement Considered Harmful. In it, he argued that the GOTO statement was an bad idea, and that programs would be 'better' without the GOTO statement. This sparked a decade-long controversy, but these days we've come to understand that Dijkstra was right. Some of the most popular languages in use today (e.g. Java and JavaScript) don't have GOTO at all.

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of all possible instructions in a high-level language without GOTO.

Since GOTO has turned out to be an entirely redundant language feature, a language without it doesn't limit your ability to express valid programs, but it does limit your ability to express invalid programs.

Do you notice a pattern?

Take something away, and make an improvement.

There's nothing new about this; Robert C. Martin told us that years ago.

You can't just arbitrarily take away anything, because that may constrain you in such a way that there are valid programs you can no longer write:

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of a language that takes away the wrong features.

You have to take away the right features.


Today, everyone seems to agree that errors should be handled via some sort of exception mechanisms; at least, everyone can agree that error codes are not the way to go (and I agree). We need something richer than error codes, and something that balances usefulness with robustness.

The problem with exceptions is that this mechanism is really only GOTO statements in disguise - and we've already learned that GOTO is considered harmful.

A better approach is to use a sum type to indicate either success or failure in a composable form.


As Robert C. Martin also pointed out, in older languages, including C and C++, you can manipulate pointers, but as soon as you introduce polymorphism, you no longer need 'raw' pointers. Java doesn't have pointers; JavaScript doesn't do pointers; C# does allow you to use pointers, but I've personally never needed them outside of interop with the Windows API.

As these languages have demonstrated, you don't need access to pointers in order to be able to pass values by reference. Pointers can be abstracted away.


Most strongly typed languages give you an opportunity to choose between various different number types: bytes, 16-bit integers, 32-bit integers, 32-bit unsigned integers, single precision floating point numbers, etc. That made sense in the 1950s, but is rarely important these days; we waste time worrying about the micro-optimization it is to pick the right number type, while we lose sight of the bigger picture. As Douglas Crockford explains, in JavaScript there's only a single number type, which is a great idea - just too bad it's the wrong single number type.

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of a language with a single, sane number type.

With the modern computer resources we have today, a programming language that would restrict you to a single, sane number type would be superior to the mess we have today.

Null pointers

Nulls are one of the most misunderstood language constructs. There's nothing wrong with the concept of a value that may or may not be present. Many great programming languages have this concept. In Haskell, it's called Maybe; in F#, it's called option; in T-SQL it's called null. What's common in all these languages is that it's an opt-in language feature: you can declare a value as being 'nullable', but by default, it isn't (nullable).

However, due to Tony Hoare's self-admitted billion-dollar mistake, mainstream languages have null pointers: C, C++, Java, C#. The problem isn't the concept of 'null', but rather that everything can be null, which makes it impossible to distinguish between the cases where null is an appropriate and expected value, from the cases where null is a defect.

Design a language without null pointers, and you take away the ability to produce null pointer exceptions.

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of a language without null pointers.

If Tony Hoare is correct that null pointers cost billions of dollars in the last few decades, getting rid of that source of defects can save you lots of money. From Turing-complete languages like T-SQL, Haskell, and F# we know that you can express all valid programs without null pointers, while a huge source of defects is removed.


A central concept in Procedural, Imperative, and Object-Oriented languages is that you can change the value of a variable while executing your program. That's the reason it's called a variable. This seems intuitive, since a CPU contains registers, and what you actually do when you execute a program is that you move values in and out of those registers. It's also intuitive because the purpose of most programs is to change the state of the world: Store a record in a database. Send an email. Repaint the screen. Print a document. Etc.

However, it turns out that mutation is also a large source of defects in software, because it makes it much harder to reason about code. Consider a line of C# code like this:

var r = this.mapper.Map(rendition);

When the Map method returns, has rendition been modified? Well, if you follow Command Query Separation, it shouldn't, but the only way you can be sure is to review the implementation of the Map method. What if that method exhibits the same problem, by calling into other methods that could mutate the state of the application? There's nothing in C# (or Java, or JavaScript, etc.) that prevents this from happening.

In a complicated program with a big call stack, it's impossible to reason about the code, because everything could mutate, and once you have tens or hundreds of variables in play, you can no longer keep track of them. Did the isDirty flag change? Where? What about the customerStatus?

Imagine taking away the ability to mutate state:

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of the set of possible instructions in a language without mutation.

Most languages don't completely take away this ability, but as Haskell (a Turing complete language) demonstrates, it's possible to write any program without implicit state mutation.

At this point, many people might object that Haskell is too difficult and unintuitive, but to me, that kind of argumentation is reminiscent of the resistance to removing GOTO. If you are used to relying on GOTO, you have to learn alternative ways to model the same behaviour without GOTO. Likewise, if you are used to relying on state mutation, you have to learn alternative ways to model the same behaviour without state mutation.

Reference Equality

In Object-Oriented languages like C# and Java, the default equality comparison is Reference Equality. If two variables point to the same memory address, the variables are considered equal. If two variables have all identical constituent values, but point to two different memory addresses, they are considered different. That's not intuitive, and many software defects are caused by this.

What if you take away Reference Equality from a language?

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of the set of possible instructions in a language without Reference Equality.

What if all data structures instead had Structural Equality?

This one I'm not entirely sure about, but in my experience, I almost never need Reference Equality. For Basic Correctness you never need it, but I wonder if you may need the occasional reference comparison in order to implement some performance optimizations... Still, it wouldn't hurt to switch the defaults so that Structural Equality was the default, and there was a special function you could invoke to compare two variables for Reference Equality.


Even in 2015, inheritance is everywhere, although it's more than 20 years ago the Gang of Four taught us to favor object composition over class inheritance. It turns out that there's nothing you can do with inheritance that you can't also do with composition with interfaces. The converse doesn't hold in languages with single inheritance: there are things you can do with interfaces (such as implement more than one), that you can't do with inheritance. Composition is a superset of inheritance.

This isn't just theory: in my experience, I've been able to avoid inheritance in the way I've designed my code for many years. Once you get the hang of it, it's not even difficult.


Many strongly typed languages (for example Java and C#) have interfaces, which is a mechanism to implement polymorphism. In this way, you can bundle various operations together as methods on an interface. However, one of the consequences of SOLID is that you should favour Role Interfaces over Header Interfaces, and as I've previously explained, the logical conclusion is that all interfaces should only have a single member.

When interfaces only have a single member, the interface declaration itself tends to mostly be in the way - the only thing that matters is the operation. In C#, you could just use a delegate instead, and even Java has lambdas now.

There's nothing new about this. Functional languages have used functions as the basic compositional unit for years.

In my experience, you can model everything with single-member interfaces, which also implies that you can model everything with functions. Again, this isn't really surprising, since functional languages like Haskell are Turing complete too.

It's possible to get by without interfaces. In fact, Robert C. Martin finds them harmful.


If you've ever done any sort of meta-programming in .NET or Java, you probably know what Reflection is. It's a set of APIs and language or platform features that enable you to inspect, query, manipulate, or emit code.

Meta-programming is an indispensable tool, so I'd be sorry to lose it. However, Reflection isn't the only way to enable meta-programming. Some languages are homoiconic, which means that a program in such languages is structured as data, which itself can be queried and manipulated like any other data. Such languages don't need Reflection as a feature, because meta-programming is baked into the language, so to speak.

In other words: Reflection is a language feature aimed at the goal of meta-programming. If meta-programming can be achieved via homoiconicity instead, it would imply that Reflection is a redundant feature.

Cyclic Dependencies

While it may be true that null pointers are the biggest single source of software defects, in my experience the greatest single source of unmaintainable code is coupling. One of the most problematic types of coupling is cyclic dependencies. In languages like C# and Java, cyclic dependencies are almost impossible to avoid.

Here's one of my own mistakes that I only discovered because I started to look for it: in the otherwise nice and maintainable AtomEventStore, there's an interface called IXmlWritable:

public interface IXmlWritable
    void WriteTo(XmlWriter xmlWriter, IContentSerializer serializer);

As you can tell, the WriteTo method takes an IContentSerializer argument.

public interface IContentSerializer
    void Serialize(XmlWriter xmlWriter, object value);
    XmlAtomContent Deserialize(XmlReader xmlReader);

Notice that the Deserialize method returns an XmlAtomContent value. How is XmlAtomContent defined? Like this:

public class XmlAtomContent : IXmlWritable

Notice that it implements IXmlWritable. Oh, dear!

Although I'm constantly on the lookout for these kinds of things, that one slipped right past me.

In F# (and, I believe, OCaml), on the other hand, this wouldn't even have compiled!

While F# has a way to introduce small cycles within a module using the and and rec keywords, there are no accidental cycles. You have to explicitly use those keywords to enable limited cycles - and there's no way to define cycles that span modules or libraries.

What an excellent protection against tightly coupled code! Take away the ability to (inadvertently) introduce Cyclic Dependencies, and get a better language!

The set of all valid programs, inside the much larger set of all possible instructions, with the overlay of the set of possible instructions in a language that disallows cycles.

This has even been shown to work in the wild: Scott Wlaschin examined C# and F# projects for cycles, and found that F# projects have fewer and smaller cycles than C# projects. This analysis was later enhanced and corroborated by Evelina Gabasova.


What I've tried to illustrate in this article is that there are many ways in which you could make a better language by taking away a particular feature from an existing language. Take away a redundant feature, and you'll still have a Turing complete language that can do (close to) anything, but with fewer options for shooting yourself in the foot.

Perhaps the ultimate programming language is a language without:

  • GOTO
  • Exceptions
  • Pointers
  • Lots of specialized number types
  • Null pointers
  • Mutation
  • Reference equality
  • Inheritance
  • Interfaces
  • Reflection
  • Cyclic dependencies

Have I identified all possible redundant features? Most likely not, so here's a great opportunity for language designers to define an even better language, by finding something new to take away!

Update September 14 2015: This article sparked a .NET Rocks! episode with even more discussion about this topic.


The idea that Javascript is a superset of "valid programs", but that C is not, could do with more explanation.

My background is in server code (Haskell/Java/Python/Javascript/Rust), web code (Javascript) and embedded code (Rust/C) (with some ARM assembly for when Rust/C can't produce the needed program unaided). This makes the idea that I could express programs in Javascript, which I couldn't in C, very interesting.

I think that Rust is going to be very important in embedded development, in a few years.

P.S. I hope I have commented in the correct way.

2015-04-14 8:07 UTC

Chris, thank you for writing. Where in this article do I claim that there are programs you could express in JavaScript, but not in C?

2015-04-14 14:41 UTC

Thanks for the post Mark. Some great points; I like especially the idea of disallowing cyclic dependencies. That'd be awesome on a legacy Java project I'm working on now!

Having working in Scala and a little Haskell, I can say I love the Maybe type. One thing I have trouble visualising still is Exceptions. I think I need to find some more examples on doing without exceptions in real applications.

I guess there's somewhat of a middle ground between having the "safest" language, vs the most performant language. In a lot of cases you don't need awesome performance so the safest language is the better one... but in some you do. You occasionally need control over how your bits are packed.

Let's hope for the future!

2015-04-16 6:24 UTC

Lachlan, thank you for writing. When it comes to working without exceptions, the point is to replace them with something stronger. Scott Wlashin's post and presentation about Railway Oriented Programming is a great place to start. You can see another example in my No Mocks presentation, and in one of my up-coming Pluralsight courses.

When it comes to the balance between safe and performant, there will always be room for languages that sacrifice safety for speed, but in my experience, this shouldn't be a common concern. As soon as you're doing any sort of significant I/O, the cost of that tends to be so much higher than any potential imperfections in pure CPU processing.

Apart from that, I don't see why, in principle, a safe language couldn't also be performant. These two traits don't seem to me to be intrinsically mutually exclusive.

2015-04-17 8:55 UTC

As one of less-is-more advocates, I feel your article makes sense instinctively. However, one thing keeps bugging me: How do you define "valid programs"?

I think I know what you try to mean with it, but once we try to be more specific, validity can only be defined in terms of a certain framework of semantics. If we have such a framework, then we can derive a programming language from it, which would be a language that only accepts valid programs and reject others. Problem solved.

In reality we don't have such a framework. We don't know what is a valid program precisely. There are programs that doesn't seem to make sense, and there are programs that are obviously useful, but there are huge gray area inbetween. It's actually a moving target---the context, the environment, the user, and other outside factors will greatly affect what's valid or not. But the fact that we don't know what valid programs shakes the ground of this whole discussion, doesn't it? Or it can end up tautology---"valid programs are defined in terms of this language, so this language fit the best to cover the valid programs."

2015-05-15 03:09 UTC

Shiro Kawai, thank you for writing. In this article, I deliberately left the definition of validity vague. Why do we write software? If we exclude Katas and the like (which we do for training), software is ultimately written in order to solve problems; in order to be used. A valid program is a program that's useful.

This doesn't change the discussion, which is pragmatic. From experience, we know that we don't need GOTO; from experience, we know that we don't need pointers; from experience, we know that we don't need null pointers; from experience, we know that we don't need inheritance; etc.

2015-05-15 5:47 UTC

Hi, thanks for the reply. I believe I get your intent. What I wanted was to point out a danger of this kind of argument. Because when you write software to solve problems, you need to articulate the problem, and the way you frame the problem is inevitably influenced by the frame of your language of choice. In other words, when you think you tighten the circle of the language features, it's not necessarily that you get it close to supposed set of "valid programs", but you may just as well start ignoring useful programs outside of your langauge circle and you don't even realize that. You just think "ok, there may be some programs that falls outside but it's not that important." How do you verify that the excluded gray area isn't that important?

(I wrote "you", but actually this is something I constanly try to remind myself, since I tend to be dragged toward feature-minimalism. I love Scheme.)

For example, you talk single-member inheritance and you brought up Haskell, but you didn't mention type classes. Isn't type classe sort of interface, in a sense that it defines a protocol consists of multiple functions? Do you say it's actually redundant? Or will you dismiss programs that can be expressed well using type classes "unimportant"?

Another controversial (and probably obscure) feature is change-class in CL. This one is such a beast that it's difficult to cope with many modern features (obviously it invalidates strict typecheckers). However, in the context where the feature is needed, I don't know if there's a replacement. If you get rid of it, you just have to give up the kind of software that require the feature. That's a reasonable trade-off, but what you do is that you cut off a part of "valid programs" in order to tighten the circle.

There's no solid circle of valid programs, and as you tighten the circle of the language, you actually shape the circle of valid programs that suit to the circle of language you draw. That's not necessarily a bad thing, but the language designers need to be aware of it, that's what I wanted to say.

2015-05-15 7:40 UTC

It's a good point that a language shapes how you approach problems. In this discussion, I assume that all languages (both existing and hypothetical) are Turing complete, but even under that assumption, there will be problems that are difficult, or perhaps even impossible, to address in a particular language.

The question is whether that isn't true for all languages?

In a sense, the most 'powerful' languages are all the dialects of machine code. By definition, they should be able to express everything a particular CPU can do, so by corollary, they should be the most complete languages available. Despite being complete, these dialects don't have inheritance, exceptions, Reflection, interfaces, or a lot of other things. Such features are features that have been added to some languages. Logically, I don't see how 'removing' such a feature constrains one's ability to express 'all valid programs'.

To be fair, that argument of mine doesn't apply to all the language features I've described. For example, CPU instruction sets most certainly allow immutability.

What I was aiming at with my article was never a formal discussion about how a hypothetical language would, or wouldn't, be able to express 'all valid programs'. The purpose was more to point out that decades of experience should by now have taught the overall programmer community that there are many language features that we can do without.

As I pointed out, you can't arbitrarily remove features. Some features are extremely important within a particular language, while it may be irrelevant in another language. While I don't know much about Type Classes, it seems to be an example of this: it's very important in Haskell, but doesn't exist at all in C# or F#.

This is also the reason why I don't think we'll ever have a single, perfect programming language. In the future, we'll also have many different languages, and they'll be good at different types of problems.

Just as it is today, it'll be important to know more than a single programming language, because, exactly as you wrote, a problem may 'fall outside' a given language, but then, if you know another language, you may realise that you can use it to solve that particular problem.

2015-05-16 14:29 UTC

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