How to interact with the system clock using strict functional programming.

A couple of years ago, I published an article called Good times with F#. Unfortunately, that article never lived up to my expectations. Not that I don't have a good time with F# (I do), but the article introduced an attempt to model execution durations of operations in a functional manner. The article introduced a Timed<'a> generic type that I had high hopes for.

Later, I published a Pluralsight course called Type-Driven Development with F#, in which I used Timed<'a> to implement a Polling Consumer. It's a good course that teaches you how to let F#'s type system give you rapid feedback. You can read a few articles that highlight the important parts of the course.

There's a problem with the implementation, though. It's not functional.

It's nice F# code, but F# is this friendly, forgiving, multi-paradigmatic language that enables you to get real work done. If you want to do this using partial application as a replacement for Dependency Injection, it'll let you. It is, however, not functional.

Consider, as an example, this function:

// (Timed<TimeSpan list> -> bool) -> (unit -> Timed<'a>) -> Timed<TimeSpan list>
// -> State
let transitionFromNoMessage shouldIdle idle nm =
    if shouldIdle nm
    then idle () |> Untimed.withResult nm.Result |> ReadyState
    else StoppedState nm.Result

The idle function has the type unit -> Timed<'a>. This can't possibly be a pure function, since a deterministic function can't produce a value from nothing when it doesn't know the type of the value. (In F#, this is technically not true, since we could return null for all reference types, and 'zero' for all value types, but even so, it should be clear that we can't produce any useful return value in a deterministic manner.)

The same argument applies, in weaker form, to the shouldIdle function. While it is possible to write more than one pure function with the type Timed<TimeSpan list> -> bool, the intent is that it should look at the time statistics and the current time, and decide whether or not it's 'safe' to poll again. Getting the current time from the system clock is a non-deterministic operation.

Ever since I discovered that Dependency Injection is impossible in functional programming, I knew that I had to return to the Polling Consumer example and show how to implement it in a truly functional style. In order to be sure that I don't accidentally call an impure function from a 'pure' function, I'll first rewrite the Polling Consumer in Haskell, and afterwards translate the Haskell code to F#. When reading, you can skip the Haskell article and go directly to the F# article, or vice versa, if you like.

Next: Pure times in Haskell.

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Tuesday, 27 June 2017 09:11:00 UTC


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