Checking for exactly one item in a sequence using C# and F# by Mark Seemann

Here's a programming issue that comes up from time to time. A method takes a sequence of items as input, like this:

public void Route(IEnumerable<string> args)

While the signature of the method may be given, the implementation may be concerned with finding out whether there is exactly one element in the sequence. (I'd argue that this would be a violation of the Liskov Substitution Principle, but that's another discussion.) By corollary, we might also be interested in the result sets on each side of that single element: no elements and multiple elements.

Let's assume that we're required to raise the appropriate event for each of these three cases.

Naïve approach in C# #

A naïve implementation would be something like this:

public void Route(IEnumerable<string> args)
{
    var countCategory = args.Count();
    switch (countCategory)
    {
        case 0:
            this.RaiseNoArgument();
            return;
        case 1:
            this.RaiseSingleArgument(args.Single());
            return;
        default:
            this.RaiseMultipleArguments(args);
            return;
    }
}

However, the problem with that is that IEnumerable<string> carries no guarantee that the sequence will ever end. In fact, there's a whole category of implementations that keep iterating forever - these are called Generators. If you pass a Generator to the above implementation, it will never return because the Count method will block forever.

Robust implementation in C# #

A better solution comes from the realization that we're only interested in knowing about which of the three categories the input matches: No elements, a single element, or multiple elements. The last case is covered if we find at least two elements. In other words, we don't have to read more than at most two elements to figure out the category. Here's a more robust solution:

public void Route(IEnumerable<string> args)
{
    var countCategory = args.Take(2).Count();
    switch (countCategory)
    {
        case 0:
            this.RaiseNoArgument();
            return;
        case 1:
            this.RaiseSingleArgument(args.Single());
            return;
        default:
            this.RaiseMultipleArguments(args);
            return;
    }
}

Notice the inclusion of the Take(2) method call, which is the only difference from the first attempt. This will give us at most two elements that we can then count with the Count method.

While this is better, it still annoys me that it's necessary with a secondary LINQ call (to the Single method) to extract that single element. Not that it's particularly inefficient, but it still feels like I'm repeating myself here.

(We could also have converted the Take(2) iterator into an array, which would have enabled us to query its Length property, as well as index into it to get the single value, but it basically amounts to the same work.)

Implementation in F# #

In F# we can implement the same functionality in a much more compact manner, taking advantage of pattern matching against native F# lists:

member this.Route args =
    let atMostTwo = args |> Seq.truncate 2 |> Seq.toList
    match atMostTwo with
    | [] -> onNoArgument.Trigger(Unit.Default)
    | [arg] -> onSingleArgument.Trigger(arg)
    | _ -> onMultipleArguments.Trigger(args)

The first thing happening here is that the input is being piped through a couple of functions. The truncate method does the same thing as the Take LINQ method does, and the toList method subsequently converts that sequence of at most two elements into a native F# list.

The beautiful thing about native F# lists is that they support pattern matching, so instead of first figuring out in which category the input belongs, and then subsequently extract the data in the single element case, we can match and forward the element in a single statement.

Why is this important? I don't know… it's just satisfying on an aesthetic level :)

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