· f

F#: Stuff I get confused about

Coming from the world of C# I’ve noticed that there are a couple of things that I sometimes get confused about when playing around with stuff in F# land.

In C# I’m used to putting the arguments in parentheses but that doesn’t quite work in F#.

For example in my twitter application I was trying to append two lists together similar to this:


let first_item = Seq.singleton("mark")
let second_item = Seq.singleton "needham"
let joined_items = Seq.append (first_item, second_item)

Which doesn’t compile with the following error message:


The type 'b * 'c' is not compatible with the type 'seq<'a>'

What we’ve done here is pass in a tuple containing ‘first_item’ and ‘second_item’ instead of passing them separately as arguments to the function.

The correct way of doing this is like so:


let joined_items = Seq.append first_item second_item

As I understand it in everything that we create in F# is an expression and when those expressions get evaluated we end up with some values.

I wrote previously how we got confused about this distinction in a coding dojo a couple of weeks ago. That particular example was around how we need to create functions which take in an argument of type ‘unit’ if they are to be picked up by the XUnit.NET test runner.

Dave explains how this works in the comments of that post:

Given this code:


let should_do_something () = Assert.AreEqual(2,2)

let square_it x = x * x

When we put brackets around the arguments we are passing to functions they stop being passed as arguments as the compiler now tried to evaluate what’s in the brackets first and pass it to the function.

To give an example from playing around with Seq.append, if we do this:


let joined_items = Seq.append (first_item second_item)

We get a compilation error over ‘first_item’:


The value is not a function can cannot be applied

Here the compiler attempts to evaluate the function ‘first_item’ with an argument ‘second_item’ but since ‘first_item’ is actually a value and not a function this is impossible.

This seems a bit restrictive to me although I’m sure there’s probably some benefits of this approach that I’m not yet aware of, maybe around type checking.

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