F# type signature gotchas

Today I was speaking with a colleague about some F#, and he pointed out a few gotchas with F# type signatures, especially if you’ve spent some time with Haskell (and not OCaml or other ML-ish language).

Aside: This post just runs through some gotchas, but if you would like more general information on reading -> style function signatures please let me know.

The example we were looking at is Seq.unfold, whose signature looks like this:

Seq.unfold : ('State -> ('T * 'State) option) -> 'State -> seq<'T>

Apostrophes for type parameters

Any type prefixed with a ' character represents a type parameter (or generic type in C# parlance). For unfold this means 'State and 'T can be any type. We can also write this in potentially more familiar .NET syntax:

Seq.unfold<'State, 'T> : ('State -> ('T * 'State) option) -> 'State -> seq<'T>

A lot of the F# code I see follows a more Haskellish (?) convention of using lowercase type variable names, more like:

Seq.unfold<'s, 't> : ('s -> ('t * 's) option) -> 's -> seq<'t>

Asterisk for tuples

Types separated by a * are tupled (or product types, which explains the * symbol). For example, (1, "abc", Foo()) is of type int * string * Foo.

So in unfold, 'T * 'State represents a tuple of 'T and 'State.

Postfix generic syntax

F# supports both .NET-style prefix generic syntax and ML-style postfix syntax. So instead of writing int option, we can also write Option<int> (both forms are equivalent). Which means we can re-write unfold as:

Seq.unfold<'s, 't> : ('s -> Option<'t * 's>) -> 's -> seq<'t>

Using unfold

With those things in mind, let’s use the unfold signature to work out what it does.

unfold :
  ('s -> Option<'t * 's>) -- A function that takes an 's and gives an optional tuple of 't and 's.
  -> 's                   -- A value of type 's
  -> seq<'t>              -- A sequence of 't values

Given a function that can take 's values and return a tuple of an element and next 's value or nothing, and a starting 's, unfold will generate a sequence of 't values until the generator function returns None (i.e. potentially infinite).

We could use this to generate a sequence of all the days since a starting date (infinite, at least until DateTime hits DateTime.MaxValue):

let daysAfterThisPost =
    DateTime(2015, 1, 22)
    |> Seq.unfold (fun d -> let d' = d.AddDays(1) in Some (d', d'))

Translating to other languages

Finally, if you’re more familiar with C# or Haskell, here are my attempted translations:

// F#
Seq.unfold : ('State -> ('T * 'State) option) -> 'State -> seq<'T>

-- Haskell
unfold :: (s -> Maybe (t,s)) -> s -> [t]

// C# (uncurried. seq = IEnumerable)
IEnumerable<T> Unfold<S,T>(Func<S, Option<Tuple<T,S>>> generator, S initial);

Haskell uses lowercase type names for generics (instead of ' characters), while concrete types have uppercase names. It also uses the same syntax for tuple types as values, so (1,2) :: (Int, Int). For some odd reason, Haskell uses :: for "type of" instead of a single :.

The C# version is a bit messier due to having to use Func instead of a shorthand for function types, and similarly for declaring tuple types. (I’ve also uncurried the C# version otherwise we end up with nested Func types everywhere, and it is the more typical form for C# functions.)

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