Question

I’m learning F# and am struggling to parse the intellisense that appears for Seq.fold:

val fold : ('State -> 'T -> 'State) -> 'State -> seq<'T> -> 'State

In C# I’m accustomed to the Aggregate extension method and can understand perfectly fine the C# declaration:

(extension) TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func);

So in order to understand/deduce the F# syntax in this case, I’ve been trying to establish an analogy between Seq.fold and Aggregate but perhaps this is the wrong approach. I understand that -> defines a function (or the signature?), but beyond that I’m having a hard time reading what appeared.

To be clear, I don’t need an example of how to use folding; I’m simply looking for a breakdown of the the F# syntax used in the Seq.fold intellisense. Thanks.

Answer 1

In F# and many other functional languages, 'a -> 'b -> 'c -> 'd is the type of function which takes parameters of type 'a, 'b, 'c as input*, and returns a 'd. So

  ('State -> 'T -> 'State)
//  1st      2nd   output

means, in C#, a function of type

State func<State,T>(State firstInput, T secondInput);

and

  ('State -> 'T -> 'State) -> 'State -> seq<'T> -> 'State
//1st input (a function)      2nd       3rd        output

means, in C#, a function of type

State func<State,T>(Func<State, T, State> firstInput,
                    State secondInput, 
                    IEnumerable<T> thirdInput);

_{*: currying is ignored for the moment.}