F#: Is the "fun" keyword necessary?

Question

I was typing the "fun" keyword and then I remembered you don't have to in C#

Wouldn't this:

List.map (x -> x + 1) [1..10]

Be just as expressive as this?:

List.map (fun x -> x + 1) [1..10]

This makes me curious as to why the "fun" keyword is necessary at all. Can someone clarify why the "fun" keyword is syntactically required?

Chuck_50742 · Answer

The language is ambiguous without it.

let x y = y z -> y z

Does x call y on the function z -> y z or does it ignore its argument and return the function y z -> y z?

user_19299 · Answer

Lots of decent speculative answers already... I'll add to the mix:

F# has a core language that's compatible with OCaml, and OCaml uses 'fun'.

user_154186 · Answer

I know as part of currying, (see this post) you can replace:

let countOneToTen = fun y List.map(fun x -> x + 1) y
countOneToTen = [1..10]

with

let countOneToTen y = List.map(fun x -> x + 1) y
countOneToTen = [1..10]

without the fun keyword.