CODEWITHSUNDEEP
ocaml
Tito
Tito

Reputation: 298

Types of OCaml functions

I started learning functional programming (OCaml), but I don't understand one important topic about functional programming: types.

Can anyone explain me this solution please?

Have a test this week and can't get reach the resolution.. 

let f a b c = a (a b c) 0;;
f: ('a -> int -> 'a) -> 'a -> int -> 'a

Upvotes: 0

Views: 462

Answers (1)

Jackson Tale
Jackson Tale

Reputation: 25812

let f a b c = a (a b c) 0;;

Your confusion involves types and type inference, i.e., when you define a function or binding, you don't need to give explicit types for its parameters, nor the function/binding itself, OCaml will figure it out if your definition of function/binding is correct.

So, let's do some manual inferences ourselves. If a human can do, then the compiler can also do.

1.

let x = 1

1 is integer, so x must be an integer. So you don't need to do int x = 1 as in other languages, right?

2.

let f x = 1

If there are multiple variable names between let and =, then it must be a function definition, right? Otherwise, it won't make sense. In Java like language, it also makes no sense to say int x y = 1, right?

So f is a function and x is must be a parameter. Since the righthand side of = is an integer, then we know f will return an integer. For x, we don't know, so x will be thought as a polymorphic type 'a.

So f: 'a -> int = <fun>.

3.

let f a b c = a (a b c) 0;;

  • f is a function, with parameters a, b, c.

  • a must be a function, because on the righthand side of =, it is a function application.

  • a takes two arguments: (a b c) and 0. 0 is an integer, so the 2nd parameter of a must be an integer type.

  • Look inside (a b c), c is the 2nd arguement, so c must be integer.

  • We can't infer on b. So b is 'a.

  • Since (a b c) can be the 1st argument of a, and (a b c) itself is an application on function a, the return type of a will have the same type of b which is 'a.

Combine information above together, you get f: ('a -> int -> 'a) -> 'a -> int -> 'a.

If you want to learn it formally, https://realworldocaml.org/ is your friend.

Upvotes: 4

Related Questions