Question

I’m a little stuck on this assignment for OCAML. I’m trying to pass in a function and a value as a parameter into another function. For example, I have function called test that takes in (fun x -> x+x) and 3 as parameters. The function test should output 6, since 3 + 3 = 6. I know I can achieve something similar this by completing:

let func x = x + x;;

let a = func;;

let test a x = (a x);;

This way when I input, test a 5, I will get 10.

But when I change the statement to this, I get only the value I placed in for x. How do I get the (fun a -> a) to take in the value x?

let test a x = ((fun a -> a) x);;

Answer 1

fun a -> a is an anonymous identity function, it will always return its parameter. You could say:

let id = fun a -> a;;
id 3;;
  => 3
id (fun x -> x + x);;
  => (fun x -> x + x)

Note that in your

let test a x = ((fun a -> a) x);;

the first a and the other two as are different variables. The first one is never used again later. You can rewrite this line for easier understanding as:

let test a x = ((fun b -> b) x);;

How do I get the (fun a -> a) to take in the value x?

You are, and that’s the problem. You’re feeding your x to an identity function, and getting x back. What you want to do is feed the x to your a function, and that’s what you did in your first attempt:

let func x = x + x;;
let test a x = a x;;
test func 3;;
  => 6