I am testing the following OCaml's online environment: https://try.ocamlpro.com/

However, my algorithms are failing because of the use of List.sort compare x, where x is a list of integers. I have tested the following basic code List.sort compare [2; 3; 1];; and seen it throws an error:

Error: This expression has type ('a -> 'a -> int) -> 'a list -> 'a list -> int
but an expression was expected of type
  ('a -> 'a -> int) -> ('a -> 'a -> int) -> int
Type 'a list is not compatible with type 'a -> 'a -> int 

The online environment specifies it is OCaml 4.13.1, so does this mean compare is deprecated? In that case, how can I substitute it?

EDIT:

I have seen the following:

If you just write List.sort compare [2; 3; 1];; in the online environment, then it returns the correct output - : int list = [1; 2; 3].

However, if you load the List module with open List, then the error above is thrown.

CodePudding user response：

As noted in the comments, by opening the List module, you've now shadowed Stdlib.compare (or Pervasives.compare) with List.compare, and the following no longer works.

sort compare [2; 3; 1]

There are a few solutions to this.

Don't open List

Without opening List, you can successfully refer to the polymorphic compare from Stdlib or Pervasives.

List.sort compare [2; 3; 1]

We can give List a shorter name if it's preferable.

module L = List

Then write:

L.sort compare [2; 3; 1]

Open List and explicitly refer to Stdlib.compare

When we open List, its compare shadows the polymorphic compare, but that function is still there.

sort Stdlib.compare [2; 3; 1]

That's pretty unweildly. You're probably just better off with List.sort compare [2; 3; 1].

Of course, in this case you don't really need the polymorphic compare. You just need to compare integers.

sort Int.compare [2; 3; 1]

That's not so bad.

Rename compare

You could bind Stdlib.compare to a different name. Either before or after opening List.

let cmp = compare

open List

let l = sort cmp [2; 3; 1]

open List

let cmp = Stdlib.compare

let l = sort cmp [2; 3; 1]