Using a "compare/cmp" function, Belt.Id confusion

1

I’m trying to write a function that returns the maximum value of an array given a comparator function. That’s one of those functions that returns -1 if the first parameter is less than the second, +1 if the first parameter is greater than the second, and 0 if they are the same. I can get it to work as shown below.

    type nonEmptyArray<'t> = NonEmptyArray(array<'t>)

    let make = xs =>
      switch xs {
      | [] => None
      | xs => xs->NonEmptyArray->Some
      }

    // Does not require a default/initial value because array has
    // at least 1 item. And will always return a meaningful result.
    let maxBy = (NonEmptyArray(n), cmp) => n->Js.Array2.reduce((maximum, i) =>
        if cmp(i, maximum) < 0 {
          maximum
        } else {
          i
        }
      , n[0])

This is pretty good, but I don’t like that my cmp function is not really documented anywhere. It’s just a function that takes two of something and returns an int. So I’m trying to reuse some well-defined comparer function type but can’t figure out how to do it. This doesn’t work…

    // SYNTAX ERROR
    type comparer<'t> = (~a:'t,~b:'t) => int
    let bigger<'t> = (a,b,cmp:comparer<'t>) =>
      if cmp(a,b)<0 {
        b
      }
      else {
        a
      }

I figure for many of my types I’ll define comparator/id modules - still a big confused what these things are - so I can use them in sets and maps. But I can’t figure out how to re-use the compare function built into these things.

    // SYNTAX ERROR
    module IntComparable = Belt.Id.MakeComparable({
      type t = int
      let cmp = (a, b) => Pervasives.compare(a, b)
    })
    let compareTwoIntegers = (a, b) => IntComparable.cmp(a, b)
2

This is a syntax error because there’s no such construct as let bigger<'t> in ReScript. Values don’t have type parameters (EDIT: not exactly true, but that’s an advanced topic), only types do.

module IntComparable = Belt.Id.MakeComparable({...

This is actually not a syntax error, it is a type error:

[E] Line 6, column 35:
This expression has type
    Belt_Id.cmp<IntComparable.t, IntComparable.identity>
  It is not a function.

The value IntComparable.cmp is an object of the abstract type Belt.Id | ReScript API , it can’t be called as a function. In fact normally it won’t be used directly, but passed into Belt Set, Map etc. constructor functions though: Belt.Set | ReScript API

let mySet = Belt.Set.make(~id=module(IntComparable))
let mySet2 = Belt.Set.add(mySet, 1)

I think the key point is that Belt’s comparator values actually don’t let you just directly use them to compare values, but only indirectly through Belt’s data structures.

3

We could use the Belt comparators with this snippet:

module IntComparable = Belt.Id.MakeComparable({
  type t = int
  let cmp = (a, b) => Pervasives.compare(a, b)
})

let cmp = Belt.Id.getCmpInternal(IntComparable.cmp)
let _ = cmp(. 1, 2)