Monomorphize compare function for builtin types

[emillon]

The [%ord: int] expression expands to Pervasives.compare. Since this function is polymorphic, it may be used with other types. For example, the following expression typechecks:

([%ord: int] : string -> string -> int)

This adds a type constraint on the Pervasives.compare call so that it becomes monomorphic. This is also more consistent with how this case is handled in the eq plugin.

Let me know what you think. Thanks!

[emillon]

Sorry, I was not invoking the test suite the correct way.

The problem with my patch is that in the following generated code:

type es =
  | ESBool of bool 
  | ESString of string 
and bool =
  | Bfoo of int* ((int -> int)[@compare fun _  -> fun _  -> 0]) 
and string =
  | Sfoo of String.t* ((int -> int)[@compare fun _  -> fun _  -> 0]) 
[@@deriving ord]
let rec (compare_es : es -> es -> Ppx_deriving_runtime.int) =
  ((let open! Ppx_deriving_runtime in
      fun lhs  ->
        fun rhs  ->
          match (lhs, rhs) with
          | (ESBool lhs0,ESBool rhs0) ->
              ((fun (a : bool)  -> fun b  -> Pervasives.compare a b)) lhs0 rhs0

bool resolves to Ppx_deriving_runtime.bool which is not the correct one. I am not too familiar with how quoting works in this project, so I am not sure how to fix that. Is it possible to quote types and not expressions?

[whitequark]

Your tests fail on Travis:

- File "src_test/test_deriving_ord.cppo.ml", line 123, characters 0-211:
- Error: This expression has type bool/2260
-        but an expression was expected of type
-          Ppx_deriving_runtime.bool = bool/5

[emillon]

Just fixed the tests (and quoting problems) by adding a call to Ppx_deriving.quote.