Mutually defined functions are comparable in some circumstances.

[camlspotter]

A Tweet has reported a strange behaviour of OCaml structural comparison:
https://twitter.com/st_toHKR/status/1256270850434592770

Here is a simpler version:

$ ocaml
        OCaml version 4.10.0

# let rec f x = x and g x = x in compare f g;;
- : int = -1

If we add another definition x = 1, the comparison fails as expected:

# let rec x = 1 and f x = x and g x = x in compare f g;;
Warning 26: unused variable x.
Exception: Invalid_argument "compare: functional value".
Raised by primitive operation at unknown location
Called from file "toplevel/toploop.ml", line 212, characters 17-27

The comparion of the first case unexpectedly succeeds because the second function does not have Tag_closure but Tag_infix. It seems that the second and later defined functions all have Tag_infix:

# let rec f x = x and g x = x and h x = x in
   let obj_tag x = Obj.tag (Obj.repr x) in
   (obj_tag f, obj_tag g, obj_tag h);;
- : int * int * int = (247, 249, 249)

This strangeness is gone if we have a non function definition in let rec:

# let rec f x = x and g x = x and x = 1 and h x = x in
   let obj_tag x = Obj.tag (Obj.repr x) in
   obj_tag f, obj_tag g, obj_tag h
;;
- : int * int * int = (247, 247, 247)

[gasche]

It is not clear to me that this is a bug. Yes, mutually-recursive functions use a specific comparison that uses Infix_tag, and the difference can be observed through Obj. If we consider comparison on function values undefined, then the behavior we observe is fine. It is a bug, however, if we consider that comparison of function values should always raise an error. Is this your assumption?

Two remarks:

• Maybe the definition of runtime comparison operators on infix tags could be changed to achieve this (consistently raising) easily. In that case I think it would be nice to do the change.
• There are discussions of getting rid of infix tags in the function representation in the future (to simplify the GC and in particular Multicore), so this behavior may change / solve itself.

[gasche]

I found the issue in compare.c; I'll propose a PR and we will see whether there is consensus to consider this a bug and fix it.