[m-users.] type error message for polymorphic predicates

[Matthias Güdemann]

Hi,

I came across an error message which could be clearer in my opinion.

The following program:

╭────
│:- module test.
│:- interface.
│:- import_module io.
│:- pred main(io::di, io::uo) is det.
│:- implementation.
│:- import_module int.
│:- pred test(float::in, float::in) is semidet.
│test(A, B) :-  A < B.
│main(!IO) :-
│    ( test(1.0, 2.0) -> print("ok\n", !IO); print("nok\n", !IO)).
╰────

gives the message (mmc -E):

╭────
│Making Mercury/cs/test.c
│test.m:008: In clause for predicate `test'/2:
│test.m:008:   in argument 1 of call to predicate `<'/2:
│test.m:008:   type error: variable `A' has type `float',
│test.m:008:   expected type was `int'.
│test.m:008:   The partial type assignment was:
│test.m:008:     A_3: float
│test.m:008:     B_4: float
│test.m:008: In clause for predicate `test'/2:
│test.m:008:   in argument 2 of call to predicate `<'/2:
│test.m:008:   type error: variable `B' has type `float',
│test.m:008:   expected type was `int'.
│test.m:008:   The partial type assignment was:
│test.m:008:     A_3: float
│test.m:008:     B_4: float
│** Error making `Mercury/cs/test.c'.
╰────

In my opinion, it would be better to warn about the undeclared type
float instead. The declaration of the predicate states that floats are
expected and not integers.

Best regards
Matthias