[m-dev.] :- external directive

[Simon Taylor]

On 10-Dec-2003, James Goddard <goddardjames at yahoo.com> wrote:
> Can anyone explain to me what the "external" directive
> does in the following code fragment?  It doesn't
> appear to be in the documentation.
> 
> (from library/builtin.m:431)
> 
> %-----------------------------------------------------------------------------%
> 
> :- external(unify/2).
> :- external(compare/3).
> :- external(compare_representation/3).

It means that these predicates are defined elsewhere (in this case
in runtime/mercury_ho_call.c).

`:- external' is a library implementation detail that users shouldn't
need to know about, so it isn't documented.

Simon.
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