[m-users.] Use of length() as function giving unexpected (to me) error

[Zoltan Somogyi]

On Sat, 30 Oct 2021 08:20:11 +0200, Volker Wysk <post at volker-wysk.de> wrote:
> > The explanation is in the error message: the compiler does not know
> > whether each occurrence of length refers to function list.length/1,
> > in which case e.g. length(Args) is an int, or to predicate list.length/2,
> > in which case length(Args) is a curried call whose type is "pred int".
> 
> But you can't have a predicate as one of the two things being unificated,
> can you? You can't compare a predicate for equality with another
> predicate..?

No, you cannot unify a predicate with another predicate. However,
the typechecker, when it looks at "length(Args)", does not know whether
the context allows the type of length(Args) to be a predicate or not,
which is why it reports the error it does.

We *could* modify the compiler to keep track of such contexts, but

- that would slow down every execution of the type checker, i.e. it would
  slow down every compilation, but

- since this kind of ambiguity is reasonably rare in Mercury programs,
  this extra effort would be wasted in the vast majority of cases.

Zoltan.