[mercury-users] Idea for another handy convention (preds, this time).

[Ralph Becket]

The language specifies that a function

:- func f(A, B, C) = Z.

lacking a mode declaration is assumed to have the conventional
mode, namely

:- mode f(in, in, in) = out is det.

This is especially handy for function lambdas.

I would like to propose a similar default for preds, namely
that a predicate

:- pred p(A, B, C).

lacking a mode declaration is assumed to have the conventional
(in most other languages) mode of

:- mode p(in, in, in) is semidet.

This would also do away with the need for excess verbiage in
many predicate lambdas, for example:

	Evens = list__filter((pred(X) :- X mod 2 = 0), Xs).

which, in my book, is preferable to

	Evens = list__filter((pred(X::in) is semidet :- X mod 2 = 0), Xs).

The default for functions is to only require explicit mode
declarations when something `unusual' is going on; I propose
we extend this idea to predicates as well.

Ralph
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