[mercury-users] Is "High order predicates" realy high order?

[Fergus Henderson]

On 21-Nov-2002, Mike Potanin <potanin at mccme.ru> wrote:
> 
> Do Mercury have syntax for pred it return another pred?

Yes.  See the "Higher-order" chapter of the Mercury language reference manual.

> Can I write?
> :-pred pp(pred (pred (X), pred (X))).
> :-mode pp(pred((pred (in) is det), (pred (out) is det)) is det) is det.

To make it parse correctly, you need to delete the space between "pred"
and the immediately following "(":

:- pred pp(pred(pred(X), pred(X))).
:- mode pp(pred((pred(in) is det), (pred(out) is det)) is det) is det.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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