[mercury-users] Higher-order predicates

[Fergus Henderson]

"Douglas Michael Auclair" <dauclair at hotmail.com> writes:
> I have some predicates, one of which is defined thus:
> 
> :- pred foo(bar, bar).
> :- mode foo(in, in) is semidet.
> :- mode foo(in, out) is nondet.
> :- mode foo(out, in) is nondet.
> 
> foo(X, Z) :- quux(X, Y), quux(Y, Z), X \= Z.
> 
> The problem is that in my main, I cannot use foo as a higher-order predicate 
> (for solutions); the compiler complains that this is not an implemented 
> feature ("Taking the address of a pred with multiple modes") and suggests 
> making a lambda expression.  But:
> 
> main -->
>   { Foof = pred(X::in, Z::out) is nondet :- foo(X, Z),
>     solutions(Foof(baz), Bars) },
>   % using Bars
>   .
> 
> doesn't work either.

The problem here is a matter of syntax: you need parentheses around
lambda expressions.  So that should be written as

	main -->
	  { Foof = (pred(X::in, Z::out) is nondet :- foo(X, Z)),
	    solutions(Foof(baz), Bars) },
	  % using Bars
	  .

-- 
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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