[mercury-users] Higher-order predicates

[Douglas Michael Auclair]

Hi,

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 compiler states that (Foof) has type 
pred((parents:bar), (parents:bar)) and that it was expecting type ((func 
V_3) = V_4).  When I make a function instead:

  { Fn = (func(X) = Z is nondet :- foo(X, Z)),
    solutions(Fn(baz), Bars) }, % ...

the compiler complains it was looking for type (pred T).  Back to square 
one.

I solved the problems with a new top-level predicate:

:- pred foo_helper(bar, bar).
:- mode foo_helper(in, out) is nondet.

foo_helper(X, Z) :- foo(X, Z).

but this solution is distasteful (it pollutes the namespace, for one, making 
the code less elegant/maintainable/comprehensible, especially when there's N 
mult-moded predicates I must process through solutions/2).

What is the way to create a higher-order predicate that's visible only in 
the local context?

Sincerely,
Doug Auclair

_________________________________________________________________
Chat with friends online, try MSN Messenger: http://messenger.msn.com
--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------