[mercury-users] Starting to understand some/all

[Douglas Auclair]

Dear all,

Now I finally understand why Mercury doesn't have the list predicates of 
any/2 and every/2!  I ran across a compilation error, however, and I need 
some help.  The compiler complained that for:

some [X] (member(X, List) => p(X))

X was free and should have been bound under the negation.  I don't 
understand the error (what negation?).  My intention is to see of any 
element of List passes p (which I think I'm doing by binding X for each of 
the elements of List and passing that value to p).  When I changed the term 
to:

not (all [x] (member(X, List) => not p(X)))

it compiled and ran fine.

Am I using some incorrectly?  I don't wish to use a negated predicate under 
a negated universal quantification test, what are the ways I can use to 
avoid this double negation?

Sincerely,
Doug Auclair

Note:  p's signature is pred p(X::in) is semidet.

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