[mercury-users] Modes problem

[Ondrej Bojar]

Hi.

I get this message:

extract.m:023: In clause for `extract(in, in, out)':
extract.m:023:   in argument 2 of call to function `std_util:pair/2':
extract.m:023:   mode error: variable `AVal' has instantiatedness `free',
extract.m:023:   expected instantiatedness was `ground'.

What I am trying to do is this (simplified version of the problem):

:- pred test(list(pair(string))::in).

test([Head|Tail]) :-
  Head = pair(A,A).

The predicate test should succeed if and only if the given list (is
nonempty and) starts with a pair of identic structures.

Here, the error is similar:

pok.m:007: In clause for `test(in)':
pok.m:007:   in argument 1 of call to function `std_util:pair/2':
pok.m:007:   mode error: variable `A' has instantiatedness `free',
pok.m:007:   expected instantiatedness was `ground'.

How should I write it? I need to get first element of the list, whatever
it is, then try to unify it with soemthing (and backtrack, if that fails).

Thanks again a lot for your help,
  Andrew.





--------------------------------------------------------------------------
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
--------------------------------------------------------------------------