[mercury-users] Modes problem

[Simon Taylor]

On Fri, May 18, 2001 at 04:09:08PM +0200, Ondrej Bojar wrote:
> 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'.

The function std_util__pair has only the default forwards mode.
:- func pair(T1, T2) = pair(T1, T2).
:- mode pair(in, in) = out is det.

In your code the output variable (Head) is bound before
the call to std_util__pair, the inputs (A) are not.

The code below will work. Data constructors can be used in any mode.

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

Simon.
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