[mercury-users] Beginner question about pairs

[Dave Slutzkin]

--- Dave Slutzkin <dave_slutzkin at yahoo.com> wrote:

I apologise for the waste of space...  As always, it
takes fifteen minutes after you post for the solution
to hit you.  My mistake was assuming that a compiler
error message is actually useful :-).  It actually
applied to a different line.  (But it seems that a
clause is parsed without any whitespace.)

> :- pred
> check_place(pair(int,int),list(pair(int,int))).
> :- mode check_place(in,in) is det.
> check_place(_,[]).
> % line 69 follows...
> check_place(X-Y,[(OldX-OldY)|Rest]) :-
> 	cptemp(X,Y,OldX,OldY) ,
> 	(if (Y=OldY) then fail else true) ,
> 	(if abs(minus(OldX,X))=abs(minus(OldY,Y)) then fail
> else true) ,
> 	check_place(X-Y,Rest).

The error:

> n_queens.m:069: In clause for predicate
> `n_queens:check_place/2':
> n_queens.m:069:   error: ambiguous overloading
> causes
> type ambiguity.
> n_queens.m:069:   Possible type assignments include:
> n_queens.m:069: V_10 :: int or (pred int)

This was because the compiler wasn't sure whether the
two expressions containing 'abs' referred to the func
(with type = int) or the pred (with type pred int)
abs.  To resolve it, I changed to the predicate
version of abs.

Sorry for posting all this, but it could be helpful to
someone out there.

Was there another way to do this?  Am I correct in my
diagnosis?

Thanks, Dave.
Benjamin St, Parkville, Melbourne, Australia.


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