[mercury-users] Predicate call uses the "wrong" mode

[Julien Fischer]

On Fri, 3 Aug 2007, Barney Fisher wrote:

> I've been investigating why some of my Mercury code is running slowly.
> I eventually found the reason: the wrong mode of a predicate was
> being called.  The following is relevant code to show the problem.
> Although, I can provide full sources if needed.
>
> :- pred atm(drug,atomid,element,int,charge).
> :- mode atm(in,out,out,out,out) is nondet.
> :- mode atm(in,out,in,out,out) is nondet.
>
> % atm/5 is a fact table
>
> :- pred atoms(drug,int,atomidlist,elementlist).
> :- mode atoms(in,in,out,in) is nondet.
>
> atoms(Drug,1,[Atom],[T]) :-
>   atm(Drug,Atom,T,_,_),
>   T \= "h".
> atoms(Drug,N1,[Atom1|[Atom2|List_a]],[T1|[T2|List_t]]) :-
>   N1 > 1,
>   N2 is N1 - 1,
>   atoms(Drug,N2,[Atom2|List_a],[T2|List_t]),
>   atm(Drug,Atom1,T1,_,_),
>   Atom1 @> Atom2,
>   T1 \= "h".
>
> When compiled like this, atoms should really call atm in mode
> :- mode atm(in,out,in,out,out) is nondet.
> because Drug and T1 are instantiated, but it actually calls atm in mode
> :- mode atm(in,out,out,out,out) is nondet.
>
> If the code is changed to be like
> :- mode atm(in,out,in,out,out) is nondet.
> :- mode atm(in,out,out,out,out) is nondet.
> then everything is fine (i.e. mode in,out,in,out,out is called at runtime.).
>
> This made the difference between my program taking 5 seconds to
> execute and 0.5 seconds!
>
> I was quite surprised by this situation because I had assumed the mode
> system would always call the most instantiated mode possible.  Is this
> a bug in mode analysis?

No.

> if not, is there a general rule for which mode
> of a predicate will be called.

Yes, they are called in declaration order

>  The code above could lead to the
> assumption that the first applicable mode is chosen but I know from
> other situations this isn't correct.

See section 4.2 of the reference manual, the bit about ``implied modes''.

Julien.
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