[mercury-users] memo and zero-arity functions

[Zoltan Somogyi]

On 29-Jul-2006, Michael Day <mikeday at bigpond.net.au> wrote:
> :- pragma foreign_proc(c,
>         value_exists,
>         [will_not_call_mercury, thread_safe], "
>     SUCCESS_INDICATOR = (Value != NULL);
> ").

No, tabling in general cannot possibly be made as efficient as this code.
The reason why your code works is that it exploits the knowledge that only
non-NULL values of the global variable are valid. If the tabled predicate
returned an integer, there would be no bit pattern one could use to indicate
"there is no value here", so this scheme wouldn't work.

I'll look into applying your optimization automatically to tabled predicates
in the cases where it is applicable.

Zoltan.
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