[m-dev.] Trailing

[Bart Demoen]

Peter Schachte writes:

> But I have another idea.  For this source code
> 
>         p(A, Z) :-
>                 (   mangle_array(A, A1),
>                     test(A1) ->
>                         q(A1, Z)
>                 ;   q(A, Z)
>                 ).
> 
> we generate code that looks like
> 
>         prev_trail = trail_pointer
>         call mangle_array/2
>         if (call test/1) {
>             notify trailed functions that they are being trimmed
>             trail_pointer = prev_trail
>             call q/2
>         } else {
>             untrail back to prev_trail
>             trail_pointer = prev_trail
>             call q/2
>         }

This has a problem if mangle_array/2 (or test/1) calls p/2. Can't you
store the prev_trail in the "environment" of p/2 ? (or was that your
intention ?) Another thing: you might want to store also the current
heap top if you want to recover heap space used during the If in case
it fails.

Putting TR and H in local variables was how if-then-else was
implemented in BIM (for certain tests only). Also HB := H temporarily
to do correct trailing ... (sorry for talking WAM :-)

Bart