“Well it just so happens that your friend here is only mostly dead.
There’s a big difference between mostly dead and all dead…
Now, mostly dead is slightly alive.
Now, all dead — well, with all dead, there’s usually only one thing that you can do.”
“Go through his clothes and look for loose change!”
— from the movie “The Princess Bride”.
To allow for backtrackable destructive updates
— that is, updates whose effect is undone on backtracking,
perhaps by recording the overwritten values on a “trail”
so that they can be restored after backtracking —
Mercury also provides “mostly unique” modes.
are equivalent to
except that only references which will be encountered during forward execution
are counted —
it is OK for
to be needed again on backtracking.
Mercury defines some standard modes for manipulating “mostly unique” values, just as it does for unique values:
% mostly unique output :- mode muo == free >> mostly_unique. % mostly unique input :- mode mui == mostly_unique >> mostly_unique. % mostly destructive input :- mode mdi == mostly_unique >> mostly_dead.