Meta-interpreting !/0

[triska]

Dear all,

as hinted in #2720 (comment), several tools may benefit from being able to meta-interpret !/0, at the very least for analytic purposes and maybe in the future also for automated suggestions and rewrites using more recent constructs with more desirable algebraic properties.

I think it is possible to meta-interpret !/0 with the standard control constructs catch/3 and throw/1. Declaratively, !/0 is true, and any faithful emulation must succeed exactly once.

I therefore suggest the following predicate to emulate !/0 with exceptions:

cut.
cut :- throw(cut).

This succeeds, and throws an exception on backtracking.

We can catch the exception with catch/3. For example:

?- catch(((X=1;X=2),cut,member(Y,"abc")), cut, false).
   X = 1, Y = a
;  X = 1, Y = b
;  X = 1, Y = c
;  false.

This suggests, as a rough outline for how meta-interpretation could be performed:

mi((Before,!,After)) :-  % needs work
        catch((mi(Before),cut,mi(After)), cut, false).

I hope this helps!

All the best,
Markus