Non-Deterministic Evaluation In Under 300 Bytes
mwh21 at cam.ac.uk
Tue Mar 21 02:11:19 CET 2000
I was playing around with Chris Tismer's stackless python today, and
wondering what I could actually use continuations for, so I pulled my
copy of On Lisp off the bookshelf and came up with:
from continuation import caller
if not cs:
raise "no answers found"
_p.append( lambda c=cs[1:],k=caller():k(choose(c)) )
global _p; _p = 
_p = 
(that's actually 317 bytes, but if you take the indentation down to 1,
it's about 280).
It can be used like so:
Python 1.5.42 (#1, Mar 19 2000, 15:06:01) ...
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> import short_amb # I have a more verbose, easier to use version...
>>> def trick(n):
... x = short_amb.choose(range(2,11,2))
... y = short_amb.choose(range(1,11))
... if x + y <> n: short_amb.choose()
... return x,y
(the example's from On Lisp).
It's not fast, but I think it's cute, and quite a nice exposition of
just how powerful continuations can be.
1) excessive use of side effects in code being non-deterministically
evaluated *will* make you're head explode.
2) call-with-current-continuation is a *really bad* interface to the
continuation world, at least from the point of actually
understanding the code (I implemented the above by writing a
call/cc function, transliterating the scheme version Graham
presents in the book, and then "unwinding" the calls to
call/cc. The result made more sense then the original - even than
the scheme!). Chris's approach is better, at least for me.
3) The above does a depth-first search of the solution space, but can
be made to do breadth first search with the addition of a single
character ("_p.pop()()" -> "_p.pop(0)()").
Anybody feel like extending the above to unification/resolution and
hence embedding (possibly the slowest implementation the world has
ever seen of) a prolog-style language in Python? I'd give it a stab,
but I don't know logic programming that well.
very few people approach me in real life and insist on proving they are
drooling idiots. -- Erik Naggum, comp.lang.lisp
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