🚀 go-pugleaf

In article <TED.90Dec6145517@aiaia.nmsu.edu>, ted@nmsu (Ted Dunning) writes:
>
>
>i have two questions about prologs which contain a freeze/2 predicate
>such as sicstus prolog.
>
>1) how can one delay a goal until _either_ of two variables are
>instantiated?
>
>something like this could be done by the following,
>
>freeze(X,Y,G) :- freeze(X,G), freeze(Y,G).
>
>but this has the nasty consequence that G is called twice.

Try

freeze(X, Y, G) :- freeze(X, Flag = 1), freeze(Y, Flag = 1), freeze(Flag, G).

>
>2) how can freeze/2 be used to do co-routining?  does this require a
>modification of what is normally taken to be co-routining?

To implement coroutining in a producer-consumer context, use the
following general scheme.

start :- consume(X), produce(X).

produce([]) :- ......./* Stop condition */.
produce([X|Y]) :- ......./* Construct X*/, produce(Y).

consume(X) :- freeze(X, consume1(X)).

consume1([]).
consume1([X|Y]) :- ......./* Use X */, consume(Y).


In Sicstus prolog you can instead of the consume/consume1-pair write

:- wait consume/1.

consume([]).
consume([X|Y]) :- ......./* Use X */, consume(Y).

which is cleaner.
-- 
Lars-Henrik Eriksson				Internet: lhe@sics.se
Swedish Institute of Computer Science		Phone (intn'l): +46 8 752 15 09
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