Re: Ann: delay pack

On 10/08/2013, at 6:22 AM, Ian Tegebo wrote:

> Wow. I find it surprising that delay/1 didn't already exist.  Could
> someone more experienced comment as to previous approaches?

There are two previous approaches.

NU Prolog had 'when' declarations where you stated conditions under
which a goal was to be *activated*, e.g.

?- append(A, Z, AZ) when A ; AZ.
?- succ(A, B, C) when ground(A), ground(B).
?- succ(A, B, C) when ground(B), ground(C).
?- succ(A, B, C) when ground(C), ground(A).

There was a program that would automatically generate these for you
for nicely behaved predicates.

Some other Prologs have 'delay' declarations where you state
conditions under which a goal is to be *suspended*, i.e., the
opposite of the NU Prolog convention:

:- delay succ(A, B, C) when var(A), var(B), var(C).
:- delay append(A, Z, AZ) when var(A), var(AZ).

I believe the "delay rule" convention is more powerful,
but I always found the "activation rule" was easier for me
to use, because it linked in with reasoning about termination.

However expressed, the information belongs with the
*definition* of a predicate, not with its uses.