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From: Ehud Lamm <mslamm@mscc.huji.ac.il>
Subject: Re: Icon Co-Expressions in Ada
Date: 1998/12/17
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Date: 1998-12-17T00:00:00+00:00
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A bit about goal directed evaluation in general. Co-Expressions are really
a special mechanism, and for some reason I slumped them together in my
post.

I'll give an example of how Goal directed eval works:
Icon has a built in function find(s1,s2) which returns (potentially) all
the indexes in s2, where s1 start (find("e","ehud") should be 1).
suppose I say
if find("e","element")>1 then...

What will happen is that find will be executed, it will return 1 which
doesn't satisfy the condition, it will then be resumed, and return the
value 3. Than the "then-clause" will be executed.

For this to work from arbitrary procedures, the language provides the
suspend statement, which is like return but after which the procedure may
be resumed.(you can have a procedure say suspend 4; suspend 5; for
example).

I was thinking about implimenting this statement. Note that you ahve to
remember exactly where the computation stopped.

Co-expression are another mechanism. The allow you a limited sort of
parallel execution, of two or more procedure that can be supended. This
allows you to take one value from one routine, another from the second
routine etc.

Hopes this clears things up, a bit.

Ehud Lamm     mslamm@pluto.mscc.huji.ac.il
P.S
If any one is interested let them browse to http://www.cs.arizona.edu/icon

If anyone wants to read more, I have a litle memo called "Integrating User
Defined Procedures With Built-In Language Features: The Case of J, Icon
and C++". It deals with Icon generators (= routines that use suspend) and
other clever programming languages features.