[Haskell-cafe] Comparison with Clean?

[Michael Vanier]

> From: Benjamin Franksen <benjamin.franksen at bessy.de>
> Date: Wed, 4 May 2005 22:47:21 +0200
> 
> On Wednesday 04 May 2005 22:22, karczma at info.unicaen.fr wrote:
> > Bryce Bockman writes:
> > > Scheme is strict, so it lacks some of the flexibility (and drawbacks)
> > > that come from Laziness, but in the book they teach you how to build a
> > > Lazy version of Scheme, which is instructive in understanding what's
> > > really going on in Lazy evaluation.
> >
> > Don't confuse categories please. SICP doesn't say how to make a lazy
> > variant of Scheme. Applicative protocol is not normal protocol, the
> > reduction is, as it is. 
> 
> We may have a different copy of SICP, but in mine (2nd edition) there is 
> Chapter 4.2 "Variantions on a Scheme -- Lazy Evaluation" and in particular 
> 4.2.2 "An Interpreter with Lazy Evaluation".
> 
> Ben

To be completely accurate: the evaluation order is Scheme is strict, not
lazy, forever and ever, amen.  That doesn't change.  What SICP shows you
how to do in chapter 4 (brilliantly, I think) is how to write a
"metacircular evaluator" which is a Scheme interpreter written in Scheme
itself.  Of course, because you have a Scheme interpreter running on top of
another Scheme interpreter, the (outer) interpreter is going to be pretty
slow, but the point of the chapter is not to build a useful interpreter but
to really understand how interpreters work.  Once you understand that, they
show that it's relatively easy to build a different kind of Scheme
interpreter, one that uses lazy evaluation instead of strict evaluation.
That's not "real Scheme" by any means, but it can be used to do real
computations.  Check out http://mitpress.mit.edu/sicp for the whole story.

We now return you to your regularly-scheduled language...

Mike