Re: [stack] disallowing recursive definitions

--- In [email protected], Don Groves <dgroves@...> wrote:
>
> 
> On Mar 3, 2008, at 19:16 , John Nowak wrote:
> 
> >
> > On Mar 3, 2008, at 3:50 PM, William Tanksley, Jr wrote:
> >
> >>> With a concatenative tail-call you're passing the stack as an
> >>> argument. In other words, you pass the entire state of the program!
> >>
> >> Yes, this is how concatenative languages are modeled. That's how
> >> *every* step in their execution is modeled, not just tail-calls or
> >> calls. If this were a problem, concatenative languages would be
> >> universally problematic, not merely problematic in the presence of
> >> tail-calls.
> >
> > Indeed, and I'd argue that this is exactly why many people find stack-
> > based languages universally problematic. They're taking issue with all
> > of this state passing, although they phrase it like "I can't keep the
> > stack in my head". Of course this isn't a problem in practice,
> > particularly when you have type inference involved and can get a quick
> > summary of what a function cares about, but it does seem to be the
> > source of many negative first impressions.
> 
> Also, there has never been a requirement to "keep it all in one's head,"
> that's what comments are for. Commenting with stack diagrams is very
> useful!
> 
> I know "real programmers" don't comment but in these languages, they
> should.
> --
> don

If code isn't readable without stack diagrams, comments seem like a
poor fix. Rather, there should be a way to make the code itself more
transparent. I think lambda expressions are a good replacement for
stack diagrams. They make the stack explicit and are often easier to
read. Compare:

#x y u v -- (x*u - y*v) (x*v + y*u)
dup4 [swap] dip * bury * swap - bury * bury * +

[x y u v] [x u * y v * - x v * y u * +] lambda

A mistake would be much easier to notice in the latter than the
former. Not being able to keep track of the stack in one's head can be
a serious problem.

Fortunately lambda expressions can be implemented as macros and
rewritten out of existence. So the language itself doesn't actually
need to be extended.

-Justin