On 3/15/07, AndrevanTonder <[EMAIL PROTECTED]> wrote:
On Thu, 15 Mar 2007, Robby Findler wrote:
> The formal semantics makes no claim to let you reason about the space
> behavior of your program!
But the body of the document does make some solid guarantees on space,
and further allows one to infer behaviour w.r.t. copying and sharing of
lambda parameters. Shouldn't one of the the purposes of an operational
(as opposed to denotational) semantics be precisely to formalize this kind of
reasoning?
The semantics does correctly model sharing (of values that aren't
procedures -- and the informal semantics does not give a complete
specification of that particular sharing anyways).
Is it possible you are misunderstanding how it works? For example,
this is modeled correctly:
((lambda (c)
((lambda (x y)
(set-car! x 3)
(car y))
c c))
(cons 1 2))
It produces 3, as you probably expect.
You can try it out. Download the semantics and edit the
"show-examples.scm" file!
Insofar as it does not, I would consider it incorrect.
That is not the usual meaning of the word "incorrect". Perhaps
"incomplete" or "not what I wanted". :)
But thanks for clarifying. I'm going to assume that the earlier uses
of "incorrect" were in this sense. I hope that's okay.
I have only seriously needed an operational semantics a few times, all of which
involved reasoning about space behaviour. An operational semantics that does
the unnecessary copying of the current specification would have been no use to
me in any of these cases.
Okay.
I see the value of the operational semantics in r6, as compared to the
denotational in r5 as:
- it covers a larger part of the language
- it comes with a stepper to promote experimentation
- it has simpler mathematical underpinnings
(making is accessible to a wider audience)
My dream is that every Scheme programmer can use the semantics that
comes with r6 to play around with parts of the language to see how
things work.
I don't mean to discredit the denotational semantics in r5 or
denotational semantics in general. I do, however, believe that this
operational semantics is a good fit for the Scheme report.
Robby
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