On Tue, 2009-09-15 at 09:17 -0500, Brian Mastenbrook wrote:
> On Sep 15, 2009, at 6:45 AM, Andre van Tonder wrote:
>
> > On Mon, 14 Sep 2009, Brian Mastenbrook wrote:
> >
> >> (define b 1)
> >> (begin
> >> (define a b)
> >> (define b 2)) -> error, attempt to use `b' before its definition
> >
> > This is certainly incorrect behavior in R5RS.
>
>
> The question is: is this behavior sensible?
I don't think so. "error, attempt to define b more than once" is
consistent, but "error, attempt to use b before its definition"
is not.
> Was the conflation of
> binding and mutation at the top-level into one syntax ever a good idea
> in the first place - especially when the SAME syntax is used only for
> binding elsewhere?
There is no mutation involved in the above code snippet. What we
have here is a conflation of binding and sequencing. BEGIN is
properly a sequencing primitive, and as such there is no good reason
why it needs to have any effect on scope.
On the contrary, if BEGIN establishes a scope, then firstly it
is redundant and should be eliminated because the only semantics
it could then express are expressible by one of the various let
and let-foo forms.
Secondly, the language needs a primitive for bundling multiple
forms which does not establish a scope, for purposes of allowing
macros to expand into multiple forms (typically definitions)
when those forms must be in the same scope as the call site.
There is no absolute reason why the primitive for bundling
multiple forms has to also be a sequencing primitive. But it's
usually convenient for it to be so, and as long as mutation
exists, we need a sequencing primitive, and as far as I know
no one has demonstrated any reason for the sequencing and
bundling primitives to be distinct.
> Would anyone who is not doing "screwy" things with macros ever notice
> the difference?
Absolutely. It is not at all screwy to have (and need!) a single
macro invocation to expand into multiple definitions. See the
highly useful object library 'Meroon,' which was written by
Christian Queinnec and featured in his excellent book
(_Les_Langages_Lisp_ in French, _Lisp_In_Small_Pieces_ in English)
for a widespread example of macros in a library that a lot of
people actually do use. It would break horribly if begin
contained its definitions in a local scope, and more importantly,
there would be no way to fix it because without an open-scoped
bundling primitive there would be no way left to implement it
in Scheme.
Bear
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