[plt-scheme] expansion of internal define v. set!

Matthias Felleisen <matthias at ccs.neu.edu> writes:

> How do you decide whether (f x) is a definition?  Bunch of options, all
> equally unappealing:
>
>   (1) You say it doesn't look like one, so skip it.
>
>   (2) You say f is define-syntax'ed so it must be expanded, at least
>       until you know whether it's a definition or not.
>
>   (3) You say that f is define-syntax'ed but you also know that
>      (define (f x) ...)  binds f and therefore shadows everything
>      following the (define ...). Of course this option presumes that
>      you understand (a portion of) the binding structure of the program
>      before you have finished expanding the whole thing. I.e., lexical
>      analysis is performed at the same time as parsing, which at least
>      on the surface sounds horribly wrong.

If macros can expand into macro definitions or binding forms, you have
to do this anyway.

> I guess that Matthew chose option 2 because define-syntax'es must be
> recognized during expansion no matter what, unlike lexical scoping. 

Lexical scoping has to be recognized as well because the lexical
bindings may shadow the syntactic ones.

> In any case, Scheme is not well-defined for this corner (I am cc'ing
> Will Clinger who is the language lawyer, aka, editor, just in case I
> am wrong).  It shows one more time how much a full definition
> matters, and probably how bad internal definitions are.  But I am
> using them, too, though at other places.

I wouldn't go so far as to say that internal definitions are `bad',
but if you write macros that expand into internal definitions, then
you are treading on thin ice.