Re: Me no understand scoping

[Jon Wilson <jsw@wilsonjc.us>]

Maciek Godek wrote:

> as for eval, it will always be there (in RnRS)

EVAL has not been in every report.  R4RS did not specify EVAL, nor did 
R3RS, RRRS, RRS.

 > besides I don't imagine not messing with lexical
 > environments :) The bad thing is that scopes aren't
 > explicitly definable and the report doesn't say
 > much about their properties. I don't know how
 > about practical performance, but I'd find it best
 > if they were implemented as (implicit) hash tables
 > that could be bound to variables and passed as
 > arguments. I think that they could perform
 > equally good as fixed arrays for memoized
 > procedures.

The view of the scheme community as a whole is that first class lexical 
environments are irreconcilable with the need to compile (optimize) 
code.  In many cases, once you have done some variety of compilation, a 
lot of the lexical environments you think you ought to have will have 
been optimized away; conversely, preserving these environments will 
prevent entire classes of compilation.

I am of the suspicion that in most cases, the environment would never be 
reified, the compiler could prove this, and optimization could proceed 
as desired.  However, I would still agree with Clinton that you should 
avoid messing with environments, first class or otherwise, whenever 
possible.  When you must, you should quarantine it: use it in a very 
limited part of your code, where you wrap it in a more mundane 
abstraction, then use that abstraction elsewhere.  CALL/CC should be 
treated the same way usually.

Broadening the topic a bit, it seems that first class envs and macros 
(and maybe not macros!) are all that would be needed to define a module 
system in scheme, rather than building it into the interpreter.  Then 
the module system could be put in a file which any r5rs+envs (or 
whatever standard you like) scheme could LOAD (most likely implicitly at 
startup) without modification to the interpreter itself.  It might even 
be the case that DEFMACROs and reified environments would be sufficient 
to make low-level hygienic macros a library too (but I'm no macrologist, 
for sure).  You would still have to deal with the compilation issues, 
but as I said, my gut feeling is that these are solvable.
Regards,
Jon