[plt-scheme] Redefining in Mzscheme with legacy R5RS support

At Wed, 20 May 2009 14:29:45 +0300, Timo Lilja wrote:
> I want to run MzScheme with legacy R5RS support enabled but I came
> across with a bit strange behaviour:
> 
>  $ mzscheme  -l scheme -l r5rs/run
>  Welcome to MzScheme v4.1.5 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
>  R5RS legacy support loaded
>  > (define (filter pred x)
>    (cond ((null? x) '())
>          ((pred (car x)) (cons (car x) (filter pred (cdr x))))
>          (else (filter pred (cdr x)))))
>  > (filter (lambda (x) x) '(1 2 3))
>  filter: expected argument of type <proper list>; given (2 3)
> 
>   === context ===
>  [...]/lib/plt/collects/scheme/private/list.ss:182:2: filter stdin::1: filter
>  [...]/lib/plt/collects/scheme/private/misc.ss:74:7
> 
> However, if I redefine the filter once more in the REPL, then
> everything works:
> 
>  > (define (filter pred x)
>    (cond ((null? x) '())
>          ((pred (car x)) (cons (car x) (filter pred (cdr x))))
>          (else (filter pred (cdr x)))))
>  > (filter (lambda (x) x) '(1 2 3))
>  (1 2 3)
> 
> So it seems that if I invoke mzscheme with -l scheme -l r5rs/run,
> then, somehow, the recursive filter function is not entirely
> redefined. It looks like the recursive call to (filter pred (cdr x))
> still uses the original mzscheme language version which doesn't accept
> mutable pairs. Is this a bug or a feature?

You're seeing the same effect as described here:

 http://www.cs.brown.edu/pipermail/plt-scheme/2006-December/015643.html


The `r5rs/run' library imports R5RS bindings into the top level in such
a way that they cannot be redefined (i.e., you get an error instead of
strange behavior if you try to redefine, say, `car'). But `-l scheme'
imports `scheme' bindings in the usual way, and the top level just
can't handle imports and redefinition gracefully.


If you really want to make this work, you can use functions like
`namespace-require/copy' that are designed to work around the hopeless
top level to some degree:

 laptop% plt-r5rs --no-prim
 Welcome to MzScheme v4.2.0.2 [3m], Copyright (c) 2004-2009 PLT Scheme Inc.
 R5RS legacy support loaded
 > (#%require (only scheme/base namespace-require/copy))
 > (namespace-require/copy 'scheme)
 > filter
 #<procedure:filter>
 > (define (filter x) (if x (filter #f) 'done))
 > (filter #t)
 done

This doesn't help with bindings to syntax, though, and it hurts
performance.