How to write portable code on R7RS

There was a discussion (or rather question about) 'cond-expand'. I was also wondering about 'cond-expand' why it has 'library' form even though it can't help to write portable script (not a library).

Since draft 8 (or 9?), R7RS dropped 'import' syntax from (scheme base) which means users can't write the code like following;
(import (scheme base) (scheme write))

 ((library (srfi :1))
  (import (srfi :1)))
 ((library (srfi 1))
  (import (srfi 1))))

(define (print . args) (for-each display args) (newline))
(print (iota 10 1))
Interestingly, this works most of the R7RS implementation (well, I only know Chibi and Sagittarius :-P) and Gauche (probably next release supports R7RS). However it's still not portable since the *correct* behaviour should be an error.

Then what is the proper way to make this portable? The answer is simple, just write the stub library like this;
;; somewhere load path whare your favourite implementation can search.
(define-library (srfi-1)
  (export iota)
   ((library (srfi :1))
    (import (srfi :1)))
   ((library (srfi 1))
    (import (srfi 1)))
    ;; To make code absolutely portable
    ;; you need 'begin' :)
    (begin (define iota ...))
For me, it's inconvenient so I will probably not write strictly portable code on R7RS. Even WG1 member is asking to implement it the way how Chibi is doing now (not sure if this is about what I'm talking about though);
> As far as I can tell, there is no way in a program to use cond-expand to
> control what libraries get imported.

That appears to be correct. I consider that an oversight on the WG's part.
Chibi actually supports this, and I would urge you to support it too.
(from http://lists.scheme-reports.org/pipermail/scheme-reports/2013-October/003802.html)
Even though it's inconvenient, however, R7RS at least provides a way to write portable library which R6RS doesn't. For this perspective, it's not so bad (well, still I don't understand why it dropped 'import' and asking to go non-standard way even if it would be de-facto. If they think it's an oversight then they should put it on errata).


grant rettke said...

There has got to me work left over for R8, right? :)

kei said...

Indeed, even though R7RS has not been decided yet...

Post a Comment