Re: [Scheme-reports] Top-level cond-expand for backwards compatibility

On Mon, Jun 10, 2013 at 5:40 AM, Christian Stigen Larsen <csl@x> wrote:

I've used the following cond-expand trick to write R7RS programs that are
backwards compatible with RnRS implementations that support SRFI-0:

(cond-expand
(r7rs
(import (scheme base)
(scheme write)))
(else
(begin)))

(display "Hello, world")
(newline)

This works out-of-the-box on a lot of implementations:

$ chibi-scheme cond-expand.scm # v0.6.1
Hello, world!

$ csi -bq cond-expand.scm # chicken v4.8.0.3
Hello, world!

$ mit-scheme --batch-mode < cond-expand.scm # v15.3
Hello, world!

$ guile cond-expand.scm # v1.8.8
Hello, world!

$ scheme cond-expand.scm # tinyscheme 1.39
Hello, world!

$ bigloo -i cond-expand.scm # v3.8c
Hello, world!

I could not get this to work with Scheme48 or Larceny and didn't attempt it
on other implementations.

It also does not work with Mickey Scheme, because I can't figure out if I
should allow it or not.

The problem is that I believe this form of usage is strictly not valid R7RS,
at least not according to the EBNF in R7RS, ch. 7.1.6 (draft 9):

<program> -->
<import declaration>+
<command or definition>+

As such, (import ...) must come before any commands and cond-expand is
defined in (scheme base) --- I'm ignoring the second definition of
cond-expand inside (import ...) blocks.

I know the document allows implementations to extend the language, given it
does not conflict with R7RS. But there's a difference between a language
and its implementation.

So, is the code above valid in the R7RS *language*?

The code is not portable R7RS. The idea is that cond-expand

and include are normal macros available in the (scheme base)

language, whereas import is a special form which should be

statically analyzable.

[I had forgotten I made a special case for cond-expand in Chibi.

This was for internal (non-portable) tests.]

Note that SRFI-0 is not available in all implementations,

so this isn't a guaranteed compatibility hack. You might get

better portability by including the R5RS source from the

R7RS program, though the exact file lookup rules are

implementation-defined.

Alex