On 09/09/2009 04:59 PM, John Cowan wrote:
> Alaric Snell-Pym scripsit:
>
>> So what this tells us from an R7RS perspective is
>> that it'd be nice to have a standard facility to ask that a block of
>> code be totally ignored (not even macro-expanded, lest we use
>> implementation-specific macros that throw up errors in other
>> implementations) if we're not on a particular implementation.
>
> That's cond-expand, SRFI-0. You are the first person to defend it in
> my virtual hearing, though it is very widely implemented. Should I add
> it to my proposals?
FWIW: The SRFI-64 reference implementation uses cond-expand heavily.
For example if an implementation supports source line numbers (very
helpful when reporting a failing test-case) then we can conditionalize
in support for that, while still doing something more basic and
portable by default.
> Note that because it works at the level of macro expansion, it can
> neuter uses of my proposal for extensible lexical syntax, but not a
> read-level mechanism like PLT's.
I agree a read-level mechanism might be more flexible - but (in the
current State of the Scheme World) less portable. So perhaps it
would be desirable to support read-level conditionals in upcoming
standards, including "small Scheme".
--
--Per Bothner
[email protected] http://per.bothner.com/
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