Re: conditional code segments

[Matt Wette <matt.wette@gmail.com>]

On 06/01/2018 06:56 PM, Mark H Weaver wrote:

> Hi Matt,
>
> Matt Wette <matt.wette@gmail.com> writes:
>
>> In C I can use `#ifdef' .. `#endif' to "comment out" code segments.
>>
>> In Scheme, one can use `#|' and '|#' which is OK but requires dealing with both ends of the
>> segment to switch on / off.  And emacs (v 24.5) scheme mode does not always fontify the buffer
>> correctly with #|...|#.
> There's another kind of comment in standard Scheme (both R6RS and R7RS)
> that's supported by Guile, but unfortunately it doesn't seem to be
> documented in our manual.  Simply put the two characters "#;" before any
> datum, and the entire datum will be skipped by the reader.  Whitespace
> may appear between "#;" and the datum.  So, for example:
>
> #;
> (let ()
>    ...)
>
> will cause the entire 'let' form (which is a datum) to be skipped.
> Emacs and paredit understand this syntax, and will act accordingly.
>
> It works not only at top-level, but also nested arbitrarily deeply
> within datums.  So, for example:
>
>    scheme@(guile-user)> (let ()
>                           (display "1\n")
>                           (display "2\n")
>                           #;
>                           (display "3\n"))
>    1
>    2
>    scheme@(guile-user)>
>
> and if you put that code in an Emacs buffer in Scheme mode, the
> (display "3\n") will be colorized as a comment, but the final closing
> parenthesis will have the default color.
>
I use #; a lot.   So for large number of forms then #;(begin ...) should work.
And I didn't know white space can separate the #; from the Scheme form.  Cool.

> I tried using cond-expand but it does not work as expected:
>    scheme@(guile-user)> (cond-expand-provide (current-module) '(abc))
>    $1 = (abc)
>    scheme@(guile-user)> (cond-expand (abc #t))
>    While compiling expression:
>    Syntax error:
>    unknown file:2:0: cond-expand: unfulfilled cond-expand in form (cond-expand (abc #t))
> It didn't work because when a 'cond-expand' form is used in module FOO,
> it looks for the features (e.g. 'abc') in all of the modules that are
> imported by FOO, but not in FOO itself.

Thanks much.