eval-case

[Ludovic Courtès]

Hello!

Andy Wingo <address@hidden> writes:

> The problem is that eval-case has really murky semantics -- to me at
> least. We've *never* needed to use it in the past, to my knowledge,
> except to distinguish toplevel expressions from lexically nested
> expressions -- an iffy use, IMO. (Any definition should be valid
> anywhere a definition is valid.)

Right, it was essentially useless, but supposedly it was added with
Guile-VM in mind---how visionary!  :-)

My suggestion to not deprecate it in 2.0 was motivated by 1.8/2.0
portability.  ‘eval-case’ in 1.8 only understands the ‘load-toplevel’
clause.  In 2.0, we could add a ‘compile’ clause.  That clause is never
true in 1.8, but in 2.0 it could amount to ‘(eval-when (compile) ...)’.
This is so that people could place things like ‘current-reader’ effects
on the right phase:

  (eval-case
    ((compile) (display "current-reader effects for 2.0\n"))
    (else      (display "current-reader effects for 1.8\n")))

(Actually this isn’t perfect because the ‘else’ branch would also been
taken in 2.0 at load-time.)

OTOH, with ‘(cond-expand (guile-2 ...))’, one can achieve this and more,
which may be a good reason to not worry about it.

Thanks,
Ludo’.