Mutating public bindings of a declarative module

["Ludovic Courtès" <ludo@gnu.org>]

Hello!

It seems that if you ‘set!’ a public variable of a declarative module,
the change is visible to all the module users, but it’s not necessarily
visible to procedures within that module, presumably because they use an
inlined or specialized variant of that thing.

I would have imagined that public bindings are considered mutable and
thus not subject to inlining; OTOH, that would obviously be a loss, so
the current approach makes sense.

Anyway, it complicates a use case for me.  In Guix, we “mock” bindings
like so:

  (define-syntax-rule (mock (module proc replacement) body ...)
    "Within BODY, replace the definition of PROC from MODULE with the definition
  given by REPLACEMENT."
    (let* ((m (resolve-interface 'module))
           (original (module-ref m 'proc)))
      (dynamic-wind
        (lambda () (module-set! m 'proc replacement))
        (lambda () body ...)
        (lambda () (module-set! m 'proc original)))))

and that allows us to write tests that temporarily modify public (or
private!) bindings.

It seems like this could be addressed by compiling selected modules with
‘user-modules-declarative?’ set to #false, or by avoiding the above hack
altogether when possible, but I thought I’d share my impressions and
listen to what people think.  :-)

Thanks,
Ludo’.