Hi!

The attached patch allows expansion-time modifications of
`current-reader' to be taken into account.  For example:

   (define-macro (install-reader!)
     (fluid-set! current-reader
                 (let ((first? #t))
                   (lambda args
                     (if first?
                         (begin
                           (set! first? #f)
                           ''ok)
                         (read (open-input-string ""))))))
     #f)
   (install-reader!)
   this-should-be-ignored

   => ok

This trick works with both the compiler and the interpreter.  I intended
to use it in Skribilo.

Furthermore, the `current-reader' fluid used at compilation-time by
default is different from the one in the compiler.  This is needed
because the REPL uses `current-reader' to install a wrapper around the
current language reader; when that language is Scheme, we enter an
infinite recursion if Scheme's reader honors `current-reader'.

The patch exposes the current compilation environment as a fluid, so
that language readers can look for the compile-time `current-reader'.
This is admittedly not very elegant, but I can't think of a better way.

Thanks,
Ludo'.