Re: how to write macroexpand

[Helmut Eller <eller dot helmut at gmail dot com>]

* Per Bothner [2010-08-12 21:37] writes:

> So my question is: What do you need macro-expand for?  The main use I
> can think
> of is when debugging macros.  Note that the --debug-print-expr command-line
> option prints out "expanded" syntax, though the syntax isn't actually valid
> Scheme - just a pretty-printed form of the Expression tree.

Below is the code for a simple macroexpand that uses the first stage of
the compiler and turns the AST back to a s-expressions which are
hopefully more readable than the debug dumps.  Unfortunately, some
fields in the IfExps class are private so we can't do that for ifs.

Another issue are resolved functions in the AST that can't be easily
mapped back to the source names.

Example usage:

(macroexpand '(do () (#f)))
=>
(let ((%do%loop #!undefined))
 (begin
  (set %do%loop
   (lambda ()
    (If (Apply (Ref/45072/Declaration[applyToArgs/1])
          (Ref/45071/Declaration[not/181])
          (Quote #f))
      (Apply line:1:15 (Ref/45076/Declaration[applyToArgs/1])
        (Ref/45075/Declaration[%do%loop/7327]))
      (Quote ))))
  (%do%loop)))

With exported fiels in IfExps the results looks so:

(let ((%do%loop #!undefined))
 (begin (set %do%loop (lambda () (if (not #f) (%do%loop) (quote ))))
  (%do%loop)))

The pretty printer doesn't turn quote back to ' which would improve
readability and it's even worse for backquote.  Note also that nothing
is printed for '#!void.

Helmut

Attachment: macroexpand.scm
Description: application/emacs-lisp