macroexpand and progn

macroexpand and progn

[Nic Ferrier]

I am writing an Emacs-Lisp to Javascript compiler. As part of that I am
converting EmacsLisp to a condensed form of EmacsLisp, removing most of
the special forms and rewriting them as more general special forms.

When I come to replace progn with a function I started to get oddness.

I did some experiments:

  (cl-defmacro nic-macroexpand-all-locally (form &environment env)
    "Macroexpand things made with macrolet."
    `(macroexpand-all ,form ',env))

  (defun nic-translate (form)
    (cl-macrolet ((progn (&rest body) `(nicblock ,body)))
      (nic-macroexpand-all-locally form)))

  (defun nic-translate2 (form)
    (cl-macrolet ((prognblah (&rest body) `(blahblock ,body)))
      (nic-macroexpand-all-locally form)))

  (nic-translate '(progn (* 10 20))) => error void-function nic-block ???
  (nic-translate2 '(prognblah (* 10 20))) => (blahblock ((* 10 20)))

In other words it looks like it's not possible to macro define `progn'.

Does anyone know if this is so and if so why?


I guess it won't be too much trouble for me, I can always add it to my
tiny Lisp language. But it's interesting.


Nic

Re: macroexpand and progn

[Stefan Monnier]

> In other words it looks like it's not possible to macro define `progn'.

Quite likely, indeed.  I'd expect trouble when trying to define macros
for any of the special forms.

> Does anyone know if this is so and if so why?

Because they're called *special*-forms?  ;-)
To make it work reliably, you'd probably need hygienic macros or packages.


        Stefan