compiler macros without byte-compilation

compiler macros without byte-compilation

[Nicolas Richard]

Hello,

Consider the following:
(progn
  (defun foobar ()
    (declare (compiler-macro
              (lambda (arg)
                "compiler-macro")))
    (message "Done !"))
  (list (symbol-function 'foobar)
        (foobar)))
hit C-x C-e :
 => ((lambda nil (message "Done !")) "compiler-macro")

Now I'm puzzled : why does this happen ? foobar wasn't compiled here !

Looking at readevalloop in lread.c, I thought it was maybe because of
eager macro expansion [which the manual only mentions as happenning at
load time], and in particular I tried changing (symbol-function
'internal-macroexpand-for-load), but that doens't seem to have any
influence on the above behaviour:

(cl-letf (((symbol-function 'internal-macroexpand-for-load) nil))
  (progn
    (defun foobar ()
      (declare (compiler-macro
                (lambda (arg)
                  "compiler-macro")))
      (message "Done !"))
    (list (symbol-function 'foobar)
          (foobar))))
 => ((lambda nil (message "Done !")) "compiler-macro")

Could someone shed some light on this ?

-- 
Nicolas Richard

Re: compiler macros without byte-compilation

[Stefan Monnier]

>  => ((lambda nil (message "Done !")) "compiler-macro")
> Now I'm puzzled : why does this happen ? foobar wasn't compiled here !

The exact cases where compiler-macros are used are unspecified and
likely to change between versions.

If you want to know the details: compiler-macros are expanded by
macroexpand-all.  So everytime we rely on macroexpand-all (i.e. "eager"
expansion) rather than on macroexpand (i.e. "lazy" expansion), compiler
macros will be expanded.

"Lazy" macroexpansion is on the way out: `eval' does not call
macroexpand-all (because it can happen that `eval' is called repeatedly
in a tight loop to evaluate very short code sequences, where going
through macroexpand-all might be detrimental for performance), but
pretty much all other paths to `eval' do or should (i.e. `load',
`eval-region', ...).


        Stefan