Re: Can the byte-compiler check whether functions passed by name are defined?

[Klaus-Dieter Bauer <bauer.klaus.dieter@gmail.com>]

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2013/7/29 Stefan Monnier <monnier@iro.umontreal.ca>

> >     (mapcar 'my-function-1 some-list)?
>
> We could do it for symbols quoted with #' but not for quoting with '.
>
>         Stefan
>

#' would allow checking for a defined function independent of the function,
the function is oassed to, true... Would require however to change coding
practice to using this syntax, with the advantage of preventing warnings
when people don't want the check but the disadvantage, that the warning is
also supressed when people just don't care. I guess it would still be the
best solution, that is backward compatible though.

Another possibilty (probably less good though):

    (eval-and-compile
      (put 'mapcar 'compiler-macro
           (lambda (&rest form)
             (let ((function (nth 1 form)))
               (when (and (macroexp--compiling-p)
          (listp function)
                          (= 2 (length function))
                          (member (nth 0 function) '(function quote))
                          (symbolp (nth 1 function))
                          (not (fboundp (nth 1 function))))
                 (warn "Function not know to be defined: %S" (nth 1
function))))
     (car form))))

    (mapcar 'message '("Hello" "World"))
    (mapcar 'foobar '("Foo" "Bar"))

Some caveats here:
1. Has to be done, though maybe through a macro, for every higher-order
function.
2. `compiler-macro' doesn't seem to be documented anywhere. The arguments
the function gets passed seem to differ from case to case...
3. (warn) doesn't emit warnings to the compilation buffer but to the
separate *Warnings* buffer, making this code only a prrof-of-concept
without practical value.
4. If the quoted funciton is defined in the same file as the higher-order
function it is passed to, the definition of the quoted function must be
both before the first use of the function and inside an (eval-and-compile
..) block, which can only be prevented by changes to the compiler code
anyway.

kind regards, Klaus

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