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

From: Klaus-Dieter Bauer
Subject: Re: Can the byte-compiler check whether functions passed by name are defined?
Date: Mon, 5 Aug 2013 10:52:43 +0200

2013/8/4 Stefan Monnier <address@hidden>:
> As mentioned, the best option is to start by adding warnings for the
> #'symbol form (should be easy: handle it in the same way we handle
> warnings for function calls).
> Then make the higher-order functions turn 'symbol into #'symbol.
> That's important for things like
>    (if foo #'a #'b)

I am not sure I understand what you have in mind. Is the following
snippet such a case?

    (mapcar (if flag 'f1 'f2) list)
    => (mapcar (if flag #'f1 #'f2) list)

I don't see much point in that, because I don't like the notion that
the above code behaves different from

    (defun select-function (flag f)
      (if flag f 'default-f))
    (mapcar (select-function flag 'f1) alist)

In my implementation still warnings would be created if the programmer
explicitly writes #'f1, #'f2 in `select-function', which I suppose
would happen with yours too. Handling cases like computed function
names seems fairly complex -- what about e.g.

    (mapcar (catch 'function
              (dolist (f function-list)
                (when (PREDICATE f)
                  (throw 'function f)))

When the simple if-case generates a warning, naively I'd expect this
to generate a warning too, but I'd expect it to become impossibly
complex then.

> Sadly, I defined the `compiler-macro' declaration to take a function
> rather than an exp that returns a function, so you can't just
> write a function macroexp--rewrite-function-arguments and then use:
>   (defun my-combine (func1 func2)
>     (declare (compiler-macro (macroexp--rewrite-function-arguments 0 1)))
> But you can do something like
>   (defun my-combine (func1 func2)
>     (declare (compiler-macro
>               (lambda (body)
>                 (macroexp--rewrite-function-arguments
>                   body (rewrite func1) (rewrite func2)))))
>   (defun my-mapcar (func list)
>     (declare (compiler-macro
>               (lambda (body)
>                 (macroexp--rewrite-function-arguments
>                   body (rewrite func) list))))
> Still, this annotation is only needed to turn a ' into a #', so it's not
> the most important.

Shouldn't the markup for user-written functions be a bit easier?
Something along the lines of using a list of positions (like specified
by my declare form) and having the rewriting, if any, done inside
byte-compile-normal-call or whichever function would be the correct
place. Especially since `compiler-macro' seems to be undocumented...

>> I couldn't define the handler in "byte-run.el" however, as when I added it
>> to the declaration of `defun-declaration-alist', it would suddenly be
>> missing again during compilation.
> Probably because you didn't re-dump Emacs (byte-run.el is preloaded
> into the `emacs' executable so if you change it, you need to rebuild
> `emacs').

Indeed I did not, since I did not expect this behaviour. Rebuilding
emacs is fairly slow on windows, so I didn't even think of this

Anyway, I find the behaviour confusing. Having code preloaded into
the binary is okay, but having code in installation directory that is
ignored even when it has been changed seems a bit awkward. Or IS it
loaded when changed and just didn't change `defun-declaration-alist',
because it is bound already before loading the changed byte-code?

  - Klaus

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