Subject: Re: 6497

From: Geoff Gole <[email-masked]>
Date: Fri, 2 Jul 2010 09:39:38 +0800

> When given an unquoted symbol as its argument `indirect-variable' will
> return the value of a non-null symbol. That it does so represents a
> subtle alteration of the generally expected semantics

No it does not. The form (indirect-variable foo) is evaluated by
fetching the value of the symbol foo and passing it to the function
definition of indirect-variable, just like every other regular
function in emacs.

See (info "(elisp)Evaluation").

> Neither are regular lisp functions they are both primitives defined in
> src/data.c

Whether they are implemented in C or lisp is irrelevant. All regular
functions have their arguments evaluated in the same way.

> Except, that they don't always. which the docstring is in error.

I guess I phrased that poorly. These functions always return the
function definition, which is just a value and not guaranteed to be a
callable function at all. The "function definition" is simply whatever
lisp value happens to be in the function slot of said symbol:

(progn
  (fset foo 3)
  (symbol-function foo)) => 3

There is no restriction on the type of the value retrieved by
symbol-function whatsoever.

> Sure they rely on the details of the function representation:

No, see above.

> (symbol-function indirect-function)
> => (void-variable indirect-function)   ;; <-- void-variable

> (indirect-function not-a-real-function)
> => (void-variable not-a-real-function) ;; <-- void-variable

> (indirect-function symbol-function)
> => (void-variable symbol-function)     ;; <-- void-variable

These results are all straightforward, predictable results of the lisp
evaluation process. The only difficulty is that emacs lisp is a
lisp-2, with separate namespaces for values and functions. Once you
understand that evaluating the symbol 'symbol-function' fetches from
the *value slot* of that symbol, while the function definition
accessed by symbol-function (or by calling the function) resides in
the *function slot*, you'll see that this is the expected behaviour.

Again, see (info "(elisp)Evaluation").

> How do these primitives reach determination that the function cell of
> `not-a-real-function' is void if they don't access som portion of the
> representation denoting that symbol is function/variable?

The primitives in question know about the C representation of
*symbols*, in particular how to test whether a function slot is empty.
They do not know or care about the representation of functions.

Why don't you go and look at the C implementation of symbol-function
and indirect-function and verify for yourself what is going on?

> - See bug#6496 re autoload objects not appearing in "What is a
>  function" node in manual.
>
> - See bug#6486 re `byte-code-function-p' requiring the user to cross
>  reference 3x info nodes in order to conclude that its return value
>  is as per `symbol-function'.

Perhaps that documentation should be changed. Documentation bugs don't
seem to get much attention, though.

> `symbol-function' and `indirect-function' are the cannonical
> interfaces by which one can access the function cell of a symbol.

The documentation of symbol-function is fine. The function itself is a
simple accessor that does nothing except fetch a value from the
function slot of a symbol, and the docstring reflects that.

> Really, symbol-function and his buddy `indirect-function' return:
>
>  - a lambda form
>  - a vector
>  - a list
>  - a cons
>  - and two types of unreadable objects

They can return many more types of value than that. This is a natural
result of the fairly large Emacs Lisp type zoo and nothing to do with
the functions themselves.