From: Geoff Gole <geoffgole@gmail.com>
To: MON KEY <monkey@sandpframing.com>
Cc: 6497@debbugs.gnu.org
Subject: bug#6497: 6497
Date: Fri, 2 Jul 2010 09:39:38 +0800 [thread overview]
Message-ID: <AANLkTins7-pIYHXOPDSZyRnQgy_BZ3r4HBznQms2e2HN@mail.gmail.com> (raw)
In-Reply-To: <AANLkTinD8CfcSSejLlAGc1imeB7rF8yCVnt3ZtuGLwlN@mail.gmail.com>
> 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.
next prev parent reply other threads:[~2010-07-02 1:39 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-06-22 23:42 bug#6497: documentation `indirect-function', `symbol-function' do not enumerate return value type MON KEY
2010-07-01 9:20 ` bug#6497: 6497 Geoff Gole
2010-07-02 0:13 ` MON KEY
2010-07-02 1:39 ` Geoff Gole [this message]
2010-07-03 4:27 ` MON KEY
2010-07-03 6:31 ` Geoff Gole
2010-07-03 19:35 ` MON KEY
2010-07-03 7:00 ` Andreas Schwab
2010-07-03 19:33 ` MON KEY
2010-07-03 19:47 ` Andreas Schwab
2011-07-09 18:25 ` bug#6497: documentation `indirect-function', `symbol-function' do not enumerate return value type Glenn Morris
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