Function pointer

Function pointer

[Decebal]

Is it possible to work with function pointers?
The following works:
    (a-function)
but this does not work:
    (setq function 'a-function)
    (function)
this gives wrong number of arguments.

Re: Function pointer

[Harald Hanche-Olsen]

+ Decebal <CLDWesterhof@gmail.com>:

> Is it possible to work with function pointers?

No. Pointers are a low level concept not supported by elisp.

> The following works:
>     (a-function)
> but this does not work:
>     (setq function 'a-function)
>     (function)
> this gives wrong number of arguments.

Ah. So you're not after pointers, just a means of passing functions
around. You should be aware, then, that symbols can have two values
associated with them: The regular value, which is obtained by just using
the symbol, and the function value, which is consulted if the symbol is
in the function place of a for. You have assigned the symbol a-function
to the symbol function above, but that symbol also has a function value,
which you inadvertently used in the last line.

The elisp manual explains this. Go to node Symbols, then sub-node Symbol
Components.

The very short answer to your question: Use funcall, as in
    (funcall function)

-- 
* Harald Hanche-Olsen     <URL:http://www.math.ntnu.no/~hanche/>
- It is undesirable to believe a proposition
  when there is no ground whatsoever for supposing it is true.
  -- Bertrand Russell

Re: Function pointer

[Decebal]

On 14 apr, 10:02, Harald Hanche-Olsen <han...@math.ntnu.no> wrote:
> + Decebal <CLDWester...@gmail.com>:
> > The following works:
> >     (a-function)
> > but this does not work:
> >     (setq function 'a-function)
> >     (function)
> > this gives wrong number of arguments.
>
> Ah. So you're not after pointers, just a means of passing functions
> around. You should be aware, then, that symbols can have two values
> associated with them: The regular value, which is obtained by just using
> the symbol, and the function value, which is consulted if the symbol is
> in the function place of a for. You have assigned the symbol a-function
> to the symbol function above, but that symbol also has a function value,
> which you inadvertently used in the last line.
>
> The elisp manual explains this. Go to node Symbols, then sub-node Symbol
> Components.
>
> The very short answer to your question: Use funcall, as in
>     (funcall function)

Works likes a charm. Now I have the tools to build the 'little'
functionality I wanted. Thanks.

RE: Function pointer

[Drew Adams]

> > >     (function)
> > > this gives wrong number of arguments.
...
> > The very short answer to your question: Use funcall, as in
> >     (funcall function)
> 
> Works likes a charm. Now I have the tools to build the 'little'
> functionality I wanted. Thanks.

BTW, the reason the error message complained about the wrong number of args is
that `function' is an existing special form - `C-h f function':

,----
| function is a special form in `C source code'.
| 
| (function arg)
| 
| Like `quote', but preferred for objects which are functions.
| In byte compilation, `function' causes its argument to be compiled.
| `quote' cannot do that.
`----

"Function" (special form) `function' expects one argument, and you gave it zero
args.