Q: Testing forthe existence of a function

Q: Testing forthe existence of a function

[grkuntzmd]

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I am trying to share my .emacs file across multiple hosts, some of which have
emacs compiled for X and some which do not.


On the ones that don't, the statement
(tool-bar-mode nil)

causes an error: Symbol's function definition is void: tool-bar-mode


I would like to add a statement of the form
(if (defined 'tool-bar-mode) (tool-bar-mode nil))

but I can't find the proper format -- is there a defined-p predicate?


I checked the elisp manual but could not find it.


Thanks


-----
G. Ralph Kuntz, MD

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Re: Q: Testing forthe existence of a function

[Giles Chamberlin]

grkuntzmd <grk@usa.net> writes:

> I would like to add a statement of the form
> (if (defined 'tool-bar-mode) (tool-bar-mode nil))

It's not the function you should be testing for but the feature  which
implements it:

(if (featurep 'tool-bar) ...)

-- 
Giles

RE: Q: Testing forthe existence of a function

[Drew Adams]

> > I would like to add a statement of the form
> > (if (defined 'tool-bar-mode) (tool-bar-mode nil))
> 
> It's not the function you should be testing for but the feature  which
> implements it: (if (featurep 'tool-bar) ...)

I disagree. Usually, it is better to test whether the function or variable that
you will use is defined. That provides finer granularity. `featurep' tells you
whether a library that provides a feature with that name has been loaded, but
different versions of libraries can have different sets of defined functions
(and it's even possible for different libraries to provide the same feature).

On the other hand, different libraries can sometimes define functions that have
the same name. In such a case, a `featurep' test can be more helpful than
`fboundp'.

In sum, it helps to know a bit about the function and library in question, and
even whether other libraries are common that might define a function with the
same name.

Re: Q: Testing forthe existence of a function

[Giles Chamberlin]

"Drew Adams" <drew.adams@oracle.com> writes:

> Usually, it is better to test whether the function or variable that
> you will use is defined. That provides finer granularity. 

Interesting.  I was thinking of my usual use case where I may or may not
have a particular library loaded, which admittedly was not the wording
of the original question.

> In sum, it helps to know a bit about the function and library in question, and
> even whether other libraries are common that might define a function with the
> same name.

Now that I can't argue with!

-- 
Giles

RE: Testing forthe existence of a function

[Drew Adams]

Function `fboundp' is what you're looking for.

--

Be aware that `fboundp' checks only whether its argument is defined, in the
sense of having something in its function cell. This does not necessarily mean
that the arg is a Lisp function - it could be also a macro or special form. If
you need to test whether the thing in question is in fact a function, use
`functionp'.

Re: Q: Testing forthe existence of a function

[Ilya Zakharevich]

On 2009-10-27, Giles Chamberlin <giles.chamberlin@tandberg.com> wrote:
> grkuntzmd <grk@usa.net> writes:
>
>> I would like to add a statement of the form
>> (if (defined 'tool-bar-mode) (tool-bar-mode nil))
>
> It's not the function you should be testing for but the feature  which
> implements it:
>
> (if (featurep 'tool-bar) ...)

If one want to guard a call to tool-bar-mode(), then one should test
for function tool-bar-mode(), not some [possibly unrelated] feature.

Hope this helps,
Ilya

Re: Q: Testing forthe existence of a function

[Colin S. Miller]

Hi Ralph,

grkuntzmd wrote:
> 
> but I can't find the proper format -- is there a |defined-p| predicate?
It's (functionp 'func-name)

> I checked the elisp manual but could not find it.
Can I ask what you searched for?

> Thanks
NP

HTH,
Colin S. Miller
-- 
Replace the obvious in my email address with the first three letters of the hostname to reply.

Re: Q: Testing forthe existence of a function

[grkuntzmd]

I searched for several things. Since I did not know the name of the
predicate, I searched under symbols, functions, conditionals, and other
places.


Colin S. Miller-2 wrote:
> 
> Hi Ralph,
> 
> grkuntzmd wrote:
>> 
>> but I can't find the proper format -- is there a |defined-p| predicate?
> It's (functionp 'func-name)
> 
>> I checked the elisp manual but could not find it.
> Can I ask what you searched for?
> 
>> Thanks
> NP
> 
> HTH,
> Colin S. Miller
> -- 
> Replace the obvious in my email address with the first three letters of
> the hostname to reply.
> 
> 


-----
G. Ralph Kuntz, MD

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View this message in context: http://www.nabble.com/Q%3A-Testing-forthe-existence-of-a-function-tp26077455p26082036.html
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Re: Q: Testing forthe existence of a function

[Colin S. Miller]

grkuntzmd wrote:
> I searched for several things. Since I did not know the name of the
> predicate, I searched under symbols, functions, conditionals, and other
> places.
> 
C-u C-h a function
found it.
In FSF Emacs, the C-u prefix is needed to search non-interactive functions.

Colin S. Miller


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