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Re: how to check if xyz-mode is on load path?

From: Michael Ekstrand
Subject: Re: how to check if xyz-mode is on load path?
Date: Mon, 02 Mar 2009 07:12:47 -0600
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.90 (gnu/linux)

Xah Lee <address@hidden> writes:
> how can i check if a package already exists in the load path?
> what i want to do is to put a package into autoload, but don't do that
> if that package is already included in user's load path (e.g. either
> the package is included in the emacs version she's got, or she already
> installed it herself).
> e.g. pseudo code:
> (when
>     (not (isPackageInstalledQ "ocaml-mode"))
>   (autoload ocaml-mode "~/xahdir/ocaml-mode" )
> )

locate-library will tell you if the library is available on the load
path; it will return a file name or nil.  featurep (on the package's
primary provided feature) or fboundp (on some relevant function's
symbol) can be used to test if it has already been loaded.

I would also pair the tests such that locate-library failing results in
the package being added to the load path, and featurep/fboundp results
in loading.  The package existing in the load path does not mean it has
been loaded/autoloaded, so just testing with locate-library in your code
above can result in the package not being loaded.

- Michael

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