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→Programs, Libraries, and Packages: add recipe to check if package is missing
Carandraug (talk | contribs) (→Find Octave configuration: check support for specific image formats) |
Carandraug (talk | contribs) (→Programs, Libraries, and Packages: add recipe to check if package is missing) |
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## Image IO with support for png files | ## Image IO with support for png files | ||
any (cellfun (@(x) ismember ("png", x), {imformats.ext})) | any (cellfun (@(x) ismember ("png", x), {imformats.ext})) | ||
=== Find if a package is installed === | |||
==== Problem ==== | |||
You have a program that uses different functions or behaves different | |||
depending on the availability of specific packages. | |||
==== Solution ==== | |||
Use {{codeline|pkg ("describe", pkg-name)}} like so: | |||
if (! isempty (pkg ("describe", "foo"))) | |||
## use functions from package foo, the prefered way | |||
elseif (! isempty (pkg ("describe", "bar"))) | |||
## use functions from package bar, not so optimal | |||
else | |||
## default case | |||
endif | |||
==== Discussion ==== | |||
It's not recommended to use this if the only purpose is to then fail | |||
in the absence of the package. In such case, simply try to load the package | |||
and Octave will already give a error message that is informative enough. | |||
There is only purpose to check this, if there is something different to | |||
do if a package is missing. The same is true for catching an error from | |||
{{codeline|pkg load}}. If you only catch an error to then throw it again | |||
then you might as well not catch it in the first place. | |||
## This contraption doesn't add anything. If 'pkg load' fails, it | |||
## will already give an error message to the user. | |||
try | |||
pkg load foo; | |||
catch | |||
error ("failed to load foo: %s", lasterr ()); | |||
end_try_catch | |||
## Again, doesn't add anything. The failure of 'pkg load' is enough | |||
if (isempty (pkg ("describe", "foo"))) | |||
error ("program: package foo is not installed"); | |||
endif | |||
Beware that an installed package is not always a guarantee that a function | |||
will be available. Some packages may disable functions at build time, or | |||
specific functions may have specific runtime requirements. | |||
== Structures == | == Structures == | ||