(octave.info)Missing Components

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37.4.4 Missing Components
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If a package relies on a component, such as another Octave package, that
may not be present it may be useful to install a function which informs
users what to do when a particular component is missing.  The function
must be written by the package maintainer and registered with Octave
using ‘missing_component_hook’.

 -- : VAL = missing_component_hook ()
 -- : OLD_VAL = missing_component_hook (NEW_VAL)
 -- : missing_component_hook (NEW_VAL, "local")
     Query or set the internal variable that specifies the function to
     call when a component of Octave is missing.

     This can be useful for packagers that may split the Octave
     installation into multiple sub-packages, for example, to provide a
     hint to users for how to install the missing components.

     When called from inside a function with the "local" option, the
     variable is changed locally for the function and any subroutines it
     calls.  The original variable value is restored when exiting the
     function.

     The hook function is expected to be of the form

          FCN (COMPONENT)

     Octave will call FCN with the name of the function that requires
     the component and a string describing the missing component.  The
     hook function should return an error message to be displayed.

     See also: Note: missing_function_hook.