Wish/suggestion: \import statement

[Urs Liska]

There has been discussion every now and then about ways to include files 
only once, with different suggestions but no final outcome (IIRC).

I would like to re-raise the issue with a slightly different perspective.

I think one thing LilyPond could use is encouraging people to use 
*libraries*.
Of course this is easily done, and I assume the majority of power users 
works that way.
But everybody is probably doing it his own way, figuring out solutions 
and reinventing the wheel. So I think it could be valuable to think 
about more consistent ways to create and maintain libraries, both 
private ones and publicly available ones.

Encouraging people to create public libraries (and perhaps on the long 
run harmonizing that to a central repository) could also help extending 
LilyPond's functionality without the urge to push everything into 
LilyPond itself.

Well, this was preliminary context which is not absolutely related to my 
suggestion. Nevertheless I'd find an addition like the following useful:

Create a new \import command.
The naming isn't mandatory, but I think this is distinguishable from 
\include.

This command should only include the to-be-imported file if it has not 
already been parsed.

Additionally it could be a good thing to *selectively* \import material 
(similar to Python's from .. import).
This would consider an "importable" file more like a real module entity 
than \include which simply includes text as if it were in the same file.
It would be nice to be able to define functions (or any other variables, 
this would also work with music variables) in a module and only parse 
selected items from a module. Or a module could include submodules, and 
from the main input file one could choose to only include specific 
submodules.
That way one could create large modules or libraries without forcing 
LilyPond to parse big amounts of code that isn't actually relevant for 
the current project.

Just one example to show what I mean:

A library with harmonic analysis tools. There are generic functions 
(like for aligning stuff) in the main module and submodules for step 
theory, pitch class theory and functional harmonics.

\import harmonic-analysis
would load everything at once (I'd even suggest this command working 
with symbolic names and not filenames).

\importFrom harmonic-analysis #'(pitch-class riemann)
would only load the two submodules.
(The submodule would have to \import the main module if it depends on 
code in it).

What do you think?

Urs