Re: what does it take to produce a new output block

[Jean Abou Samra]

Hi,

A minor point of procedure: even though you changed the subject, it
appears that you started this thread from a previous thread about time
signatures in your email client. The email contains headers that make it
appear in that thread for others. In the future, start a new thread
every time.


Le 31/05/2022 à 23:02, Flaming Hakama by Elaine a écrit :
> Hi,
>
> I am interesting in seeing what it would take to enhance lilypond to output
> in this text format
>
> https://www.irealpro.com/ireal-pro-file-format
>
> My first idea would be to add something along the lines of the midi block,
> where if you add this new output, you will get output in this text format,
> which is essentially an HTML link.
>
> \score {
>    … music …
>    \layout { }
>    \midi { }
>    \ireal { }
> }
>
>
> I suppose it should support similar features for command line and scheme:
>
> -direal-extension=html
> #(ly:set-option 'ireal-extension "html")
>
>
> To look into implementing this, where would I get started?



I would start by writing Scheme engravers living in \layout
that take the content and format it together. The only disadvantage
is that won't have ireal output without at the same time doing
layout output (well, unless you go for calling (exit) at the
end of the translation step if that's really a problem ...).
The big advantage is that you can start doing this right now
in a .ly file, without touching LilyPond's code at all, post
it on the user list and get feedback.

There is no official documentation for engravers, but there
is an unofficial resource (by me):

https://extending-lilypond.readthedocs.io/en/latest/translation.html



> Is there any documentation that describes what exactly is a block, what it
> takes as input, etc.?


Well, LilyPond doesn't really have a notion of a "block".
The objects you create with \midi and \layout are called
"output definitions" (output defs for shorts), a category
which actually also encompasses \paper blocks. I guess the
closest you get as documentation about output defs is

https://lilypond.org/doc/v2.23/Documentation/notation/modifying-context-plug_002dins.html

but this is not really descriptive of the internals, and
I don't think it needs to be: output defs are an overarching
concept where many tweaks take place; the way these tweaks
are arranged internally isn't really relevant to the user.

Most importantly, \layout and \midi are hardcoded. You
cannot create you own type of "block" without touching
the C++ sources.



> Where would I find the code for the layout and midi blocks?


A bit all over the place, really. lily/lexer.ll and lily/parser.hh
have the parsing logic, with some bits in lily/lily-lexer.cc
and lily/lily-parser.cc. The Output_def class is found in
lily/output-def.cc. The Scheme interfaces for manipulating
it are in lily/output-def-scheme.cc. A central element of output
defs is context defs (the \context blocks); that class is
in lily/context-def.cc with Scheme interfaces in
lily/context-def-scheme.cc. Then you have files like
lily/engraver.cc, lily/score-engraver.cc, lily/performer.cc,
lily/score-performer.cc which define the classes of
translators that \layout and \midi respectively run.

But as I said above, I don't think the code for \layout
and \midi is the most relevant part of LilyPond to
look at for solving your problem, at least until you get
to a state where it's working fully and you want to contribute
it to LilyPond with that kind of interface. Start with
learning how to code an engraver first.

Best,
Jean