LilyPond Contributor's Guide: 10.13.3 Music functions and GUILE debugging

### 10.13.3 Music functions and GUILE debugging

Ian Hulin was trying to do some debugging in music functions, and came up with the following question

HI all, I’m working on the Guile Debugger Stuff, and would like to try debugging a music function definition such as:

>     conditionalMark = #(define-music-function (parser location) ()
>         #{ \tag #'instrumental-part {\mark \default}  #} )

It appears conditionalMark does not get set up as an equivalent of a Scheme

>     (define conditionalMark = define-music-function(parser location () ...

although something gets defined because Scheme apparently recognizes

>     #(set-break! conditionalMark)

later on in the file without signalling any Guile errors.

However the breakpoint trap is never encountered as define-music-function passed things on to ly:make-music-function, which is really C++ code ly_make_music_function, so Guile never finds out about the breakpoint.

Han-Wen answered as follows:

You can see the definition by doing

>     #(display conditionalMark)

noindent inside the ‘`.ly`’ file.

The breakpoint failing may have to do with the call sequence. See ‘`parser.yy`’, run_music_function(). The function is called directly from C++, without going through the GUILE evaluator, so I think that is why there is no debugger trap.

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