Re: Question about scheme engravers

[Eric Knapp]

Thanks, Carl.

I am making a bit progress, but I haven't cracked the fingerings in a
chord, yet. I'll post another with my source to the devel list.

-Eric

On Mon, Jan 18, 2010 at 4:49 PM, Carl Sorensen <address@hidden> wrote:
> On 1/18/10 2:41 PM, "Eric Knapp" <address@hidden> wrote:
>
>> Hello, everyone.
>>
>> I'm working on creating engravers in scheme. I have the example from
>> here:
>>
>>   http://codereview.appspot.com/181109/show
>>
>> In the example file named, "scheme-engraver.ly" there is this piece of
>> code:
>>
>> (cons 'rest-event (lambda (engraver event)
>>            (let*
>>             ((x (ly:engraver-make-grob engraver 'TextScript event)))
>>             (display (list "caught event" event "\ncreate:\n" x "\n"))
>>             (ly:grob-set-property! x 'text "hi"))
>>            ))
>>
>> If there's a rest in the notation code like this, "<c-2\6>1 r2", then
>> the above display function will output this:
>>
>>     (caught event #<Prob: Stream_event C++: Stream_event((music-cause
>>     . #<Prob: Music C++: Music((length . #<Mom 1/2>) (elements)
>>     (duration . #<Duration 2 >) (origin . #<location
>>     scheme-engraver.ly:23:11>))((display-methods #<procedure #f (rest
>>     parser)>) (name . RestEvent) (types general-music event
>>     rhythmic-event rest-event)) > ) (length . #<Mom 1/2>) (elements)
>>     (duration . #<Duration 2 >) (origin . #<location
>>     scheme-engraver.ly:23:11>))((class . rest-event)) >
>>
>> Here's my question. How do I access elements in the event? For
>> example, the duration of the rest is shown above as, "(duration .
>> #<Duration 2>)". I would like to have an "if" statement look at the
>> duration variable and do something based on the value. What would an
>> if for that look like?
>
> First, you want to extract the music from the stream event, I think
>
> (ly:event-property event 'Music)  (but I'm not sure about this step)
>
> Once you have the music, then you get the elements of the music:
>
> (ly:music-property music 'elements)
>
> Now, elements is a list of music elements, so you'll want to get the
> duration from the first element of elements:
>
> (ly:music-property (car elements) 'duration)
>
>>
>> As you can probably tell, I'm also learning scheme. I've been looking
>> for a reason to learn scheme and the new capability of writing scheme
>> engravers has hooked me!
>
> Looks like you're doing a good job of it!
>
>>
>
>> Thanks for the help in advance, this is getting exciting.
>
>
> I think so!
>
> HTH,
>
> Carl
>
> P.S.  Questions at this level of detail probably belong on -devel, rather
> than -user
>
>
>