Re: Optional arguments of a music-function?

[Thomas Morley]

2012/7/17 David Kastrup <address@hidden>:
> Thomas Morley <address@hidden> writes:
>
>> Hi,
>>
>> I tried to adapt the syntax-trickery David showed with the new
>> \tweak-definition to an own function.
>>
>> Trying a code like the tiny example below fails.
>>
>> proc =
>> #(define-music-function (parser location name arg)((string?) ly:music?)
>>   (if name
>>   #{ \once \override $name #'color = #red  $arg #}
>>   #{ \parenthesize $arg #}))
>>
>> \relative c' {
>>         c1
>>         \proc "NoteHead" c
>>         \proc c
>> }
>>
>> Obviously I did sth wrong.
>
> Not obviously.  Unobviously, however, we have the situation that
> optional arguments can only be properly checked by the parser when they
> can be parsed without lookahead, namely are a "closed" expression, since
> only then the parser can push the value back onto its token stack: if it
> has already looked at a lookahead token, it does not have this option
> since the parser state already depends on the value of the lookahead
> token.
>
> A pitch can't be parsed without lookahead since it may be followed by
> ''' and/or ,,, and so lookahead is needed to make the decision.  On the
> other hand, we have other situations where the user is not supposed to
> change the meaning of a predicate when its _value_ rather than its type
> changes, so a predicate that fails or succeeds depending on the _octave_
> of a pitch, or on the presence of a postevent or similar, is not kosher
> anyway.
>
>> Well, I could do:
>>
>> #(define-public (string-or-music? x)
>>   (or (string? x) (ly:music? x)))
>>
>> procII =
>> #(define-music-function (parser location arg)(string-or-music?)
>>   (if (string? arg)
>>   #{ \once \override $arg #'color = #red  #}
>>   #{ \parenthesize $arg #}))
>>
>> \relative c' {
>>         c1
>>         \procII "NoteHead" c
>>         \procII c
>> }
>>
>> But I'd prefer the first version.
>> How  to do?
>
> Wait a few days.
>
> It's the same problem as with
> <URL:http://code.google.com/p/lilypond/issues/detail?id=2067#c19>, and
> the idea to look at the pitch identifier only at first (rather than the
> whole pitch) did occur to me only now.  It is conceivable that I can
> rein this in.
>
> Until I do, the syntactical possibilities for a music argument following
> an optional argument are restricted.  You can always remove the
> restriction by explicitly turning this into one lookahead-free token,
> say with
>
> \procII ##{ c4 #}
>
> but this is not really pretty.
>
> --
> David Kastrup
>
>
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Can't say that I understood your explanations in all details.
But I'm sure I can wait a few days.

Thanks,
  Harm