Re: Difference between # and $

[David Kastrup]

"Phil Holmes" <address@hidden> writes:

> ----- Original Message ----- 
> From: "David Kastrup" <address@hidden>
> To: "Phil Holmes" <address@hidden>
> Cc: "Urs Liska" <address@hidden>; <address@hidden>
> Sent: Monday, March 31, 2014 2:44 PM
> Subject: Re: Difference between # and $
>
>
>> "Phil Holmes" <address@hidden> writes:
>>
>>> I've read
>>> http://lilypond.org/doc/v2.19/Documentation/extending/lilypond-scheme-syntax
>>> (as no doubt, Urs has) and tried to use $ in place of #.  Can't get it
>>> to compile.  So, taking the following note doubler, how would $ be
>>> used instead of #?
>>>
>>> dubble = #(define-music-function( parser location arg )
>>>  (ly:music?)
>>>  #{ $arg $arg #}
>>> )
>>>
>>> { c'' \dubble c' }
>>
>> Huh?  Which # would you even want to replace here?  #{ ... #} is inside
>> of Scheme.  $arg already uses a $.
>>
>> This code works fine as written.
>
> Er - yes.  I know it works fine.  I ran it.  However, the page I refer
> to above says "Another way to call the Scheme interpreter from
> LilyPond is the use of dollar $ instead of a hash mark for introducing
> Scheme expressions". So my presumption was that
> "#(define-music-function" is "a hash mark for introducing Scheme
> expressions" and could be replaced by a $.  But if I do that, it fails
> to compile.

Well, as mentioned in the documentation, $xxx is the same as \xxx.  It's
accurate that one effect that is not explicitly listed is that if $xxx
or \xxx happen to evaluate to a music function, that music function is
called.

If you wrote

dubble = $(define-music-function( parser location arg )
 (ly:music?)
 #{ $arg $arg #}
)

{ c'' c' }

{ \dubble }

Then this would be similar to

dubble = { { c'' c' } { c'' c' } }

{ \dubble }

because the music function is called right after its definition.  The
same happens with your example, but the \dubble occuring inside of your
music function argument is not yet defined.

-- 
David Kastrup