Re: Scheme function problems

[Nicolas Sceaux]

Kieren Richard MacMillan <address@hidden> writes:

> Hello, all --
>
> Can anyone tell me why the code
>
> \version "2.8.1"
> barpadding = #(define-music-function (parser location padding music)
> (number? ly:music?)
>       #{
>               \once \override Score.BarLine #'space-alist =
>               #'((first-note extra- 
> space . $padding))
>               $music
>       #}
> )
> { c'1 \barpadding #32.0 { c' c' } c' }
>
> returns
>
> ERROR: Wrong type (expecting real number): lilyvartmpa

The reason of the error is the following:

When using a $variable inside scheme context in a #{ #} expression, the
"$padding" is replaced by a symbol (of the form
lilyvartmpNNN), which is bound to value given to padding. That is,
this is equivalent to:

#(define lilyvartmpa <value of padding>)

{
  \once \override Score.BarLine #'space-alist =
    #'((first-note extra-space . lilyvartmpa))
  {..music..}
}

'((first-note extra-space . lilyvartmpa)) is a quoted, literal list, and
when these data are used, a symbol 'lilyvartmpa is found where a number
is expected, hence the cryptic message.

Don't use a quoted list where you want to introduce a variable. You can
build the list by calling list, cons, etc, or use the dedicated feature:
backquote.

#`((first-note extra-space . ,lilyvartmpa))

is like writing:

#(list (cons 'first-note (cons 'extra-space lilyvartmpa)))

where lilyvartmpa will be evaluated.

So, as Jan explained, a solution is:

#`((first-note extra-space . ,$padding))
 ^
 |
 a backquote, not a quote.

nicolas