Re: Replace ly:music elements in a Scheme function

[David Kastrup]

paolo prete <address@hidden> writes:

> Hello David,
> thanks for the snippet but I ask if it is possible to modify only the body
> of my function, inside lambda (x y)( ...  ) so to obtain the same result.
> Something like:
>
>
> %%%%%%%%
>
> fun = #(define-music-function (parser location music1 music2) (ly:music?
> ly:music?)
> (let
>     (
>       (musicList1 (ly:music-property music1 'elements))
>       (musicList2 (ly:music-property music2 'elements))
>     )
>     (map
>         (lambda (x y)
>           ;
>           ;  place HERE the code for copying the WHOLE y element to the
> whole x element
>           ;  (where y and x have the same position in their corresponding
> lists)
>           ;
>         )
>      musicList1 musicList2
>     )
> )
> #{ $music1 #})

No.  x and y are values stored in lists.  The relation to their lists is
not passed into your function, so you cannot change it there.

You'd need to use something like pair-for-each to modify the first list
in-place, and that gets old fast as music structures change.

There is an actual reason functions like map-some-music are provided.

-- 
David Kastrup