Re: Variables as variables

[David Kastrup]

Johan Vromans <address@hidden> writes:

> In LilyPond terms, in
>
>   namedMusic = { ... }
>
> 'namedMusic' is called a variable, identifier or macro.
>
> Normal LilyPond behaviour is
>
>   aaa = { c4 d e f }
>   bbb = { a4 a a a \aaa b4 b b b }
>   \score { \bbb }
>
> This produces { a4 a a a c4 d e f b4 b b b } .
>
> Now I want to re-use bbb with a different value of aaa. Like a real
> variable as known from other programming languages.
>
>   aaa = { c4 d e f }
>   bbb = { a4 a a a \aaa b4 b b b }
>   \score { \bbb }    % a4 a a a c4 d e f b4 b b b
>   aaa = { f4 e d c }
>   \score { \bbb }    % a4 a a a f4 e d c b4 b b b

That's not "like a real variable as known from other programming
language".  If I write

aaa = 3
bbb = aaa + 1
print ( bbb )
aaa = 5
print ( bbb )

I know of no programming language which will output 6 the second time
round.  You are not thinking of a variable, but of a parametric
procedure.

> This works, but it's not elegant.
>
>   bbb = 
>   #(define-music-function
>       (parser location)
>       ()
>     #{
>       a4 a a a \aaa b4 b b b
>     #})

That's delayed execution.  In my opinion, it makes more sense as a
parametric procedure:
bbb = 
#(define-music-function
    (parser location aaa)
    (ly:music?)
  #{
    a4 a a a #aaa b4 b b b
  #})

You can call it as \bbb \aaa or \bbb { f4 e d c } then.

> Do you have better (simpler, more elegant) suggestions?

I don't think that there are "more elegant" solutions available here.
While the given solutions might conceivably gain a bit more syntactic
sugar at one point of time, what actually _happens_ is unlikely to fall
in any different categories than what the current solutions do.

-- 
David Kastrup