Re: Scoped variables

[Aaron Hill]

On 2019-11-07 7:33 am, Peter Toye wrote:
> This raises a further question which I can't see answered in the
> documentation: exactly what characters are allowed in a Lilypond
> variable name? NR says that the convention is alphabetic characters
> only, but you seem to have found that digits and full-stops are
> allowed. Is this documented, or did you experiment?

Only the part up to the first period or comma is the name of the 
variable.  Each part after is defining the keys for a (possibly nested) 
association list:

%%%%
Variable . Key , 1 = "foo"
#(format #t "\n~a" Variable)
#(format #t "\n~a" #{ \Variable.Key.1 #})
%%%%
====
Parsing...
((Key (1 . foo)))
foo
Success: compilation successfully completed
====

This creates a top-level variable named 'Variable' that has the 
equivalent Scheme definition:

%%%%
#(define Variable '((Key . ((1 . "foo")))))
#(format #t "\n~a" Variable)
#(format #t "\n~a" #{ \Variable.Key.1 #})
%%%%
====
Parsing...
((Key (1 . foo)))
foo
Success: compilation successfully completed
====

It seems that numbers are permitted for naming of the association list 
keys, but the top-level variable name must be alphabetic only unless it 
is quoted:

%%%%
"12?!" = "blah"
#(format #t "\n~a" #{ \"12?!" #})
#(format #t "\n~a" (eval (string->symbol "12?!") 
(interaction-environment)))
%%%%
====
Parsing...
blah
blah
Success: compilation successfully completed
====

An interesting detail with using numbers is that the resulting key is 
numeric rather than a symbol.  To get a symbol, you would need to use 
quotes:

%%%%
Thing, 1 = "one"
Thing, "2" = "two"
#(format #t "\n~a" Thing)
#(format #t "\n~a" (map (lambda (x) (symbol? (car x))) Thing))
%%%%
====
Parsing...
((#{2}# . two) (1 . one))
(#t #f)
Success: compilation successfully completed
====

Mind you, I am uncertain why you want #{2}# over just 2.

Finally, LilyPond only prepends new association list entries rather than 
overwriting any existing ones:

%%%%
Thing, Key = 12
Thing, Key = -34
#(format #t "\n~a" Thing)
#(format #t "\n~a" #{ \Thing.Key #})
%%%%
====
Parsing...
((Key . -34) (Key . 12))
-34
Success: compilation successfully completed
====

But since lookup returns the first matching key, it will appear as if it 
is overwriting.


-- Aaron Hill