Better method for inputting alists?

[Aaron Hill]

LilyPond makes it easy to define a top-level alist and extend it with 
new keys:

%%%%
\version "2.22.0"

foo.one = 1
foo.two = "2"
foo.nested.red = 3,4
foo.nested.blue = ##t

\void \displayScheme \foo.nested.red
%%%%

This pattern works well enough.  However, especially when nested more 
deeply, it does result in redundant and error-prone typing as each new 
entry must specify the full path of keys.

Borrowing syntax for context mods, one can get an improved approach:

%%%%
\version "2.22.0"

alist =
#(define-scheme-function (with) (ly:context-mod?)
  (define (assign? mod) (eq? 'assign (car mod)))
  (define (assign->pair mod) (cons (cadr mod) (caddr mod)))
  (map assign->pair (filter assign? (ly:get-context-mods with))))

foo = \alist \with {
  one = 1
  two = "2"
  nested = \alist \with {
    red = 3,4
    blue = ##t
  }
}

\void \displayScheme \foo.nested.red
%%%%

The above technique admittedly abuses the \with construct, so would it 
make sense for the parser to support alists directly?  Not sure what the 
syntax should be; but here's a possibility prepending a colon to the 
brace:

%%%%
foo = :{ one = 1 two = "2" nested = :{ red = 3,4 blue = ##t } }
%%%%

Functions could take advantage of such abbreviated syntax to support 
named arguments without having to resort to the same \with hack.  The 
\override markup command in particular would become nicer-looking:

%%%%
\markup \override :{ offset = 15 thickness = 3 } \undertie "lorem ipsum"
%%%%


-- Aaron Hill