Re: Scheme: Syntax for storing a variable in an alist

[Aaron Hill]

On 2020-10-18 5:36 pm, Matthew Fong wrote:
> Hello everyone,
>
> I am likely in over my head with nesting pairs. I read the 
> documentation
> link from Jean, and I don't completely understand the internal
> representation of this nesting in scheme.
>
> dictHymn.HymnOne.ID = "210"
> dictHymn.HymnOne.Meter = "8.8.8.8"
> dictHymn.HymnTwo.ID = "344"
> dictHymn.HymnTwo.Meter = "12.12.12.8"
>
> Would it be something like:
> ( ( HymnOne . (( ID . 210 ) ( Meter . "8.8.8.8" )) )
>   ( HymnTwo . (( ID . 344 ) ( Meter . "12.12.12.8" )) ) )

Yes, it ends up being an alist of alists.


> assoc-get doesn't seem to work here with the syntactic sugar, likely
> because I may not understand how nested pairs work.
> dictHymnValue = #(ly:assoc-get 'ID dictHymn.HymnOne "error")
> \markup \dictHymnValue

The dot notation I showed is part of LilyPond syntax, not Scheme.

ly:assoc-get does not support nested alists directly, so you need to 
explicitly perform multiple lookups:

(NOTE: lily-library.scm defines an alias for ly:assoc-get omitting the 
ly: prefix.)

%%%%
\markup #(assoc-get 'ID (assoc-get 'HymnOne dictHymn '()) "error")
\markup #(assoc-get 'Tune (assoc-get 'HymnTwo dictHymn '()) "error")
\markup #(assoc-get 'Meter (assoc-get 'HymnThree dictHymn '()) "error")
%%%%

If you end up nesting alists more deeply, it can be cumbersome to do all 
of the individual assoc-get calls.  Here's a recursive wrapper that 
accepts a list of keys to lookup one after the other:

%%%%
#(define (assoc-get* key alist . args)
  (if (and (list? key) (not (null? (cdr key))))
    (apply assoc-get* (cdr key) (assoc-get (car key) alist '()) args)
    (apply assoc-get (if (pair? key) (car key) key) alist args)))

foo.one.two.three.four = "!"
\markup #(assoc-get* '(one two three four) foo "?")
\markup #(assoc-get* '(one two five six seven) foo "?")
%%%%


-- Aaron Hill