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

[Jean Abou Samra]

Le 18/10/2020 à 21:42, Matthew Fong a écrit :
> Hello everyone,
>
> I am having a bit of an issue storing a variable in an alist, and I 
> was wondering if you all could point out what I am doing wrong. If I 
> write the string directly, there's no issue retrieving it ...
>
> my-alist = #'((1  . "A") (2 . "B") (3 . "C"))
> value = #(ly:assoc-get 2 my-alist " error")
> \markup \value
>
> hymnID = "210"
> hymnMeter = "8.8.8.8"
> hymnAlist = #'(("ID" . \hymnID ) ("Meter" . \hymnMeter ))
> hymnValue = #(ly:assoc-get "ID" hymnAlist "error")
> \markup \hymnValue
>
>
> Many thanks,
> mattfong

Hello,

Take a look at the output of:

hymnID = "210"
hymnMeter = "8.8.8.8"
hymnAlist = #'(("ID" . \hymnID ) ("Meter" . \hymnMeter ))
#(display (ly:assoc-get "ID" hymnAlist "error"))

This prints

  \hymnID

The reason is that you created the alist using quoting, and a piece of 
text that you quote is a symbol. Note that prefixing something with a 
backslash is not the way to use a LilyPond variable from Scheme; you 
just need the naked name. Here, the values (as opposed to keys) in 
hymnAlist are symbolic values representing the identifiers “\hymnID” and 
“\hymnMeter” (backslashes are valid in Scheme identifiers!).

You can read more about quoting here:

https://scheme-book.ursliska.de/scheme/quoting/README.html

The correct way to go about it is to use quasi-quoting:

hymnID = "210"
hymnMeter = "8.8.8.8"
hymnAlist = #`(("ID" . ,hymnID ) ("Meter" . ,hymnMeter ))
\markup #(ly:assoc-get "ID" hymnAlist)

(note ` instead of ' and the commas).

Alternatively,

hymnAlist = #(list (cons "ID" hymnID ) (cons "Meter" hymnMeter ))

has the same effect.

Best,
Jean