Using conditionals in LilyPond scheme functions

Question

I have a software development background, but I'm new to LilyPond. I'm trying to create a function that will allow me to pass in a number and then return some text depending on which number is passed in.

Example: \getStanza 2

#(define-scheme-function
    (stanza)
    (number?)
  #{
    % conditional code goes here
  #}

I've tried this:

(
  cond ((#stanza 1) \lyricsStanzaOne)
  cond ((#stanza 2) \lyricsStanzaTwo)
  cond ((#stanza 3) \lyricsStanzaThree)
)

And this:

\if (#stanza 1) \lyricsStanzaOne
\if (#stanza 2) \lyricsStanzaTwo
\if (#stanza 3) \lyricsStanzaThree

But I get an "ignoring non-music expression" warning and the lyrics for stanza one are returned, preceded by the word "if" or "cond" depending on which code block I try to use.

The LilyPond documentation doesn't provide much in the way of examples, so it's been a lot of trial and error just to get this far.

Answer 1

Lilypond somewhat consists of two languages, the Lilypond markup language and guile. These can be somewhat interchanged, as scheme code can be injected in lilypond language code by using # or $ (these have somewhat different meanings!), and lilypond language can be parsed and evaluated in guile using #{ ... #}.

If you use the Lilypond syntax A = B this will also define a scheme binding for A. So you can simply reference these things from scheme code.

Finally a cond in scheme works like this: (cond (condidion1 value1) (condition2 value2) ... (else default-value)) where the conditions are boolean values (and the else part is optional).

So one way to do what you want would be

getStanza =
#(define-scheme-function
    (stanza)
    (number?)
  (cond ((= stanza 1) lyricsStanzaOne)
        ((= stanza 2) lyricsStanzaTwo)
        ((= stanza 3) lyricsStanzaThree)
        (else "")))


lyricsStanzaOne = "abc"
lyricsStanzaTwo = "def"
lyricsStanzaThree = "ghi"

\getStanza 2

Using the conditional markup statements is probably not what you want, as the evaluation happens after parsing during rendering, so it will lead to an overly complicated structure. That one is specific for markups, so it needs to have a \markup statement. The way to do it would be

getStanza = #(define-scheme-function
    (stanza) (number?)
    #{
      \markup {
        \if #(lambda (. args) (= stanza 1)) \lyricsStanzaOne
        \if #(lambda (. args) (= stanza 2)) \lyricsStanzaTwo
        \if #(lambda (. args) (= stanza 3)) \lyricsStanzaThree
      }
    #})

lyricsStanzaOne = "abc"
lyricsStanzaTwo = "def"
lyricsStanzaThree = "ghi"

\getStanza #1