Implementing a counter in markup text

This snippet shows how to implement a counter in LilyPond for use in markup texts. Whenever you call the \counter "name" markup function, the counter named name will be increased by one and the new value will be printed as a text markup. The first call will print “1” by default. Example:

\markup {
  Initial value of the counter: \counter #"mycounter".
  Second call: \counter #"mycounter" }

The snippet also implements a \setcounter #"name" newvalue markup command to set the the counter to a specific value and print it out.

One can have an arbitrary number of simultaneous counters, each indicated by its own name (as a string).

The storage of the counter values is implemented as a global alist counter-alist (Scheme/Guile's concept of a hash), where the counter command simply extracts the current value, increases it by one and stores it back in the alist.

\version "2.24"

#(define counter-alist '())

#(define-markup-command (counter layout props name) (string?)
  "Increases and prints out the value of the given counter named @var{name}.
  If the counter does not yet exist, it is initialized with 1."
  (let* ((oldval (assoc-ref counter-alist name))
         (newval (if (number? oldval) (+ oldval 1) 1)))
  (set! counter-alist (assoc-set! counter-alist name newval))
  (interpret-markup layout props
    (markup (number->string newval)))))

#(define-markup-command (setcounter layout props name value) (string? number?)
  "Set the given counter named @var{name} to the given @var{value} and prints
  out the value. The counter does not yet have to exist."
  (set! counter-alist (assoc-set! counter-alist name (- value 1)))
  (interpret-markup layout props (make-counter-markup name)))

\markup { Initial value of the counter: \counter #"mycounter". }
\markup { Second call: \counter #"mycounter" }
\markup { It is increasing: \counter #"mycounter" }
\markup { A second counter: \counter #"myothercounter" }
\markup { First counter is increasing: \counter #"mycounter" }
\markup { Second counter is increasing: \counter #"myothercounter" }
\markup { Setting to a specific value works, too: }
\markup { Set first counter to 15: \setcounter #"mycounter" #15 }
\markup { It is increasing: \counter #"mycounter" }
\markup { don't rush it: }
\markup { Three calls in a row: \counter #"mycounter", \counter #"mycounter" and \counter #"mycounter" }
\markup { It is increasing: \counter #"mycounter" }
\markup { Second counter is increasing: \counter #"myothercounter" }