helper function that should take a list argument doesn't do anything

[Janek Warchoł]

% Hi people,

% here's what i want to do:

<<
  {
    \overrideProperty #"Score.NonMusicalPaperColumn"
    #'line-break-system-details  #'((alignment-distances . (30)))
    a a a a
  }
  { b b b b }
>>

% the override is very long, so i wanted to create a helper music
function.  I tried this

staffdist =
#(define-music-function (parser location distances)
                        (list?)
                        #{
                          \overrideProperty #"Score.NonMusicalPaperColumn"
                          #'line-break-system-details
#'((alignment-distances . #'distances))
                        #})

<<
  {
    \staffdist #'(30)
    a a a a
  }
  { b b b b }
>>

%{
and it doesn't do anything.  I tried adding/deleting apostrophes and
parentheses, but it didn't work.  I don't see any example function
operating on lists in the Extending manual, so i'm a bit lost.
I had no problem with a similar function which operates on a pair and
overrides beam positions.  I have no idea why this doesn't work.

what i'm doing wrong?
Janek
%}