Function template (or whatever it's called)

[Urs Liska]

I'm providing users with the option to configure some behaviour by 
providing custom styling functions. These functions must have a certain 
interface and will typically retrieve the same information from their 
arguments. I would like to simplify this task by providing a "function 
template". I think this may be achieved by macros or some other 
syntactic extension mechansim, but I have no idea about how to go 
forward with that.

This is what currently a user would have to do:

\version "2.19.80"

myFunc =
#(define-music-function (mus)(ly:music?)
   (let*
    ((elts (ly:music-property mus 'elements))
     (cnt (length elts)))
    (ly:message "The music has ~a elements" cnt)
    mus))

{
  \myFunc { c' e' }
}

Create a music function with one ly:music? argument and extract some 
information from the music expression (of course this is just an 
example, in the real case it's a custom object attached to the music).

What I would like to have is what define-music-function does here but 
with a pre-set argument list and essentially the let* bindings 
transparently done so that something like this is possible:

userFunc =
#(define-my-custom-function
   (ly:message "The music has ~a elements" cnt))

where one ly:music? argument is implicitly expected and the bindings 
like 'cnt' have already been prepared.

Is this possible? I would assume so, and I'd also would assume it's 
quite easy - once you know what to look for.

Thanks
Urs