Is string interpolation possible?

Hello,
I'm trying midish and I found it very useful so far.

What I haven't been able to do is composing a string with a variable
inside, for example I'm trying to write a function to save a song in a
deeply nested directory without typing it every time, i.e.

proc savesong songname {
    save "long/directory/nested/midish-songs/${songname}"
}

I'm missing something?
 Thanks
-- 
Emiliano Grilli
Linux user #209089
http://www.emillo.net