Include file from music function 2.15.18

[Jay Anderson]

In working on a large set of hymns pre-2.15.18 I could do something like this (a simplified example):

===============

\version "2.15.18"

function =

#(define-music-function (parser location num) (number?)

  (let ((file (string-append "hymn" (number->string num) ".ily")))

    #{ \include $file #}))

\score

{

  \new Staff \relative c'

  {

    \function #0

  }

}

===============

Error in 2.15.18:

===============

GNU LilyPond 2.15.18

Processing `2.15.18.ly'

Parsing...

<string>:1:10: error: invalid character: `$'

 \include 

          $file 

<string>:1:10: error: syntax error, unexpected $undefined

 \include 

          $file 

2.15.18.ly:6:5: In procedure variable-ref in _expression_ (ly:parse-string-_expression_ clone " \\include $file "):

2.15.18.ly:6:5: variable is unbound: #<variable 7f862c8af440 value: #<undefined>>

===============

The 'let' isn't breaking it because I can do silly things like this:

functionB =

#(define-music-function (parser location num) (number?)

  (let ((str (string-append "hymn" (number->string num) ".ily")))

    #{ c_$str #}))

Essentially, I want to build up the name of the file to include and include it. Is this a bug or should this be done in a fundamentally different way now? Thanks.

-----Jay