Add \eval to Lilypond?

[Alan Stern]

The facility for generating Lilypond code from within Scheme is
fairly limited.  The #{ ... #} syntax only recognizes music
expressions, so it can't be used in a \layout block, for instance.

Is it possible to add a \eval primitive to Lilypond?  It could
take as an argument either a literal string or a Scheme expression
that returns a string, and then it would feed the contents of that
string directly into the parser.

As an example, we could put in a source file

\eval #(if OutputMidi "\\midi { \\tempo 4 = 120 } " "")

Then the setting of the single variable OutputMidi would control
whether or not the \midi block was parsed and hence whether a MIDI
output file was created.   Currently there's no way to make such a
decision at runtime.  Much more complicated code could easily be
constructed, in which the strings are calculated and assembled
on-the-fly.

This ought to be reasonably simple to implement for someone who is
already familiar with Lilypond's internal operation (which I am not).
Is anyone interested?

Alan Stern