if/then/else in Lilypond with config file

[cpf]

> Is there a possibility to do something like this?
>
>     if( custom_cfg_flag)
>     {
>         % default score combination
>         % (e.g. with header and chords)
>     }
>     else
>     {
>         % some other combination
>         % (e.g. only melody)
>     }

I have also always wanted LilyPond to provide me with
the code-writing niceties which C offers. Here is a
method which enables me to use blocks of preprocessor
directives in the manner which I am used to:

#define BEGRIFF

#ifdef BEGRIFF
  #define INTEGER_CONSTANT 1
#else
  #define INTEGER_CONSTANT 43
#endif

My operating system is Linux. For its GNU Compiler Suite
there is the preprocessor ``gpp''. This preprocessor can
be invoked with the ``-T'' option, to instruct it to
understand macros which are written in the style that
TeX (and LilyPond) use.

The first shell script processes a the file
``if-else-1a-input.ly''.

-----if-else-1a-input.ly-----
% Within a region which is commented, you can embed your
% macro definitions.
% \define{conditionalPackage}
% \ifdef{conditionalPackage}
%   \define{musicPassage}{\musicPassageAlpha}
% \else
%   \define{musicPassage}{\musicPassageBeta}
% \endif
\version "2.12.3"
\header { title = "Using if/else Constructs by Means of GPP" }
musicPassageAlpha = { c'4 d' e' f' }
musicPassageBeta = { b,4 a, g, f, }
\score {
  <<
    \new Staff \musicPassage
  >>
  \layout { }
}
% EOF
-----------------------

Here is the shell script that does the work:

-----if-else-1a.sh-----
#! /bin/bash
# Invokes ``[[gpp]]'' in order to process an input file
# which has a macro definition in it formatted in the
# manner which TeX formats things.
INPUT_FILE=if-else-1a-input.ly
OUTPUT_FILE=if-else-1a-output.ly
gpp -T -o $OUTPUT_FILE $INPUT_FILE && \
  echo 'Output file "'$OUTPUT_FILE'" successfully written.'
# EOF
-----------------------

The second shell script processes a the file
``if-else-1b-input.ly''. This document has an ``undef''
preprocessor directive in it.

-----if-else-1b-input.ly-----
% Within a region which is commented, you can embed your
% macro definitions.
% This version puts an ``undef'' preprocessor
% directive in the macro definition.
% \define{conditionalPackage}
% \undef{conditionalPackage}
% \ifdef{conditionalPackage}
%   \define{musicPassage}{\musicPassageAlpha}
% \else
%   \define{musicPassage}{\musicPassageBeta}
% \endif
\version "2.12.3"
\header { title="Using if/else Constructs by Means of GPP" }
musicPassageAlpha = { c'4 d' e' f' }
musicPassageBeta = { b'''4 a''' g''' f''' }
\score {
  <<
    \new Staff \musicPassage
  >>
  \layout { }
}
% EOF
-----------------------

Here is the shell script that does the work:

-----if-else-1b.sh-----
#! /bin/bash
# Invokes ``[[gpp]]'' in order to process an input file
# which has a macro definition in it formatted in the
# manner which TeX formats things.
INPUT_FILE=if-else-1b-input.ly
OUTPUT_FILE=if-else-1b-output.ly
gpp -T -o $OUTPUT_FILE $INPUT_FILE && \
  echo 'Output file "'$OUTPUT_FILE'" successfully written.'
# EOF
-----------------------