4.3.16 Using Preprocessor to Improve the Configuration.

Before parsing its configuration file, dicod preprocesses it. The built-in preprocessor handles only file inclusion and #line statements (see Pragmatic Comments), while the rest of traditional preprocessing facilities, such as macro expansion, is supported via m4, which is used as an external preprocessor.

The detailed description of m4 facilities lies far beyond the scope of this document. You will find a complete user manual in http://www.gnu.org/software/m4/manual. For the rest of this subsection we assume the reader is sufficiently acquainted with m4 macro processor.

The external preprocessor is invoked with -s flag, instructing it to include line synchronization information in its output. This information is then used by the parser to display meaningful diagnostic. An initial set of macro definitions is supplied by the pp-setup file, located in $prefix/share/dico/version/include directory (where version means the version of GNU Dico package).

The default pp-setup file changes quote characters to ‘[’ and ‘]’, and renames all m4 built-in macros so they all start with the prefix ‘m4_’. The latter has an effect similar to GNU m4 --prefix-builtin option, but has an advantage that it works with non-GNU m4 implementations as well.

As an example of how the use of preprocessor may improve dicod configuration, consider the following fragment taken from one of the installations of GNU Dico. This installation offers quite a few Freedict dictionaries. The database definition for each of them is almost the same, except for the dictionary name and eventual description entry for several databases that miss it. To avoid repeating the same text over again, we define the following macro:

# defdb(NAME[, DESCR])
# Produce a standard definition for a database NAME.
# If DESCR is given, use it as a description.
m4_define([defdb], [
database {
        name "$1";
        handler "dictorg database=$1";m4_dnl
m4_ifelse([$2],,,[
        description "$2";])
}
])

It takes two arguments. The first one, NAME, defines the dictionary name visible in the output of SHOW DB command. Optional second argument may be used to supply a description string for the databases that miss it.

Given this macro, the database definitions look like:

defdb(eng-swa)
defdb(swa-eng)
defdb(afr-eng, Afrikaans-English Dictionary)
defdb(eng-afr, English-Afrikaans Dictionary)