hpp is a Haskell pre-processor that is also a C90-compatible pre-processor (with the addition of a -c-compat flag). It is packaged as both a library and an executable.

To use as a Haskell preprocessor for resolving #ifdef conditionals and simple macro expansion while still allowing multi-line string literals, an invocation might look like,

hpp -DDEBUG Foo.hs

To use as a C preprocessor, an invocation might look like,

hpp -DDEBUG -c-compat foo.c