CPP with multi-line string literals

When you enable CPP language extension and compile a file that contains multi-line string literals, you’ll find that it fails to compile.

{-# LANGUAGE CPP #-}

test = "abc \
          \def"

This is because cpp expands backslashes at the end of lines. To workaround this problem, you can use cpphs instead.

{-# LANGUAGE CPP #-}
{-# OPTIONS_GHC -pgmP cpphs -optP --cpp #-}

test = "abc \
          \def"

You can specify an alternative preprocessor with -pgmP and options to the preprocessor with -optP. You need to use --cpp to tell cpphs to process command line options for cpp.