So haddock ignores {-# LANGUAGE CPP #-}, which makes it crash on any file that uses it. But if you pass --optghc=-cpp, it runs CPP on everything, which makes it crash on any file that uses string gaps, or happens to contain a /*. /* is rare and easily fixed, but not string gaps. It looks like a workaround would be to manually inspect the files for LANGUAGE CPP and run two haddock passes, but then I would have to get the two passes to cooperate creating a single TOC and index. Isn't there some way to run haddock on files that use CPP? In the broader scheme, it seems perverse to be using CPP in the first place. I use it to configure imports and exports, e.g. to swap out a driver backend on different OSes, and to export more symbols when testing. Would it make sense to have a haskell version of CPP that provides only these features (e.g. just #ifdef, #else, #endif, and #define) and leaves out the problematic C comments and backslash expectations?