ghc runs preprocessor too much

[guest]

When an external preprocessor has been specified with -F -pgmF, and then using ghc --make, ghc runs the preprocessor on every .hs file, even those for which compilation is skipped. This seem unnecessary and it slows down recompilation significantly when you have many files.

-- Lennart

[simonmar]

I don't see how to avoid this easily: we need to parse the imports of every source file in order to construct the dependency tree, and we can't parse the imports without preprocessing the source file first.

Perhaps keeping a cache file containing the imports for each source file with the last-modified date of the source?

[guest]

Yes, you're right. It's difficult to fix. It would take a cache to avoid, which is a pain.