Frontend plugin to print locally used module info
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ghc-usage is a simple frontend plugin which outputs the minimum export list for every module and signature necessary to compile the package. This is useful for taking a large signature and paring it down to precisely the identifiers you use.

For example, suppose you have:

signature A where
    f :: Int
    g :: Bool
module B where
    h = f

Clearly, only f is actually used. Thus, ghc-usage will output:

signature A (
) where

The benefit of having a reduced signature is that you allow more implementations (because you require fewer function implementations.)

This tool also outputs minimal export lists for modules, which might be useful if you're trying to make modules more abstract/find unnecessary exports. (Though, if this is a library, obviously there might be external clients using these libraries!)

How to use

ghc-usage is only compatible with GHC 8.2 and later.

Install using cabal install ghc-usage, and then run ghc-usage --interactive A B C. If you are working in a Cabal project, you can run cabal repl -w ghc-usage or cabal new-repl -w usage to get the correct command line arguments.

GHC 8.2 is probably not the default ghc executable in your path. In this case, you will need to also pass --with-ghc-pkg=/path/to/ghc-pkg-8.2; otherwise Cabal will complain about a version mismatch.