Expression evaluation mode in haskell for scripting

Regarding modules: ghc -e uses your ~/.ghci file, so in this case, you'd add :m +Data.List to it (import Data.List(nub) is also supported since GHC 7 or so).

Regarding packages: You can use ghc-pkg hide somepackage and ghc-pkg expose somepackage to define the default set of visible packages (packages are exposed by default though; maybe I misunderstand your question).

You might find eddie useful.

In addition to the standard configuration per the .ghci file (apart from your global ~/.ghci, you can have specialised .ghci files in some directories, which will then be read before that), you can also pass a Haskell source file as command line argument, ghc -e "expression" path/to/Source.hs. expression will then be evaluated in the context of module Source.

Like FunctorSalad said, ghc -e loads the ~/.ghci so you can use it to add any boilerplate that you might use in your script. In your case you could add

let script f = getContents >>= return fimport Data.List

to you ~/.ghci file and then run:

git diff ... | ghc -e "script.nub.map(fst.break(=='/').head.words).lines"