If you want to hack on Cabal, don't be intimidated!
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Read the guide to the source code.
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Subscribe to the mailing list.
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Browse the list of open issues.
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There are other resources listed on the development wiki.
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See Cabal/tests/README.md for information about writing package tests.
Of particular value are the open issues list and the cabal-devel mailing list, which is a good place to ask questions.
Cabal's policy is to support being built by versions of GHC that are up to 3 years old.