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Summary: GHCi's `:info` command was pretty-printined GADT constructors suboptimally in the following ways: 1. Sometimes, fields were parenthesized when they did not need it, e.g., ```lang=haskell data Foo a where MkFoo :: (Maybe a) -> Foo a ``` I fixed this by refactoring some code in `pprIfaceConDecl` to be a little smarter with respect to GADT syntax. See `pprFieldArgTy` and `pprArgTy`. 2. With `-fprint-explicit-kinds` enabled, there would be times when specified arguments would be printed without a leading `@` in GADT return types, e.g., ```lang=haskell data Bar @k (a :: k) where MkBar :: Bar k a ``` It turns out that `ppr_tc_app`, the function which pretty-prints these return types, was not using the proper machinery to print out the arguments, which caused the visibilities to be forgotten entirely. I refactored `ppr_tc_app` to do this correctly. Test Plan: make test TEST=T16030 Reviewers: goldfire, bgamari, simonpj Reviewed By: simonpj Subscribers: simonpj, rwbarton, carter GHC Trac Issues: #16030 Differential Revision: https://phabricator.haskell.org/D5440
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