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Ryan Scott authored
GHC previously had a handful of special cases for pretty-printing equalities in a more user-friendly manner, but they were far from comprehensive (see #15039 for an example of where this fell apart). This patch makes the pretty-printing of equalities much more systematic. I've adopted the approach laid out in https://ghc.haskell.org/trac/ghc/ticket/15039#comment:4, and updated `Note [Equality predicates in IfaceType]` accordingly. We are now more careful to respect the properties of the `-fprint-explicit-kinds` and `-fprint-equality-relations` flags, which led to some improvements in error message outputs. Along the way, I also tweaked the error-reporting machinery not to print out the type of a typed hole when the type is an unlifted equality, since it's kind (`TYPE ('TupleRep '[])`) was more confusing than anything. Test Plan: make test TEST="T15039a T15039b T15039c T15039d" Reviewers: simonpj, goldfire, bgamari Reviewed By: simonpj Subscribers: rwbarton, thomie, carter GHC Trac Issues: #15039 Differential Revision: https://phabricator.haskell.org/D4696
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