#15845: TH eta-reduces away explicit foralls in data family instances -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: 8.8.1 Component: Template Haskell | Version: 8.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #9692, #14179 | Differential Rev(s): Phab:D5294 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott <ryan.gl.scott@…>): In [changeset:"63a817074a8d49798bfd46a6545906fff143e924/ghc" 63a81707/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="63a817074a8d49798bfd46a6545906fff143e924" Fix #15845 by defining etaExpandFamInstLHS and using it Summary: Both #9692 and #14179 were caused by GHC being careless about using eta-reduced data family instance axioms. Each of those tickets were fixed by manually whipping up some code to eta-expand the axioms. The same sort of issue has now caused #15845, so I figured it was high time to factor out the code that each of these fixes have in common. This patch introduces the `etaExpandFamInstLHS` function, which takes a family instance's type variables, LHS types, and RHS type, and returns type variables and LHS types that have been eta-expanded if necessary, in the case of a data family instance. (If it's a type family instance, `etaExpandFamInstLHS` just returns the supplied type variables and LHS types unchanged). Along the way, I noticed that many references to `Note [Eta reduction for data families]` (in `FamInstEnv`) had slightly bitrotted (they either referred to a somewhat different name, or claimed that the Note lived in a different module), so I took the liberty of cleaning those up. Test Plan: make test TEST="T9692 T15845" Reviewers: goldfire, bgamari Reviewed By: goldfire Subscribers: rwbarton, carter GHC Trac Issues: #15845 Differential Revision: https://phabricator.haskell.org/D5294 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15845#comment:2> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler