#15845: TH eta-reduces away explicit foralls in data family instances
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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: |
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Comment (by Ryan 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
}}}
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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15845#comment:2
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