• Ryan Scott's avatar
    Fix #15845 by defining etaExpandFamInstLHS and using it · 63a81707
    Ryan Scott authored
    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
    63a81707
all.T 20.4 KB