[GHC] #15845: TH eta-reduces away explicit foralls in data family instances

2 Nov 2018

      #15845: TH eta-reduces away explicit foralls in data family instances
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Template       |           Version:  8.7
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #9692, #14179
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following code:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Language.Haskell.TH

 data family F1 a b
 data instance F1 [a] b = MkF1

 data family F2 a
 data instance F2 a = MkF2

 $(do i1 <- reify ''F1
      i2 <- reify ''F2
      runIO $ mapM_ (putStrLn . pprint) [i1, i2]
      pure [])
 }}}
 {{{
 $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.7.20181101: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 data family Bug.F1 (a_0 :: *) (b_1 :: *) :: *
 data instance forall (a_2 :: *). Bug.F1 ([a_2]) b_3 = Bug.MkF1
 data family Bug.F2 (a_0 :: *) :: *
 data instance Bug.F2 a_1 = Bug.MkF2
 }}}

 The output here is quite baffling:

 * In the `F1` instance, we have an explicit `forall` which quantifies
 `a_2` but not `b_3`!
 * In the `F2` instance, there isn't an explicit `forall` at all despite
 the fact that there ought to be one, since there is a type variable `a_1`
 in this instance.

 The culprit in both of the bullet points above is the fact that GHC eta-
 reduces its internal representation of data family instance axioms. This
 is the same thing which caused #9692 and #14179, in fact. Luckily, the
 same fix for those tickets will also work here.

 Patch incoming.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15845
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