[GHC] #11339: Possible type-checker regression in GHC 8.0

2 Jan 2016

      #11339: Possible type-checker regression in GHC 8.0
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following code (extracted from hackage:microlens) type-checks on GHC
 7.10 but '''not''' on GHC 8.0/8.1:

 {{{#!hs
 {-# LANGUAGE RankNTypes, ScopedTypeVariables #-}

 module Failing where

 import Control.Applicative ( Const(Const, getConst) )
 import Data.Functor.Identity ( Identity(Identity) )

 type Traversal s t a b = forall f. Applicative f => (a -> f b) -> s -> f t

 failing :: forall s t a b . Traversal s t a b -> Traversal s t a b ->
 Traversal s t a b
 failing left right afb s = case pins t of
   [] -> right afb s
   _  -> t afb
   where
     -- t :: (a -> f b) -> f t

     -- TYPECHECKS with GHC 7.10, but not with GHC 8.x:
     Bazaar { getBazaar = t } = left sell s

     -- FAILS TO TYPECHECK with GHC 7.10 and GHC 8.x:
     -- t = getBazaar (left sell s)

     sell :: a -> Bazaar a b b
     sell w   = Bazaar ($ w)

     pins :: ((a -> Const [Identity a] b) -> Const [Identity a] t) ->
 [Identity a]
     pins f   = getConst (f (\ra -> Const [Identity ra]))

 newtype Bazaar a b t = Bazaar { getBazaar :: (forall f. Applicative f =>
 (a -> f b) -> f t) }

 instance Functor (Bazaar a b) where
   fmap f (Bazaar k) = Bazaar (fmap f . k)

 instance Applicative (Bazaar a b) where
   pure a = Bazaar $ \_ -> pure a
   Bazaar mf <*> Bazaar ma = Bazaar $ \afb -> mf afb <*> ma afb
 }}}

 The following error is emitted on GHC 8.0:

 {{{
 failing.hs:13:11: error:
     • Couldn't match type ‘f’ with ‘Const [Identity a]’
       ‘f’ is a rigid type variable bound by
         the type signature for:
           failing :: forall (f :: * -> *). Applicative f => Traversal s t
 a b -> Traversal s t a b -> (a -> f b) -> s -> f t
         at failing.hs:11:1
       Expected type: a -> Const [Identity a] b
         Actual type: a -> f b
     • In the first argument of ‘t’, namely ‘afb’
       In the expression: t afb
       In a case alternative: _ -> t afb
     • Relevant bindings include
         t :: (a -> Const [Identity a] b) -> Const [Identity a] t (bound at
 failing.hs:18:26)
         sell :: a -> Bazaar a b b (bound at failing.hs:24:5)
         pins :: ((a -> Const [Identity a] b) -> Const [Identity a] t) ->
 [Identity a] (bound at failing.hs:27:5)
         afb :: a -> f b (bound at failing.hs:11:20)
         right :: Traversal s t a b (bound at failing.hs:11:14)
         left :: Traversal s t a b (bound at failing.hs:11:9)
         failing :: Traversal s t a b -> Traversal s t a b -> Traversal s t
 a b (bound at failing.hs:11:1)
 }}}

 I don't understand why `Bazaar t = ...` vs `t = getBazaar ...` makes a
 difference in GHC 7.10 at all. So I'm not sure if this is a regression or
 actually something that got fixed in GHC 8.0...

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11339
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