[GHC] #14091: When PolyKinds is on, suggested type signatures seem to require TypeInType

5 Aug 2017

      #14091: When PolyKinds is on, suggested type signatures seem to require TypeInType
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           Reporter:  jberryman      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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:
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 We compile the following with -Wall

 {{{
 {-# LANGUAGE PolyKinds #-}

 import Data.Bits
 import Data.Word

 data Hash128 a = Hash128 { hashWord128_0 :: !Word64, hashWord128_1 ::
 !Word64 }
     deriving (Show, Read, Eq)

 -- These instances copied from 'FixedPoint-simple':
 instance FiniteBits (Hash128 a) where
       finiteBitSize ~(Hash128 a b) = finiteBitSize a + finiteBitSize b

 instance Bits (Hash128 a) where
       popCount (Hash128 h l) = popCount h + popCount l
       bit i | i >= 64    = Hash128 (bit $ i - 64) 0
             | otherwise = Hash128 0 (bit i)
       complement = pointwise complement
       (.&.) = pointwise2 (.&.)
       (.|.) = pointwise2 (.|.)
       xor = pointwise2 xor
       setBit (Hash128 h l) i
               | i >= 64   = Hash128 (setBit h (i - 64)) l
               | otherwise = Hash128 h (setBit l i)
       shiftL (Hash128 h l) i
               | i > finiteBitSize l = shiftL (Hash128 l 0) (i -
 finiteBitSize l)
               | otherwise     = Hash128 ((h `shiftL` i) .|. (l `shiftR`
 (finiteBitSize l - i))) (l `shiftL` i)
       shiftR (Hash128 h l) i
               | i > finiteBitSize h = shiftR (Hash128 0 h) (i -
 finiteBitSize h)
               | otherwise     = Hash128 (h `shiftR` i) ((l `shiftR` i) .|.
 h `shiftL` (finiteBitSize h - i))
       isSigned _ = False
       testBit (Hash128 h l) i
               | i >= finiteBitSize l = testBit h (i - finiteBitSize l)
               | otherwise      = testBit l i
       rotateL w i = shiftL w i .|. shiftR w (128 - i)
       rotateR w i = shiftR w i .|. shiftL w (128 - i)
       bitSize _ = 128
       bitSizeMaybe _ = Just 128

 pointwise op (Hash128 a b) = Hash128 (op a) (op b)

 pointwise2 op (Hash128 a b) (Hash128 c d) = Hash128 (op a c) (op b d)
 }}}

 get a warning like:

 {{{
     Top-level binding with no type signature:
       pointwise2 :: forall k1 k2 k3 (a1 :: k2) (a2 :: k1) (a3 :: k3).
                     (Word64 -> Word64 -> Word64)
                     -> Hash128 a1 -> Hash128 a2 -> Hash128 a3
 }}}

 but that's not a valid signature. Pasting it in causes GHC to suggest
 RankNTypes with an error, and then to suggest TypeInType, at which point
 it compiles.

 I want to just be able to paste in the signature from the warning.

 Related to #6065

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14091
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