[GHC] #11459: Rather terrible error message due to excessive kind polymorphism

19 Jan 2016

      #11459: Rather terrible error message due to excessive kind polymorphism
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When fixing up `cassava` for GHC 8.0 I found I needed to enable
 `PolyKinds` due to an unrelated change encountered a rather vexing error.

 Consider this,
 {{{#!hs
 {-# LANGUAGE DataKinds, PolyKinds, KindSignatures, RankNTypes #-}

 module Hi where

 -- | Failure continuation.
 type Failure f r   = String -> f r
 -- | Success continuation.
 type Success a f r = a -> f r

 newtype Parser a = Parser {
       unParser :: forall f r.
                   Failure f r
                -> Success a f r
                -> f r
     }

 runParser :: Parser a -> Either String a
 runParser p = unParser p left right
   where
     left !errMsg = Left errMsg
     right !x = Right x
 }}}

 With GHC 7.10 this failed with the quite comprehensible,
 {{{
 Hi.hs:21:20:
     A newtype constructor cannot have existential type variables
     Parser :: forall a (k :: BOX).
               (forall (f :: k -> *) (r :: k).
                Failure f r -> Success a f r -> f r)
               -> Parser a
     In the definition of data constructor ‘Parser’
     In the newtype declaration for ‘Parser’
 }}}

 However, with 8.0 the compiler curtly informs you that,
 {{{
 Hi.hs:29:26: error:
     • Couldn't match kind ‘GHC.Prim.Any’ with ‘*’
       When matching the kind of ‘Either String’
     • In the second argument of ‘unParser’, namely ‘left’
       In the expression: unParser p left right
       In an equation for ‘runParser’:
           runParser p
             = unParser p left right
             where
                 left !errMsg = Left errMsg
                 right !x = Right x
 }}}

 As expected, adding a kind signature to `Parser`'s type variables fixed
 the issue but the error doesn't help the user realize this nearly as much
 as it could.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11459
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