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Rather terrible error message due to excessive kind polymorphism

    • Closed Issue created by Ben Gamari @bgamari

    When fixing up cassava for GHC 8.0 I found I needed to enable PolyKinds due to an unrelated change (namely in order to apply Proxy to something of kind GHC.Generics.Meta, which will be quite a common refactoring in 8.0) encountered a rather vexing error.

    Consider this,

    {-# 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

    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.

    Edited by Ben Gamari
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