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

[GHC]

#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
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by bgamari:

Old description:

> 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.

New description:

 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,
 {{{#!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.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11459#comment:1>
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