[GHC] #11438: Code does not compile without ScopedTypeVariables

15 Jan 2016

      #11438: Code does not compile without ScopedTypeVariables
-------------------------------------+-------------------------------------
           Reporter:  wereHamster    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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:
-------------------------------------+-------------------------------------
 Feel free to change the summary, I have no idea how to summarize this
 issue.

 The code below fails to compile with `Couldn't match type A with B` and
 `The function X is applied to one argument, but its type Y has none` and
 some more (full output is attached below the code). However, simply
 enabling a language feature (`ScopedTypeVariable`) makes the code compile.
 If this is not a bug in the compiler then the message should be improved,
 because nothing in it points to the solution.

 I always thought of ScopedTypeVariables as allowing me to sprinkle `:: T`
 throughout the code, in more places than GHC would normally allow. I did
 not expect that merely enabling this language feature without any other
 changes in the source code would have any effect on the output.

 Dependencies: servant, servant-server

 {{{#!hs
 {-# LANGUAGE DataKinds         #-}
 {-# LANGUAGE TypeFamilies      #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE TypeOperators     #-}

 -- {-# LANGUAGE ScopedTypeVariables  #-}

 module Main where

 import Data.Proxy
 import Servant.API
 import Servant.Server

 data Credentials = Credential !String

 instance (HasServer sublayout) => HasServer (Credentials :> sublayout)
 where

     type ServerT (Credentials :> sublayout) m =
         Credentials -> ServerT sublayout m

     route Proxy subserver request respond = do
         let mbSessionIdString = lookup "cookie" [("cookie", "session id")]
             mbCredentials     = fmap Credential mbSessionIdString

         case mbCredentials of
             Nothing -> error "No credentials supplied"
             Just cred -> route (Proxy :: Proxy sublayout) (subserver cred)
 request respond
 }}}

 {{{
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:30:60:
     Couldn't match type ‘ServerT
                            sublayout
 (either-4.4.1:Control.Monad.Trans.Either.EitherT ServantErr IO)’
                    with ‘ServerT
                            layout0
 (either-4.4.1:Control.Monad.Trans.Either.EitherT ServantErr IO)’
     NB: ‘ServerT’ is a type function, and may not be injective
     The type variable ‘layout0’ is ambiguous
     Expected type: Credentials -> Server layout0
       Actual type: ServerT
                      (Credentials :> sublayout)
                      (either-4.4.1:Control.Monad.Trans.Either.EitherT
 ServantErr IO)
     Relevant bindings include
       subserver :: Server (Credentials :> sublayout)
         (bound at Bug.hs:24:17)
       route :: Proxy (Credentials :> sublayout)
                -> Server (Credentials :> sublayout)
                -> Servant.Server.Internal.RoutingApplication
         (bound at Bug.hs:24:5)
     The function ‘subserver’ is applied to one argument,
     but its type ‘Server (Credentials :> sublayout)’ has none
     In the second argument of ‘route’, namely ‘(subserver cred)’
     In the expression:
       route (Proxy :: Proxy sublayout) (subserver cred) request respond
 Failed, modules loaded: none.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11438
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler