[GHC] #13953: forall

[GHC]

#13953: forall
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           Reporter:  zaoqi          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:  forall         |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 --Copyright (C) 2017  Zaoqi

 --This program is free software: you can redistribute it and/or modify
 --it under the terms of the GNU Affero General Public License as published
 --by the Free Software Foundation, either version 3 of the License, or
 --(at your option) any later version.

 --This program is distributed in the hope that it will be useful,
 --but WITHOUT ANY WARRANTY; without even the implied warranty of
 --MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 --GNU Affero General Public License for more details.

 --You should have received a copy of the GNU Affero General Public License
 --along with this program.  If not, see <http://www.gnu.org/licenses/>.
 {-# LANGUAGE DataKinds, TypeOperators, KindSignatures, GADTs,
 MultiParamTypeClasses, FlexibleInstances, FlexibleContexts,
 AllowAmbiguousTypes,
 UndecidableInstances, IncoherentInstances, NoMonomorphismRestriction,
 ScopedTypeVariables #-}

 module Data.U where

 data U :: [*] -> * where
     UOne :: x -> U (x : xs)
     USucc :: U xs -> U (x : xs)

 class Usuccs a b where
     usuccs :: U a -> U b
 instance Usuccs a a where
     usuccs = id
 instance Usuccs xs ys => Usuccs (x : xs) (x : ys) where
     usuccs (UOne x) = UOne x
     usuccs (USucc xs) = USucc (usuccs xs)
 instance Usuccs xs (x : xs) where
     usuccs = USucc
 instance Usuccs xs ys => Usuccs xs (y : ys) where
     usuccs x = USucc (usuccs x)

 u :: forall x xs. Usuccs '[x] xs => x -> U xs
 u x = usuccs (UOne x :: U '[x])

 class T a b where
     t :: U a -> U b
 instance Usuccs '[x] t => T '[x] t where
     t (UOne x) = u x
 instance (T xs t, Usuccs '[x] t) => T (x ': xs) t where
     t (UOne x) = u x
     t (USucc xs) = t xs

 instance Eq x => Eq (U '[x]) where
     UOne x == UOne y = x == y
 instance (Eq x, Eq (U xs)) => Eq (U (x : xs)) where
     UOne x == UOne y = x == y
     USucc xs == USucc ys = xs == ys
     _ == _ = False

 instance Show x => Show (U '[x]) where
     show (UOne x) = "(u " ++ showsPrec 11 x ")"
 instance (Show x, Show (U xs)) => Show (U (x : xs)) where
     show (UOne x) = "(u " ++ showsPrec 11 x ")"
     show (USucc xs) = show xs
 }}}
 {{{
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Prelude> :load U.hs
 [1 of 1] Compiling Data.U           ( U.hs, interpreted )
 Ok, modules loaded: Data.U.
 *Data.U> :t u
 u :: Usuccs '[x] xs => x -> U xs
 }}}

 ------------------------------------------------------

 {{{#!hs
 --Copyright (C) 2017  Zaoqi

 --This program is free software: you can redistribute it and/or modify
 --it under the terms of the GNU Affero General Public License as published
 --by the Free Software Foundation, either version 3 of the License, or
 --(at your option) any later version.

 --This program is distributed in the hope that it will be useful,
 --but WITHOUT ANY WARRANTY; without even the implied warranty of
 --MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 --GNU Affero General Public License for more details.

 --You should have received a copy of the GNU Affero General Public License
 --along with this program.  If not, see <http://www.gnu.org/licenses/>.
 {-# LANGUAGE DataKinds, TypeOperators, KindSignatures, GADTs,
 MultiParamTypeClasses, FlexibleInstances, FlexibleContexts,
 AllowAmbiguousTypes,
 UndecidableInstances, IncoherentInstances, NoMonomorphismRestriction,
 ScopedTypeVariables #-}

 module Data.U where

 data U :: [*] -> * where
     UOne :: x -> U (x : xs)
     USucc :: U xs -> U (x : xs)

 class Usuccs a b where
     usuccs :: U a -> U b
 instance Usuccs a a where
     usuccs = id
 instance Usuccs xs ys => Usuccs (x : xs) (x : ys) where
     usuccs (UOne x) = UOne x
     usuccs (USucc xs) = USucc (usuccs xs)
 instance Usuccs xs (x : xs) where
     usuccs = USucc
 instance Usuccs xs ys => Usuccs xs (y : ys) where
     usuccs x = USucc (usuccs x)

 u :: Usuccs '[x] xs => x -> U xs
 u x = usuccs (UOne x :: U '[x])

 class T a b where
     t :: U a -> U b
 instance Usuccs '[x] t => T '[x] t where
     t (UOne x) = u x
 instance (T xs t, Usuccs '[x] t) => T (x ': xs) t where
     t (UOne x) = u x
     t (USucc xs) = t xs

 instance Eq x => Eq (U '[x]) where
     UOne x == UOne y = x == y
 instance (Eq x, Eq (U xs)) => Eq (U (x : xs)) where
     UOne x == UOne y = x == y
     USucc xs == USucc ys = xs == ys
     _ == _ = False

 instance Show x => Show (U '[x]) where
     show (UOne x) = "(u " ++ showsPrec 11 x ")"
 instance (Show x, Show (U xs)) => Show (U (x : xs)) where
     show (UOne x) = "(u " ++ showsPrec 11 x ")"
     show (USucc xs) = show xs
 }}}
 {{{
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Prelude> :load U.hs
 [1 of 1] Compiling Data.U           ( U.hs, interpreted )

 U.hs:39:7: error:
     • Could not deduce (Usuccs '[x0] xs) arising from a use of ‘usuccs’
       from the context: Usuccs '[x] xs
         bound by the type signature for:
                    u :: Usuccs '[x] xs => x -> U xs
         at U.hs:38:1-32
       The type variable ‘x0’ is ambiguous
       Relevant bindings include u :: x -> U xs (bound at U.hs:39:1)
     • In the expression: usuccs (UOne x :: U '[x])
       In an equation for ‘u’: u x = usuccs (UOne x :: U '[x])

 U.hs:39:15: error:
     • Couldn't match type ‘x’ with ‘x1’
       ‘x’ is a rigid type variable bound by
         the type signature for:
           u :: forall x (xs :: [*]). Usuccs '[x] xs => x -> U xs
         at U.hs:38:6
       ‘x1’ is a rigid type variable bound by
         an expression type signature:
           forall x1. U '[x1]
         at U.hs:39:25
       Expected type: U '[x1]
         Actual type: U '[x]
     • In the first argument of ‘usuccs’, namely ‘(UOne x :: U '[x])’
       In the expression: usuccs (UOne x :: U '[x])
       In an equation for ‘u’: u x = usuccs (UOne x :: U '[x])
     • Relevant bindings include
         x :: x (bound at U.hs:39:3)
         u :: x -> U xs (bound at U.hs:39:1)
 Failed, modules loaded: none.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13953>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler