[GHC] #13920: 自動選擇實例

[GHC]

#13920: 自動選擇實例
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           Reporter:  zaoqi          |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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:
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 https://github.com/zaoqi/U.hs/blob/master/Data/U.hs:{{{#!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 #-}
 module Data.U (
     U(),
     t,
     u
     ) where

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

 class T1 a b where
     t1 :: (U a) -> (U b)
 instance T1 a a where
     t1 = id
 instance T1 xs (x : xs) where
     t1 = USucc
 instance T1 (x : y : xs) (y : x : xs) where
     t1 (UOne x) = USucc (UOne x)
     t1 (USucc (UOne x)) = UOne x
     t1 (USucc (USucc xs)) = USucc (USucc xs)
 instance T1 xs ys => T1 (x : xs) (x : ys) where
     t1 (UOne x) = UOne x
     t1 (USucc xs) = USucc (t1 xs)
 t = t1 . t1 . t1 . t1 . t1 . t1 . t1 . t1

 uone :: a -> U '[a]
 uone = UOne
 u x = t (uone x)

 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
 }}}

 {{{#!hs
 *Data.U> (u 'c') :: U [Int,Char]

 <interactive>:12:2: error:
     • No instance for (T1 a30 '[Int, Char]) arising from a use of ‘u’
     • In the expression: (u 'c') :: U '[Int, Char]
       In an equation for ‘it’: it = (u 'c') :: U '[Int, Char]
 *Data.U> t1 (uone 'c') :: U [Int,Char]
 (u 'c')
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13920>
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