Re: [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 request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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@@ -57,1 +57,1 @@
- {{{#!hs
+ {{{

New description:

 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
 }}}

 {{{
 *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')
 }}}

--

Comment (by bgamari):

 I spent a few minutes looking at this: It seems like you want `(u 'c') ::
 U [Int,Char]` to behave like `t1 (uone 'c') :: U [Int,Char]`, but they are
 quite different.

 Can you provide some more description of the problem? What does GHC do and
 how does this differ from what you would want it to do?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13920#comment:2>
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