忽略默認約束類型

Question

定義默認約束時，我遇到了一個奇怪的問題。如果約束是單位，則不選擇默認實例。在所有其他情況下，它按預期工作。

{-# LANGUAGE TypeFamilies, ConstraintKinds #-} 
import qualified GHC.Exts as E 

class Expression a where 
    type Constr a v :: E.Constraint 
    --type Constr a v =()   -- with this line compilation fails 
    --type Constr a v = v ~ v  -- compiles 
    wrap :: Constr a v => a -> Maybe v 

instance Expression() where 
    wrap() = Just undefined 

main = print (wrap() :: Maybe Int) 

有人可以澄清typechecker行爲的原因嗎？

Answer 1

這與7.4相關類型默認的錯誤0.1。幾周前，我被告知#haskell這是一個已知的bug，但我在GHC trac上找不到它。

Answer 2

不是一個真正的答案，但是這是不是ConstraintKinds

class Expression a where 
    type Type a v 
    type Type a v =() 
    wrap :: (Type a v) ~() => a -> Maybe v 

instance Expression() where 
    wrap() = Just undefined 

main = print (wrap() :: Maybe Int) 

不能編譯，但

class Expression a where 
    type Type a v 
    type Type a v = v 
    wrap :: (Type a v) ~ v => a -> Maybe v 

instance Expression() where 
    wrap() = Just undefined 

main = print (wrap() :: Maybe Int) 

確實