爲什麼在GHC 7.10中進行類型檢查的代碼不再在GHC 8.0.1中進行類型檢查？

Question

我有以下代碼：

{-# LANGUAGE DefaultSignatures#-} 

import Control.Monad.Trans.Class 
import Control.Monad.Trans.Maybe 

class Monad m => MonadMaybe m where 
    liftMaybe :: Maybe a -> m a 
    default liftMaybe :: (MonadTrans t, MonadMaybe m) => Maybe a -> t m a 
    liftMaybe = lift . liftMaybe 

instance MonadMaybe m => MonadMaybe (MaybeT m) 

使用光榮格拉斯哥Haskell的編譯系統，版本8.0.1.20161117，這無法編譯：

foo.hs:11:10: error: 
    • Couldn't match type ‘m’ with ‘MaybeT m’ 
     ‘m’ is a rigid type variable bound by 
     the instance declaration at foo.hs:11:10 
     Expected type: Maybe a -> MaybeT m a 
     Actual type: Maybe a -> MaybeT (MaybeT m) a 
    • In the expression: Main.$dmliftMaybe @MaybeT m 
     In an equation for ‘liftMaybe’: 
      liftMaybe = Main.$dmliftMaybe @MaybeT m 
     In the instance declaration for ‘MonadMaybe (MaybeT m)’ 
    • Relevant bindings include 
     liftMaybe :: Maybe a -> MaybeT m a (bound at foo.hs:11:10) 

但是在GHC 7.10，這個編譯沒有問題。

GHC 8是否正確，或者我發現了一個錯誤？

Answer 1

儘管GHC 7.10接受它，但您的代碼實際上沒有意義。討論請參閱https://ghc.haskell.org/trac/ghc/ticket/12784。