【发布时间】：2016-11-28 17:16:18
【问题描述】：
我有以下代码：
{-# 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 Glorious Glasgow Haskell 编译系统，版本 8.0.1.20161117，编译失败：
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 中，编译没有问题。
GHC 8 是否正确，还是我发现了错误？
【问题讨论】：
-
出于好奇，如果将默认签名中的
m更改为另一个类型变量，如n，或者添加显式forall，是否有效？我想知道 GHC 是否出于某种原因在这里将m视为范围类型变量。 -
您的代码还在 GHC 8.0.1 的稳定版本中进行类型检查。
-
@AlexisKing，这基本上就是问题所在。类声明的头部（这里是
m）的出现总是在主体中，否则m和方法的类型之间没有关系！ -
@ReidBarton 没错，当然，这是有道理的——回想起来似乎很明显。 :)