类型类扣除错误

Question

我有这样的功能：

sTest :: (MonadState s m, MonadError e m) => m() 
sTest = do 
    s <- get 
    throwError "abc" 
    put s 

编译时，我得到一个类型类扣除错误：如果我改变throwError抛出Int代替

• Could not deduce (MonadError [Char] m) 
    arising from a use of ‘throwError’ 
    from the context: (MonadState s m, MonadError e m) 
    bound by the type signature for: 
       sTest :: (MonadState s m, MonadError e m) => m() 
    at ... 
• In a stmt of a 'do' block: throwError "abc" 
    In the expression: 
    do { s <- get; 
     throwError "abc"; 
     put s } 
    In an equation for ‘sTest’: 
     sTest 
     = do { s <- get; 
       throwError "abc"; 
       put s } 

（并添加约束条件），则编译成功：

sTest :: (Num e, MonadState s m, MonadError e m) => m() 
sTest = do 
    s <- get 
    throwError 123 
    put s 

任何人都可以解释我吗？

更新＃1

我创建了错误一种新的类型：

data EvalError = VarNotFound 
    | PlusParamsMustBeIntVal 
    | FirstAppParamMustBeFunVal 
    | OtherError String 
    deriving (Show) 

instance Error EvalError where 
    strMsg = OtherError 

sTest :: (Error e, MonadState s m, MonadError e m) => m() 
sTest = do 
    s <- get 
    throwError $ OtherError "abc" 
    put s 

但是编译依然没有成功：

• Could not deduce (MonadError EvalError m) 
    arising from a use of ‘throwError’ 
    from the context: (Error e, MonadState s m, MonadError e m) 
    bound by the type signature for: 
       sTest :: (Error e, MonadState s m, MonadError e m) => m() 
    at ... 
• In a stmt of a 'do' block: throwError $ OtherError "abc" 
    In the expression: 
    do { s <- get; 
     throwError $ OtherError "abc"; 
     put s } 
    In an equation for ‘sTest’: 
     sTest 
     = do { s <- get; 
       throwError $ OtherError "abc"; 
       put s } 

Answer 1

throwError :: MonadError e m => e -> m a 

这里throwError "abc"品牌e等于String，所以你需要MonadError String m。

sTest :: (MonadState s m, MonadError String m) => m()