使用类型约束时类型类推导错误

Question

I have this function:

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

When compiling, I got a typeclass deduction error:

• 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 }

If I change the throwError to throw an Int instead (and add a Num constraint) then the compilation is success:

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

Can anyone explain me about this?

Update #1

I create a new type for the errors:

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

But the compilation is still not success:

• 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"使得e等于String ，所以你需要MonadError String m 。

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