Couldn't match type `IOException` with `RequestError`

[Kendrick Tan]

I'm just trying to make a very simple 'EchoBot' that echos whatever the input query is. E.g.
/echo hello. Bot replies with hello.

However, I'm having some trouble replying to the user, I'm getting this error on compile:

    • Couldn't match type ‘IOException’ with ‘RequestError’
        arising from a functional dependency between:
          constraint ‘Control.Monad.Error.Class.MonadError RequestError IO’
            arising from a use of ‘say’
          instance ‘Control.Monad.Error.Class.MonadError IOException IO’
            at <no location info>
    • In the first argument of ‘void’, namely ‘(say m config)’
      In a stmt of a 'do' block: void (say m config)
      In the expression:
        do { putStrLn ("+ Outgoing message: " <> show (encode m));
             void (say m config);
             return "" }

Code:

testTo :: Command -> IO Text
testTo cmd = do
    putStrLn ("+ Incoming command: " <> show cmd)
    config <- slackConfig
    message <- runExceptT (messageOfCommand cmd)
    case (debug, message) of
      (False, Right m) -> do
          putStrLn ("+ Outgoing message: " <> show (encode m))
          void (say m config) -- <<< Error occurs here
          return ""
      (False, Left e) ->
          return ("popslave encountered an error " <> (e ^. packed))
      (True, Right m) -> do
          putStrLn ("+ Outgoing message: " <> show (encode m))
          return ""
      (True, Left e) -> do
          putStrLn ("+ Outgoing ERROR: " <> e)
          return ""
    where
        debug = False

Any help would be appreciated.

[Kendrick Tan]

Solved it, credits to @TRManderson

testTo :: Command -> IO Text
testTo cmd = do
    putStrLn ("+ Incoming command: " <> show cmd)
    config <- slackConfig
    message <- runExceptT (messageOfCommand cmd)
    case (debug, message) of
      (False, Right m) -> do
          putStrLn ("+ Outgoing message: " <> show (encode m))
          eitherResult <- runExceptT $ say m config
          case eitherResult of
            (Left e)  -> return "error"
            (Right _) -> return ""
          return ""
      (False, Left e) ->
          return ("popslave encountered an error " <> (e ^. packed))
      (True, Right m) -> do
          putStrLn ("+ Outgoing message: " <> show (encode m))
          return ""
      (True, Left e) -> do
          putStrLn ("+ Outgoing ERROR: " <> e)
          return ""
    where debug = False