[It is best to post questions only to haskell-cafe.] On Mon, Dec 19, 2005 at 03:53:53PM +1300, Karl Grapone wrote:
I'm having trouble making use of MonadError within another Monad, in this case IO. I've blundered around for a while, trying various combinations of things, but I don't think I've fully cottoned-on to nesting of monads.
Looking at the signature of f,
f :: IO (Either String String)
"Either String String" is just an ordinary value produced in the IO monad--the monad structure of IO and "Either String" are completely independent. With that in mind, here is a way to make part of your code type-check: g n = do c <- getChar cs <- g (n-1) return $ do c' <- mapErrs (h c) cs' <- cs return (c':cs') The outer do is a computation in the IO monad, the inner do is a computation in the "Either String" monad, and the net effect is an IO computation returning an "Either String" computation--which is just what the type signature says. I had to change your code in 3 other places to make it type-check; hopefully you can now find them. ;-) When people speak of nesting monads, they often mean using monad transformers. If you were using the ErrorT monad transformers, your signature would look like f :: ErrorT String IO String You might want to try rewriting your code that way, but I would suggest making it work with the current type signatures first. Andrew