On 12/19/05, Andrew Pimlott
[It is best to post questions only to haskell-cafe.]
Roger.
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.
snip
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. snip 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. ;-)
No problem, with a little shove in the right direction :). Not entirely sure why I didn't arrive at it myself. I'm using StateT successfully elsewhere and was overeager in using similar methods which, as you point out below, is a different situation. I still find monadic signatures a little confusing, I believe ghci was, at some point, deriving types for f or g that had MonadError in place of Either String, so I'll have to think carefully about why it is so different from the ErrorT type you show below.
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.
At one point I was playing with types like 'Either String (IO String)', which to me seems very similar to the ErrorT type. Intuitively they don't seem like they'd be correct... I feel like determining that there is an error involves interaction with the outside world, consequently the error part should be inside the IO monad. Now that you've cleared up the simpler case for me I'll try out the ErrorT case. Thanks for the help. Karl