Excerpts from Jose Iborra's message of Sun Nov 29 10:41:50 -0500 2009:
There is indeed an Monad instance for Either in mtl, declared in the module Control.Monad.Error. I can't explain why your compiler cannot find it. Can you paste a blurb of code somewhere?
{-# LANGUAGE PackageImports, FlexibleContexts #-} import "mtl" Control.Monad.Error import Control.Monad.Failure import Control.Monad.Failure.MTL data MyError = MyError String instance Error MyError where strMsg = MyError failureFunction :: MonadFailure MyError m => Integer -> m Integer failureFunction 0 = failure $ MyError "Cannot use zero" failureFunction n = return (n - 1) -- instantiate eitherFunction :: Either MyError Integer eitherFunction = failureFunction 23 Which results in: either.hs:17:17: No instance for (MonadFailure MyError (Either MyError)) arising from a use of `failureFunction' at either.hs:17:17-34 Possible fix: add an instance declaration for (MonadFailure MyError (Either MyError)) In the expression: failureFunction 23 In the definition of `eitherFunction': eitherFunction = failureFunction 23
You need to import Control.Monad.Failure.MTL in order to bring the MTL instances into scope. The reason for this is that we provide instances both for MTL and transformers in the same package. These have to live in different modules to avoid a conflict due to the duplicated monad instance for Either.
Great, that fixed it! Where is this documented, or is this one of those conventions that I'm supposed to know about? ;-)
Very likely. Existing error handling packages such as control-monad-exception and attempt already provide this feature to convert other error forms into their specific error types. If this can be abstracted cleanly for a generic form of failure, then I would definitely support including it in control-monad-failure.
I was thinking about this, and I think the answer is basically yes, especially if we assume that we're dealing with the monads Error e => Either e or Either String which cover a big swath (most specifically Parsec, which I care about). This is very much a "encylopedia" style problem. I don't know if Haskell is powerful enough to get us the ability to have such conversions be transparent though. Cheers, Edward