Hello folks,
I took advantage of Thanksgiving weekend to port my application to use
Control.Monad.Failure, and learned (slightly painfully) that I still
needed to pick some mechanism to instantiate my failure monads as.
After the experience, I have three questions/comments:
1. Why isn't there an instance for Either in mtl? (There is one for
Transformers. The error message left me very puzzled there: the docs
clearly claimed the instance existed, and only a little source code
diving elucidated the situation.) Copying the instance declaration
from the transformers version seems to fix it.
2. I was having difficulty instantiating MonadFailure as an ErrorT
for an arbitrary monad. Here is an example:
{-# LANGUAGE PackageImports, FlexibleContexts #-}
import "mtl" Control.Monad.Error
import "mtl" Control.Monad.State
import Control.Monad.Failure
data MyError = MyError String
instance Error MyError where
strMsg = MyError
type MyMonad = ErrorT MyError (State Integer)
failureFunction :: MonadFailure MyError m => Integer -> m Integer
failureFunction 0 = failure $ MyError "Cannot use zero"
failureFunction n = return (n - 1)
-- instantiate
monadicFunction :: MyMonad Integer
monadicFunction = failureFunction 23
Which results in the following error:
failure.hs:19:18:
No instance for (MonadFailure
MyError (ErrorT MyError (State Integer)))
arising from a use of `failureFunction' at failure.hs:19:18-35
Possible fix:
add an instance declaration for
(MonadFailure MyError (ErrorT MyError (State Integer)))
In the expression: failureFunction 23
In the definition of `monadicFunction':
monadicFunction = failureFunction 23
Which seems to contradict the documentation and source code, which states:
Instances: [...]
(Error e, Monad m) => MonadFailure e (ErrorT e m)
How do I misunderstand?
3. In a motivating example, one of the goals of MonadFailure is to let
us mix the error code of third-party modules into the generic failure mode.
Control.Monad.Failure appears to give the machinery for instantiating a generic
failure monad, but it doesn't have any facilities for the opposite direction:
that is, marshalling a specific error form into the generic error form. Am I
mistaken, and if not, would it be a welcome addition to the library?
Cheers,
Edward
