[Haskell-cafe] Why does the transformers/mtl 'Error' class exist?

John Millikin jmillikin at gmail.com
Thu Apr 15 22:18:53 EDT 2010

Both the transformers[1] and mtl[2] define a class named 'Error', for
use with MonadError and ErrorT. This class is required for the
instance of Monad (and therefore MonadTrans, MonadIO, etc). However, I
can't figure out why this class exists. Its presence means that
instead of something like:

data NameError = ErrorFoo | ErrorBar
validateName :: Monad m => Text -> m (Either Error Text)
validateName x = runErrorT $ do
    when (some condition) $ throwError ErrorFoo
    when (other condition) $ throwError ErrorBar
    return x

I have to define this, which is more verbose, no more useful, and adds
a "fake" class to the Haddock docs (with a warning not to use it):
data Error = ErrorFoo | ErrorBar

instance Error NameError where
    strMsg = error

-- validateName ...

Is there any good reason why the 'Error' class can't just be removed?

[1] http://hackage.haskell.org/packages/archive/transformers/
[2] http://hackage.haskell.org/packages/archive/mtl/

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