It's used in the implementation of "fail" for those monads.
class Monad m where
...
fail :: String -> m a
fail = error -- default implementation
which is then used to desugar do-notation when pattern matching fails:
do
Left x <- something
return x
=>
something >>= \v -> case v of { Left x -> return x ; _ -> fail
"Pattern match failure ..." }
You can argue about whether "fail" belongs in Monad (and many people
have), but that's why it is how it is.
-- ryan
On Thu, Apr 15, 2010 at 7:18 PM, John Millikin
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/0.2.0.0/doc/html/Co... [2] http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Mon... _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe