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

Ryan Ingram schrieb:

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.

I also prefered to not support the fail method in this way and wrote: http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/htm...