Re: [Haskell-cafe] ErrorT vs Either

Probably because in the instance of Monad Either, fail has not been overloaded, and still has its default implementation: fail = error Whereas runErrorT explicitely catches the exception. 2011/5/16 Gracjan Polak <gracjanpolak@gmail.com>

Hi all,

A why question: Why:

Control.Monad.Error Prelude> runErrorT (fail "msg") :: IO (Either String Int) Left "msg"

but

Control.Monad.Error Prelude> (fail "msg") :: (Either String Int) *** Exception: msg

?

-- Gracjan

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