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



_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe