[Haskell-cafe] ErrorT vs Either
gracjanpolak at gmail.com
Mon May 16 23:41:44 CEST 2011
Thanks Daniel, Yves and Edward for explanation. Two things come to my mind now.
1. It should be unified. Why? Because conceptually:
runIdentity (runErrorT (fail "msg")) :: Either String Int
fail "msg" :: Either String Int
*** Exception: msg
Should be the same as Identity monad should not add structure.
2. I need a Failure monad that works well with pattern match failures (that call
fail). I'd like to use it like this:
runErrorT $ do
Active <- getStatus -- ensure proper status
Just elm <- lookup stuff there -- lookup element
when (condition) $ fail "wrong!" -- check condition
return 1234 -- return useful value
sort of... Any ideas what could be used in place of Either monad? Basically I
need working pattern match failures (I guess that means I need working fail
method that is not equal to error).
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