Yes, however, ap is not equal to (<*>) for the given Applicative. I don't believe such a monad is possible. Neil Brown wrote:
Martijn van Steenbergen wrote:
Hello,
Mr. McBride and mr. Paterson define in their Applicative paper:
data Except e a = OK a | Failed e instance Monoid e => Applicative (Except e) where ...
Sometimes I'd still like to use >>= on Excepts but this "feels" wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Isn't the Except type just Either by another name? OK = Right, Failed = Left. Therefore the monad is just the same as the Either monad, and is useful as an error monad:
instance Monad (Except e) where (OK x) >>= f = f x Failed e >>= _ = Failed e return = OK
This obeys all the monad laws.
Thanks,
Neil. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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