On Tuesday 19 September 2006 09:40, Deokhwan Kim wrote:

Albert Lai wrote:

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Deokhwan Kim writes:

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Where is the Monad instance declaration of Either e?

It is in Control.Monad.Error as well. Strange: the doc doesn't state it.

Thanks a lot, Albert! I found the declaration in libraries/mtl/Control/Monad/Error.hs of the ghc source distribution:

instance (Error e) => Monad (Either e) where return = Right Left l >>= _ = Left l Right r >>= k = k r fail msg = Left (strMsg msg)

I think I have a problem that can be solved by using Either as a Monad. In my program I have the following function `update` which I would like to write shorter: type BoundVarsMap = [Binding] type Binding = (Name, Maybe Exp) type Name = String data Exp = Var Name | App Exp Exp | Abs Name Exp | ... -- update n e bvsMap returns Just the bvsMap where the first occurence of -- (n, _) has been updated to (n, Just e). Nothing is returned if n was not -- found in the bvsMap update :: Name -> Exp -> BoundVarsMap -> Maybe BoundVarsMap update _ _ [] = Nothing update n e ((bn, me):bvsMap) | n == bn = Just ((bn, Just e):bvsMap) | otherwise = update n e bvsMap >>= return . ((bn, me):) Example: update "b" (Var "z") [("a", Nothing), ("b", Nothing), ("c", Nothing)] => Just [("a",Nothing),("b",Just (Var "z")),("c",Nothing)] Example: update "x" (Var "z") [("a", Nothing), ("b", Nothing), ("c", Nothing)] => Nothing My question is: can I write `update` shorter by abstracting over the recursion on the list? I thought I could do it by somehow folding some function over the bvsMap which encodes the n == bn case as Left ... and the otherwise case as Right ... Then somehow using sequence to combine the resulting list. But I can't get it to work. It's late now here in Holland (~02:00) and I'm losing my concentration ;-) So I will just ask if somebody knows a shorter version. Thanks, Bas van Dijk