[Haskell-cafe] Either e Monad

Bas van Dijk basvandijk at home.nl
Wed Sep 20 19:58:03 EDT 2006

On Tuesday 19 September 2006 09:40, Deokhwan Kim wrote:
> Albert Lai wrote:
>  > Deokhwan Kim <dk at ropas.snu.ac.kr> writes:
>  >> 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 

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 

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.


Bas van Dijk

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