[Haskell-cafe] IO (Either a Error) question

[Brandon S. Allbery KF8NH]

On May 8, 2010, at 01:16 , Ivan Lazar Miljenovic wrote:
> David Menendez <dave at zednenem.com> writes:
>> On Sat, May 8, 2010 at 12:15 AM, Ivan Lazar Miljenovic
>>> Well, any time you have a do-block like this you're using failable
>>> patterns:
>>>
>>> maybeAdd       :: Maybe Int -> Maybe Int -> Maybe Int
>>> maybeAdd mx my = do x <- mx
>>>                   y <- my
>>>                   return $ x + y
>>
>> This is true in the sense that the translation for the do syntax in
>> the Haskell report uses fail.
>
> Huh?  What about "maybeAdd (Just 2) Nothing" ?

Isn't that handled by the definition of (>>=) in Maybe, as opposed to  
by invoking fail?

 > instance Monad Maybe where
 >   -- ...
 >   Nothing >>= _ = Nothing
 >   (Just x) >>= f = f x

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH


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