On Fri, May 20, 2011 at 02:31:54PM +0200, Kees Bleijenberg wrote:
I was playing with winghci and I tried:
mapM id [Just 1, Just 2, Just 3] result: Just [1,2,3]
I don't understand this answer.
From http://members.chello.nl/hjgtuyl/tourdemonad.html#mapM http://members.chello.nl/hjgtuyl/tourdemonad.html#mapM mapM mf xs takes a monadic function mf (having type Monad m => (a -> m b)) and applies it to each element in list xs; the result is a list inside a monad.
A few things I've found: mapM :: (a -> m b) -> [a] -> m [b] So in this case: a = Maybe Int (second arg in mapM id [Just1, Just 2, Just 3] and b = Int and m = Maybe. So id is :: Maybe Int -> Maybe Int
mapM id [Just 1, Nothing, Just 3] result: Nothing. My first guess for the result: Just [Just 1, Nothing, Just 3]
when I do: mapM id [1,2,3] I get an error (id has wrong type, which makes sense)
Can somebody explain what is going on here?
You have to types right: a = Maybe Int m = Maybe b = Int 'mapM' is implemented like this: mapM f as = sequence (map f as) So working through the examples: mapM id [Just 1, Just 2, Just 3] = sequence (map id [Just 1, ...]) = sequence [Just 1, ...] = Just [1, ...] mapM id [Just 1, Nothing, Just 3] = sequence (map id [Just 1, Nothing, ...]) = sequence [Just1, Nothing, ...] = Nothing /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus@therning.org jabber: magnus@therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay