On 30 October 2010 16:30, Mark Spezzano
Not exactly. If you use the type with Maybe Int like so:
sequence [Just 1, Nothing, Just 2]
then the result is Nothing.
Whereas sequence [Just 1, Just 2, Just 3] gives
Just [1, 2, 3]
Why?
I assume there's special implementations of sequence and sequence_ depending on the type of monad used. If it's a sequence_ [putStrLn "hello", putStrLn "goodbye"] then this prints out hello and goodbye on separate lines.
It seems to work differently for different types.
The definition of the monad. In the Maybe monad, as soon as you get a Nothing the entire thing returns Nothing. sequence [ma,mb,mc] = do { a <- ma; b <- mb; c <- mc; return [a,b,c] } = ma >>= \ a -> mb >>= \ b -> mc >>= \ c -> return [a,b,c] However, for Maybe: instance Monad Maybe where ... Nothing >>= f = Nothing Just x >>= f = f x ... So yes, the behaviour of the Monad is dependent upon the Monad. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com