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. Mark On 30/10/2010, at 3:42 PM, Bardur Arantsson wrote:
On 2010-10-30 07:07, Mark Spezzano wrote:
Hi,
Can somebody please explain exactly how the monad functions "sequence" and "sequence_" are meant to work?
I have almost every Haskell textbook, but there's surprisingly little information in them about the two functions.
From what I can gather, "sequence" and "sequence_" behave differently depending on the types of the Monads that they are processing. Is this correct? Some concrete examples would be really helpful.
sequence [m1,m2,m3,m4,...] = do x1 <- m1 x2 <- m2 x3 <- m3 x4 <- m4 ... return [x1,x2,x3,x4,...]
sequence_ [m1,m2,m3,m4,...] = do m1 m2 m3 m4 ... return ()
Cheers,
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