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, > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
