That is a result of the implementation of the specific Monad instance, and
that does depend on the type, as you say (but it isn't determined for
sequence(_) specifically).
Nothing >>= f = Nothing
Just x >>= f = f x
is why a Nothing "pollutes" the sequenced lists of Maybes. If Maybe is a
Monad representing computations that can fail (to produce a result), then if
you sequence a bunch of such computations together, if any one computation
fails, your entire computation fails. This reflects the natural behavior of
the Maybe monad, where if you use "x <- maybe computation", the only way to
produce that x is if the computation returned Just.
In other monads, sequence will behave in the "right way" for that monad.
On Sat, Oct 30, 2010 at 1:30 AM, Mark Spezzano wrote: 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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