| Understood. Thanks. A little further on in the tutorial they define a "parent" function. The mplus operator is used to combine monadic values from
separate computations into a single monadic value. Within the context
of our sheep-cloning example, we could use Maybe's
mplus to define a function,
parent s = (mother s) `mplus` (father s),
which would return a parent if there is one, and Nothing is the sheep has no parents at all.
For a sheep with both parents, the function would return one or the
other, depending on the exact definition of mplus in the
Maybe monad.But I get this when I try to use it: sheep.hs:30:22: Not in scope: `mplus' [michael@localhost ~]$ And here's the sheep.hs file, attempting to use "parent" near the bottom ========= {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern@nomaware.com> Time-stamp: <Mon Nov 10 11:59:14 2003> License: GPL -} {- DESCRIPTION Example 1 - Our first monad Usage: Compile the code and execute the resulting program. It will print Dolly's maternal grandfather. -} -- everything you need to know about sheep data Sheep = Sheep {name::String, mother::Maybe Sheep, father::Maybe Sheep} -- we show sheep by name instance Show Sheep where show s = show (name s) -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x parent s = (mother s) `mplus` (father s) -- now we can use `comb` to build complicated sequences maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = (Just s) `comb` mother `comb` father fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandmother s = (Just s) `comb` father `comb` mother `comb` mother mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father -- this builds our sheep family tree breedSheep :: Sheep breedSheep = let adam = Sheep "Adam" Nothing Nothing eve = Sheep "Eve" Nothing Nothing uranus = Sheep "Uranus" Nothing Nothing gaea = Sheep "Gaea" Nothing Nothing kronos = Sheep "Kronos" (Just gaea) (Just uranus) holly = Sheep "Holly" (Just eve) (Just adam) roger = Sheep "Roger" (Just eve) (Just kronos) molly = Sheep "Molly" (Just holly) (Just roger) in Sheep "Dolly" (Just molly) Nothing -- print Dolly's maternal grandfather main :: IO () main = let dolly = breedSheep in do print (parent dolly) -- END OF FILE Michael --- On Wed, 4/29/09, Anton van Straaten <anton@appsolutions.com> wrote:
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