Re: [Haskell-cafe] chr/ord?

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    Time-stamp:    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 wrote: From: Anton van Straaten Subject: Re: [Haskell-cafe] chr/ord? To: "haskell-cafe@haskell.org" Date: Wednesday, April 29, 2009, 12:33 PM michael rice wrote:

Since I'm trying to learn Monads, let's look at this as a teaching moment. The example code (see below), which I pulled off YAMT (Yet Another Monad Tutorial ;-)), is the source of my 'comb' function.

I understand the code as it now stands, and I understand that the Prelude (>>=) would replace the 'comb'. Adding whatever statements are needed, how would you "specialize" the (>>=) to Maybe and solve this particular problem.

Saying that "comb is just (>>=) specialized to Maybe" just means that you can define comb like this:   comb :: Maybe a -> (a -> Maybe b) -> Maybe b   comb = (>>=) Which also of course means that you can typically use (>>=) instead of comb.  Although in some cases, being more specific about the type can be useful. You can do this sort of specialization for any polymorphic function, e.g.:   -- id is predefined in Haskell, definition given as example   id :: a -> a   id x = x   intID :: Int -> Int   intId = id In that case, the compiler basically specializes the function for you, providing a version of it that's specific to Ints. However, (>>=) is defined by the Monad type class, and as it happens there's also already a definition for it that's specific to the Maybe type.  You can see GHC's source for it here: http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-Maybe.html#M... Not surprisingly, that definition is essentially identical to the definition of comb:   (Just x) >>= k      = k x   Nothing  >>= _      = Nothing So defining "comb = (>>=)" just uses that definition. Anton _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe