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#Maybe
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
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