[Haskell-cafe] chr/ord?

[Anton van Straaten]

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