On Fri, Jul 30, 2010 at 8:54 PM, michael rice
Thanks all,
Now that I have a (very) rudimentary understanding of Haskell, I figured I'd back up and have a closer (conceptual) look at type definitions to see what they have in common, and just happen to pick Maybe and List.
I also noticed Maybe has a list of "Instances"
Monad Maybe Functor Maybe Typeable1 Maybe MonadFix Maybe MonadPlus Maybe etc.
while List has none, at least I don't see any in Data.List. Same reason?
From "Learn You A Haskell:"
"If a type is a part of a typeclass, that means it supports and implements the behavior the typeclass describes."
I'm way out on a limb here, but isn't Monad a typeclass? and if, as we say above, that Maybe is an instance of Monad, wouldn't there have to be
instance Monad Maybe where return = ... -- return for Maybe >>= = ... -- bind for Maybe etc.
somewhere? Where? It's not in Data.Maybe. Is there some kind of scheme for defining this stuff, i.e., this goes here, that goes there?
It *is* in Data.Maybe: http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/src/Data-M... Generally speaking a type class instance should go either where the type is defined, or where the class is defined, although there are exceptions. In any case, if you can get the instance loaded in ghci, you can find out where its defined: ghci> :i Maybe data Maybe a = Nothing | Just a -- Defined in Data.Maybe instance (Eq a) => Eq (Maybe a) -- Defined in Data.Maybe instance Monad Maybe -- Defined in Data.Maybe instance Functor Maybe -- Defined in Data.Maybe instance (Ord a) => Ord (Maybe a) -- Defined in Data.Maybe instance (Read a) => Read (Maybe a) -- Defined in GHC.Read instance (Show a) => Show (Maybe a) -- Defined in GHC.Show