On Friday 30 July 2010 21:54:20, michael rice wrote:
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?
Data.List provides only functions for working with lists, while Data.Maybe also contains the definition of the type. Type class instances are documented - at the type class - at the data type Since the list datatype isn't documented like the other types (probably because it's not defined in valid Haskell but built-in), the list instances appear only at the type classes (and at the Monad documentation, you find "Monad []" listed as the first instance.
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?
Yes.
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?
Yes, there is. In GHC at least, it's defined in Data.Maybe: instance Monad Maybe where (Just x) >>= k = k x Nothing >>= _ = Nothing (Just _) >> k = k Nothing >> _ = Nothing return = Just fail _ = Nothing
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?
Instance declarations should be in the same module where - the class is defined, or - the type is defined if possible.