Re: [Haskell-cafe] Questions about the Functor class and it's use in "Data types à la carte"

Excellent! This has all been very helpful. Thanks a lot everybody! :-) -Corey On 12/14/07, Benja Fallenstein wrote:

Hi Corey,

On Dec 14, 2007 8:44 PM, Corey O'Connor wrote:

...

The reason I find all this odd is because I'm not sure how the type class Functor relates to the category theory concept of a functor. How does declaring a type constructor to be an instance of the Functor class relate to a functor? Is the type constructor considered a functor?

Recall the definition of functor. From Wikipedia:

"A functor F from C to D is a mapping that

* associates to each object X in C an object F(X) in D, * associates to each morphism f:X -> Y in C a morphism F(f):F(X) -> F(Y) in D

such that the following two properties hold:

* F(idX) = idF(X) for every object X in C * F(g . f) = F(g) . F(f) for all morphisms f:X -> Y and g:Y -> Z."

http://en.wikipedia.org/wiki/Functor

We consider C = D = the category of types. Note that any type constructor is a mapping from types to types -- thus it associates to each object (type) X an object (type) F(X).

Declaring a type constructor to be an instance of Functor means that you have to provide 'fmap :: (a -> b) -> (f a -> f b)" -- that is, a mapping that associates to each morphism (function) "fn :: a -> b" a morphism "fmap fn :: f a -> f b".

Making sure that the two laws are fulfilled is the responsibility of the programmer writing the instance of Functor. (I.e., you're not supposed to do this: instance Functor Val where fmap f (Val x) = Val (x+1).)

Hope this helps with seeing the correspondence? :-) - Benja

-- -Corey O'Connor