Simon Marlow wrote:
On 21 March 2005 01:46, Iavor Diatchki wrote:
On Mon, 21 Mar 2005 00:29:38 +0100, Thomas Jäger
It is already annyoing enough that `Funtor' isn't a subclass of `Monad' although every monad must also be functor.
Yes it is!
I think you are right. Does anyone remember why "Functor" is not a superclass of "Monad"?
I think it is because it doesn't need to be.
Superclasses are never really needed, are they? But they are useful because they make types smaller and more readable. For a given type T which has a Monad instance, you can always declare instance Functor T where fmap = liftM so it is not a big burden to have to declare a Functor instance to accompany a Monad instance. And the Haskell libraries already contain superclass relationships that make less sense than the Functor=>Monad relationship. For example, why is Show and Eq superclasses of Num? Presumably only because it makes types more readable. There are types for which you can declare sensible instances in the Num class, but not so sensible Show or Eq instances: instance Num b => Num (a->b) where (f+g) x = f x + g x negate f x = negate (f x) ... instance Num a => Num (IO a) where ... So, in other words, I think the fact that Functor is not a superclass of Monad is a poorly motivated library design inconsistency...
The current situation is slightly more flexible: you don't *have* to provide a Functor instance for every Monad instance. On the other hand, it means you occasionally have to write an additional Functor context in types. You can always get around that by defining
class (Functor m, Monad m) => Monad' m where {}
Introducing Monad' does not strike me as particularly appealing solution, because 1. For automatically inferred types, it seems pointless, because the compiler would presumably still infer types containing "(Functor m, Monad m) => ..." rather than "Monad' m => ...". 2. To be able to use Monad' in explicitly given type signatures, you would be forced to declare three instances (Functor, Monad, Monad') per type, while if Functor was a superclass of Monad you would only need to declare two instances (Functor, Monad). -- Thomas H