All Monads are Functors

[Taral]

On 8/13/06, Lennart Augustsson <lennart at augustsson.net> wrote:
> If I remember right, Functor was a superclass of Monad in Haskell
> early on, but it was taken away.  I think this was the wrong
> decision, but I seem to remember that the rationale was that it would
> be too onerous to require programmers to write a Functor instance
> every time they want a Monad instance.  Bah!

There was a proposal (Re: [Haskell] A collection of related proposals
regarding monads) to allow instance definitions to be combined,
allowing a more fine-grained hierarchy (Functor, PointedFunctor,
Applicative, Monad) without changing the instance definition
requirements.

In my opinion, an instance definition of a subclass should allow the
superclass's methods to be defined as if they were part of the
subclass, e.g.:

instance Monad [] where
    fmap = map
    return x = [x]
    join = concat

It's so pretty! (But a little inefficient. You'd probably want to
define ap/lift2 in there.)

-- 
Taral <taralx at gmail.com>
"You can't prove anything."
    -- Gödel's Incompetence Theorem