Re: [Haskell-cafe] Retrospective type-class extension

I've compared and clearly the former is significantly superior :) I'm rather interested if there are any sound suggestions to resolve the general issue of retrospective type-class extension. Miguel Mitrofanov wrote:

That won't be a great idea; if I just want my monad to be declared as one, I would have to write

instance Functor MyMonad where fmap = ... instance Pointed MyMonad where pure = ... instance Applicative MyMonad where (<*>) = ... instance Monad MyMonad where join = ...

Compare this with

instance Monad MyMonad where return = ... (>>=) = ...

and take into account that (>>=) is usually easier to write than join.

Limestraël wrote:

...

Then it would be:

class Functor f where fmap :: (a -> b) -> f a -> f b

class (Functor f) => Pointed f where pure :: a -> f a

class (Pointed f) => Applicative f where (<*>) :: f (a -> b) -> f a -> f b

class (Applicative f) => Monad f where join :: f (f a) -> f a

This would be a great idea, for the sake of logic, first (a monad which is not a functor doesn't make sense), and also to eliminate redudancy (fmap = liftM, ap = (<*>), etc.)

2010/5/20 Tony Morris mailto:tonymorris@gmail.com>

Ivan Miljenovic wrote: > On 20 May 2010 14:42, Tony Morris mailto:tonymorris@gmail.com> wrote: > >> We all know that "class (Functor f) => Monad f" is preferable but its >> absence is a historical mistake. We've all probably tried once: >> >> instance (Functor f) => Monad f where >> > > Do you mean the reverse of this (instance (Monad m) => Functor m where) ? > Yes.

-- Tony Morris http://tmorris.net/

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