Re: proposed change to transformers package

While I personally prefer the (:*:) and (:+:) notation, both require an additional extension, TypeOperators, and to Ross's point (:+:) fails to be an Applicative/Monad transformers, so the scope of the package starts to stretch there. That said, there are always ideal monad coproducts, but then you need the notion of ideal monads. ;) data Ideal m a = Return a | Ideal (m a) data Mutual m n a = Mutual (m (Mutual n m a)) data (m :+: n) a = Coproduct { runCoproduct :: Either (m a) (n a) } type IdealCoproduct m n = Ideal (Mutual m n :+: Mutual n m) given a definition for class MonadIdeal m where idealize :: m (Ideal m a) -> m a which describes a monad, that has a separate return. You can define instance MonadIdeal m => Monad (Ideal m) and then you can define an 'ideal monad coproduct' of any two ideal monads from there. This covers, Maybe, Either, Identity, and a bunch of others. wherever the 'Return' constructor can be cleanly separated from the rest of the monad. However, this drifts out of library/platform territory and into esoterica. -Edward Kmett On Mon, Mar 29, 2010 at 5:42 AM, Nicolas Pouillard < nicolas.pouillard@gmail.com> wrote:

On Fri, 26 Mar 2010 19:28:18 +0000, Ross Paterson wrote:

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On Fri, Mar 26, 2010 at 05:24:18AM -0700, Nicolas Pouillard wrote:

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Could we have functors products, sums, fixpoints as well? It would really avoid to redefine them each time.

Do you mean

data Product f g a = Product (f a) (g a)

Yes, I was thinking of using :*: instead of Product:

data (:*:) f g a = (:*:) (f a) (g a)

...

with Functor, Foldable, Traversable and Applicative instances?

Yes.

...

Not sure if the other two count as transformers.

Why not:

data (:+:) f g a = Inl (f a) | Inr (g a)

And sure, no applicative nor monad instance for this one.

And Fix like in category-extras:

http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html/...

Best regards,

-- Nicolas Pouillard http://nicolaspouillard.fr _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries