proposed change to transformers package

Nicolas Pouillard nicolas.pouillard at
Mon Mar 29 05:42:57 EDT 2010

On Fri, 26 Mar 2010 19:28:18 +0000, Ross Paterson <ross at> wrote:
> On Fri, Mar 26, 2010 at 05:24:18AM -0700, Nicolas Pouillard wrote:
> > 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?


> 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:

Best regards,

Nicolas Pouillard

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