Apologies

[Gabriel Gonzalez]

On 05/20/2013 10:41 PM, Carter Schonwald wrote:
> could you elaborate on the other useful generalizations please? :-)
>
>
For example, mapM defines a functor in the Kleisli category:

mapM return = return

mapM (f >=> g) = mapM f >=> mapM g

So then why not have a generalized Functor class that works on arbitrary 
categories:

class CFunctor c f where
     cmap :: c a b -> c (f a) (f b)

instance (Traversable t) => CFunctor Kleisli t where ...

Or consider that `mapM` leaks space for large traversables.  Another 
alternative generalization that doesn't leak space is to use pipes.  
This is not as far-fetched as it sounds, considering that `pipes` are 
`ListT` done right when viewed through the `RespondT` newtype:

-- i.e. the same as `ProduceT` in `pipes`
type ListT = RespondT ProxyCorrect C () ()

-- Just pretend we lived in a parallel Haskell world with this definition
type [] = ListT Identity

Then `mapM` becomes `(RespondT . lift .)` when viewed through the lens 
of the `pipes`-based `ListT m` (except with no more space leaks).  Then 
you'd have the theoretically pure version that does not leak space, but 
as much as I like `pipes` not a single beginner would ever think to use 
that.

APIs are like monad tutorials: it's better to teach people starting from 
something simple and concrete than something abstract.  If the Prelude 
is not for teaching purposes, then what is?

> On Tue, May 21, 2013 at 1:41 AM, Gabriel Gonzalez 
> <gabriel439 at gmail.com <mailto:gabriel439 at gmail.com>> wrote:
>
>     I want to apologize to wren (and everybody else).  I was tired and
>     overreacted when the discussion started veering into making more
>     breaking changes.
>
>     I don't mind the Foldable changes too much.  My main concerns are:
>
>     * loss of monomorphism makes teaching more difficult
>     * They can be generalized in other useful ways
>
>     However, I don't consider those concerns show-stoppers.
>
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