Sjoerd Visscher sjoerd at w3future.com
Sun Jun 27 08:16:25 EDT 2010

```Allowing Functor i also makes defining Thingy directly (without going though Monoidal) easy:

newtype Thingy i a = Thingy { runThingy :: forall b. i (a -> b) -> i b }

instance Functor i => Functor (Thingy i) where
fmap f m = Thingy \$ runThingy m . fmap (. f)

instance Functor i => Applicative (Thingy i) where
pure x = Thingy \$ fmap (\$ x)
mf <*> mx = Thingy \$ runThingy mx . runThingy mf . fmap (.)

Not allowing Functor i and adding Yoneda also works.

On Jun 27, 2010, at 1:43 PM, Sjoerd Visscher wrote:

> Hi Max,
>
> This is really interesting!
>
>> 1. There exist total functions:
>>
>>> lift :: X d => d a -> D a
>>> lower :: X d => D a -> d a
>>
>> 2. And you can write a valid instance:
>>
>>> instance X D
>>
>> With *no superclass constraints*.
>
> All your examples have a more specific form:
>
>> lift :: X d => d a -> D d a
>> lower :: X d => D d a -> d a
>> instance X (D d)
>
> This might help when looking for a matching categorical concept. With your original signatures I was thinking of initial/terminal objects, but that's not the case.
>
>> 2. Is there a mother of all idioms? By analogy with the previous three
>> examples, I tried this:
>>
>>> -- (<**>) :: forall a. i a -> (forall b. i (a -> b) -> i b)
>>> newtype Thingy i a = Thingy { runThingy :: forall b. i (a -> b) -> i b }
>>
>> But I can't see how to write either pure or <*> with that data type.
>> This version seems to work slightly better:
>>
>>> newtype Thingy i a = Thingy { runThingy :: forall b. Yoneda i (a -> b) -> i b }
>>
>> Because you can write pure (pure x = Thingy (\k -> lowerYoneda (fmap
>> (\$ x) k))). But <*> still eludes me!
>
> It's usually easier to switch to Monoidal functors when playing with Applicative. (See the original Functional Pearl "Applicative programming with effects".)
>
> Then I got this:
>
> newtype Thingy i a = Thingy { runThingy :: forall b. Yoneda i b -> Yoneda i (a, b) }
>
> (&&&) :: Thingy i c -> Thingy i d -> Thingy i (c, d)
> mf &&& mx = Thingy \$ fmap (\(d, (c, b)) -> ((c, d), b)) . runThingy mx . runThingy mf
>
> instance Functor (Thingy i) where
>  fmap f m = Thingy \$ fmap (first f) . runThingy m
>
> instance Applicative (Thingy i) where
>  pure x = Thingy \$ fmap (x,)
>  mf <*> mx = fmap (\(f, x) -> f x) (mf &&& mx)
>
> Note that Yoneda is only there to make it possible to use fmap without the Functor f constraint. So I'm not sure if requiring no class constraints at all is a good requirement. It only makes things more complicated, without providing more insights.
>
> I'd say that if class X requires a superclass constraint Y, then the instance of X (D d) is allowed to have the constraint Y d. The above code then stays the same, only with Yoneda removed and constraints added.
>
> greetings,
> --
> Sjoerd Visscher
> http://w3future.com
>
>
>
>
> _______________________________________________