# Diagonalization/ dupe for monads and tuples?

chessai chessai1996 at gmail.com
Thu Sep 17 05:12:36 UTC 2020

```dup and dedup? Just throwing out names. Not sure I'm a fan of diag/coding.

On Thu, Sep 17, 2020, 00:10 Emily Pillmore <emilypi at cohomolo.gy> wrote:

> Proposing the following names then to start, and we can talk about
> expanding this at a later date:
>
> ```
> -- In Data.Tuple
> diag :: a → (a,a)
> diag a = (a, a)
>
> -- In Data.Either
> codiag :: Either a a → a
> codiag = either id id
> ```
>
> Thoughts?
> Emily
>
>
>
> On Thu, Sep 17, 2020 at 12:55 AM, Asad Saeeduddin <masaeedu at gmail.com>
> wrote:
>
>> `Either a a -> a` is another very handy operation. That and `a -> (a, a)`
>> together make up 90% of use cases for `duplicate :: p x (t x x)`.
>> On 9/17/20 12:49 AM, Edward Kmett wrote:
>>
>> For what it's worth, I'd just like to see a no-nonsense
>>
>> dup : a -> (a,a)
>> dup a = (a,a)
>>
>> in Data.Tuple, where it is out of the way, but right where you'd expect
>> it to be when looking for something for working with tuples.
>>
>> Yes, bipure and id &&& id exist, and generalize it on two incompatible
>> axes, and if we had a proper cartesian category we'd be able to supply this
>> in full generality, as a morphism to the diagonal functor, but all of these
>> require a level of rocket science to figure out.
>>
>> I'd also happily support adding the equivalent in Data.Either for Either
>> a a -> a, which invites bikeshedding names.
>>
>> -Edward
>>
>> On Wed, Sep 16, 2020 at 6:10 PM Emily Pillmore <emilypi at cohomolo.gy>
>> wrote:
>>
>>> Just to clarify, that's not the "real" diagonal in the space, but it is
>>>
>>>
>>> On Wed, Sep 16, 2020 at 9:08 PM, Emily Pillmore <emilypi at cohomolo.gy>
>>> wrote:
>>>
>>>> operations: as the data of a cartesian monoidal category, and the laws are
>>>> correct in this case. I think we can get away with the additional
>>>> abstraction to `Biapplicative` in this case, though.
>>>>
>>>> wouldn't the existence of appropriate `forall a. a -> t a a` and
>>>> `forall a. x -> Unit t` functions pigeonhole it into being "the" cartesian
>>>> monoidal structure on `->` (and thus naturally isomorphic to `(,)`)?
>>>>
>>>>
>>>> Only if you chose that particular unit and that particular arrow. But
>>>> there are other places where having a general `Biapplicative` contraint
>>>> would make it useful. For example,  i'd like to use this in `smash` with
>>>> `diag :: a → Smash a a`, defining the adjoining of a point to `a` and
>>>> creating a diagonal in the subcategory of pointed spaces in Hask, so that I
>>>> don't have to redefine this as `diag' = quotSmash . view _CanIso . diag .
>>>> Just`.
>>>>
>>>> Cheers,
>>>> Emily
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Wed, Sep 16, 2020 at 6:35 PM, Asad Saeeduddin <masaeedu at gmail.com>
>>>> wrote:
>>>>
>>>>> Whoops, I just realized I've been responding to Carter specifically
>>>>> instead of to the list.
>>>>>
>>>>> I was having some trouble understanding the `unJoin` stuff but I read
>>>>> it a few more times and I think I understand it a little better now.
>>>>>
>>>>> In my personal experience the "abstracted version" of `x -> (x, x)` I
>>>>> use most often looks like this:
>>>>>
>>>>> ```
>>>>>
>>>>> class SymmetricMonoidal t i p => CartesianMonoidal t i p
>>>>>
>>>>>   where
>>>>>
>>>>>   duplicate :: p x (x `t` x)
>>>>>
>>>>>   discard :: p x i
>>>>>
>>>>>
>>>>> -- Laws:
>>>>>
>>>>> -- duplicate >>> first  discard = fwd lunit
>>>>>
>>>>> -- duplicate >>> second discard = fwd runit
>>>>>
>>>>>
>>>>> -- where
>>>>>
>>>>> -- lunit :: Monoidal t i p => Iso p x (i `t` x)
>>>>>
>>>>> -- runit :: Monoidal t i p => Iso p x (x `t` i)
>>>>>
>>>>> ```
>>>>>
>>>>> monoidal structure gives us a cartesian monoidal structure.
>>>>>
>>>>> This doesn't really seem to be relevant to what you folks are looking
>>>>> for, but it does bring up a question. If some `Bifunctor` `t` happens to
>>>>> form a monoidal structure on `->`, wouldn't the existence of appropriate
>>>>> `forall a. a -> t a a` and `forall a. x -> Unit t` functions pigeonhole it
>>>>> into being "the" cartesian monoidal structure on `->` (and thus naturally
>>>>> isomorphic to `(,)`)?
>>>>> On 9/16/20 5:26 PM, Emily Pillmore wrote:
>>>>>
>>>>> Nice!
>>>>>
>>>>> That's kind of what I was going for with Carter earlier in the day,
>>>>> thanks Matthew.
>>>>>
>>>>> I think a diagonalization function and functor are both very sensible
>>>>> additions to `bifunctors` and `Data.Bifunctor`. The theory behind this is
>>>>>
>>>>> Certainly the function `diag :: a → (a,a)` is something I've seen
>>>>> written in several libraries, and should be included in `Data.Tuple` as a
>>>>> `base` function. The clear generalization of this function is `diag ::
>>>>> Biapplicative f ⇒ a → f a a`. I'm in favor of both existing in their
>>>>> separate capacities.
>>>>>
>>>>> Thoughts?
>>>>>
>>>>> Emily
>>>>>
>>>>>
>>>>> On Wed, Sep 16, 2020 at 3:49 PM, Carter Schonwald <
>>>>> carter.schonwald at gmail.com> wrote:
>>>>>
>>>>>> Is the join bipure definition taking advantage of the (a->) monad
>>>>>> instance?  Slick!
>>>>>>
>>>>>>
>>>>>> On Wed, Sep 16, 2020 at 3:39 PM Matthew Farkas-Dyck <
>>>>>> strake888 at gmail.com> wrote:
>>>>>>
>>>>>>> We also have
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> diag = join bipure
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> diag = unJoin . pure
>>>>>>>
>>>>>>>   where
>>>>>>>
>>>>>>>     newtype Join f a = Join { unJoin :: f a a } deriving (Functor)
>>>>>>>
>>>>>>>     deriving instance Biapplicative f => Applicative (Join f)
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> The latter seems on its face potentially related to the instance for
>>>>>>>
>>>>>>> lists of fixed length, but i am not sure how deep the connection may
>>>>>>>
>>>>>>> be.
>>>>>>>
>>>>>>> _______________________________________________
>>>>>> Libraries mailing list
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>>
>>>>> _______________________________________________
>>>>> Libraries mailing list
>>>>>
>>>>
>>> _______________________________________________
>>> Libraries mailing list
>>
>>
> _______________________________________________
> Libraries mailing list