# \`transformers\` vs \`arrows\` packages

**URL:** <https://discourse.haskell.org/t/transformers-vs-arrows-packages/9664>\
**Category:** Learn\
**Created:** [June 2, 2024, 12:24pm UTC](https://discourse.haskell.org/t/transformers-vs-arrows-packages/9664 "2024-06-02T12:24:31Z")\
**Posts on this page:** 1\
**Showing post:** 5

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**Author:** ![jaror](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/jaror/32/3271_2.png) [@jaror](https://discourse.haskell.org/u/jaror)\
**Post date:** [June 2, 2024, 2:45pm UTC](https://discourse.haskell.org/t/transformers-vs-arrows-packages/9664/5 "2024-06-02T14:45:19Z")

</div>

> Meaning code like this isn’t possible using just applicatives:
> 
> ```haskell
> getFriendsUsernames :: Id -> DB [Username]
> getFriendsUsernames user = do
> ids <- getFriends user
> traverse getUsername ids
> 
> ```

It is possible in pretty much the same way as they do with their free profunctors (and in the same way as I did with FRP in the link I included in my previous comment):

```haskell
data DB a b where
  GetUsernames :: DB (Set Id) -> DB (Map Id Username)
  GetFriends :: DB Id -> DB (Set Id)
  Pure :: a -> DB a
  Ap :: DB (a -> b) -> DB a -> DB b

getFriendsUsernames :: Id -> DB [Username]
getFriendsUsernames user = 
  Ap (Pure toList) (GetFriends (GetUsernames (Pure user)))

```

I think there may be a drawback of using applicatives like this instead of arrows/profunctors. In particular, I think arrows could support sharing better. But I haven’t seen a concrete example of that yet.

Edit: Actually, it seems the `traverse'` function may be a problem for applicatives, because the signature would have to include a higher order function: `Traverse' :: Traversable f => (DB a -> DB b) -> DB (f a) -> DB (f b)`, but then you wouldn’t be able to inspect that function that we’re traversing with. Although, it still remains to be shown that this is actually a problem.

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