Apply typeclass by pedrovgs · Pull Request #1404 · arrow-kt/arrow

pedrovgs · 2019-04-12T18:13:51Z

📌 References

Issue: Closes Apply and Divide typeclasses #284

🎩 What is the goal?

Add Apply typeclass to the project and update Applicative typeclass implementation to be based on it.

Disclaimer: I'm creating this PR as a draft because the implementation is not finished. There are some pending tasks related to the extensions and documentation I'd like to review with the team before. Formatting the code properly is also a task I have pending but I can't spend more time on this issue for now.

How is it being implemented?

Crating the brand new Apply typeclass was easy. We only needed to create the new Apply type and move all the code from Applicative but the just related code. The hard part was related to the laws and the usage of simulated higher kinded types.

These are some points I'd like to review with the team:

It was weird to see how the laws already described for Aplicative doesn't match with the one I've found in other implementations like Cat's Apply or Scalaz's Apply typeclasses.
I've updated the AplicativeLaws implementation to be divided in ApplyLaws and ApplyCartesianLaws. This change let us write some of our laws without coupling the implementation to any concrete type. I'm afraid I couldn't implement this for the cartesian laws because of the internal map implemented. Ideally, we'd like to don't even couple this implementation to the Applicative instance passed as parameter, but there is no way to lift any instance into Kind<F, A> w/o an Applicative instance. I've reviewed the laws implementation with @delr3ves thinking about how could reduce the coupling but it wasn't possible for this scenario.
As @raulraja already told us: "import arrow.effects.extensions.io.applicative will no longer be available in that location but instead in:import arrow.effects.extensions.io.apply.[functionName]". ⚠️ This breaking change will not be important now because we are not in version > 1.0.0 but it's important to keep it in mind.

About the rest of the task. I have pending to write the documentation and implement the extensions for the types already providing an Applicative instance. However I have questions about these tasks:

Should we create a new page for the Apply typeclass is it ok if we just update the Applicative page?
As Apply is only needed to be able to implement other typeclasses for now, is adding all the extensions worth it?

Please, review the current implementation and my questions about the docs and the extensions. As soon as we consider this implementation correct I'll update the PR with the rest of the code 😃

How can it be tested?

🤖 All the code already using the ApplicativeLaws will now use the ApplyLaws.

…ws associated

…ss how many extensions we will add

pedrovgs · 2019-04-12T18:51:39Z

Finally, I had to rever the ApplyLaw changes to one closer to the original laws 😞

raulraja

Looking good so far!

raulraja · 2019-04-12T20:11:12Z

modules/core/arrow-core-extensions/src/main/kotlin/arrow/core/extensions/try.kt

+@extension
+interface TryApply : Apply<ForTry> {
+  override fun <A, B> TryOf<A>.ap(ff: TryOf<(A) -> B>): Try<B> =
+          fix().ap(ff)


I believe in Arrow we use 2 spaces but we are in the process of adding Ktlint // cc @aballano . You can leave or change this but eventually, it's gonna get reformated.

Yeap, I couldn't find any tool to automatically format the code and I decided to left the formatting issue as is until I get the answers about the docs and the extensions 😃 I'll format the code properly before finishing this PR. Don't worry 😃

Still working on it! #1405

pakoito · 2019-04-12T20:31:06Z

modules/core/arrow-typeclasses/src/main/kotlin/arrow/typeclasses/Apply.kt

+import arrow.core.*
+import java.math.BigDecimal
+
+interface Apply<F> : Functor<F> {


Cool, I'll never need to open this file anymore.

pakoito · 2019-04-12T20:31:49Z

modules/core/arrow-test/src/main/kotlin/arrow/test/laws/ApplyLaws.kt

+
+object ApplyLaws {
+
+    fun <F, A> laws(AP: Applicative<F>, gen: Gen<A>, EQ: Eq<Kind<F, A>>): List<Law> =


We should keep the laws separated between those that use Apply and those that use Applicative.

Then I realised all of them needed both lol

I'd keep them as ApplicativeLaws tbh.

But do they? If just is only used to lift into Kind<F, A> then it might be better to either use a constructor function or a Gen<Kind<F, A>> directly. And all laws that strictly need just belong to applicative anyway. Although I am not too familiar with applicative laws, so idk how many that leaves o.O

This is an issue we have in more places.

We should indeed be using Gen<Kind<F, A>> directly here. There is some low hanging fruit in the test setup.

Hi all, I commented one interesting point about the laws in the PR description:

It was weird to see how the laws already described for Aplicative doesn't match with the one I've found in other implementations like Cat's Apply or Scalaz's Apply typeclasses.

I've updated the AplicativeLaws implementation to be divided in ApplyLaws and ApplyCartesianLaws. This change let us write some of our laws without coupling the implementation to any concrete type. I'm afraid I couldn't implement this for the cartesian laws because of the internal map implemented. Ideally, we'd like to don't even couple this implementation to the Applicative instance passed as parameter, but there is no way to lift any instance into Kind<F, A> w/o an Applicative instance. I've reviewed the laws implementation with @delr3ves thinking about how could reduce the coupling but it wasn't possible for this scenario.

Finally, as FunctorLaws need some dependencies I can't fix for now without rewriting most of the laws and this issue was supposed to bee "noob friendly" I decided not to refactor them and just move them to the ApplyLaw folder.

Are we going to keep the laws in the ApplicativeLaw folder and remove this part of the PR? I tried to use Gen<Kind<F, A>>, and a method to lift any value into Kind, but it is not so easy without refactoring the rest of the code laws.

1Jajen1 · 2019-04-12T20:49:15Z

modules/core/arrow-typeclasses/src/main/kotlin/arrow/typeclasses/Apply.kt

+
+import arrow.Kind
+import arrow.core.*
+import java.math.BigDecimal


This import seems weird, is it used anywhere?

You are right, I have pending the imports optimization and the code formatting for this PR.

1Jajen1 · 2019-04-12T20:50:52Z

Looks good so far 👍 I'd be in favor of adding the instances as it could be confusing not having them. Although I don't think it is super important, but at least a ticket for adding them should be added

pedrovgs · 2019-04-15T06:09:35Z

Thanks for your comments guys! Do we all agree if we finish the following task before considering this PR as final?

Format code and optimize imports.
Recover ApplicativeLaw changes.
Add the missing extensions for Apply.
Update Applicative documentation with a small section about Apply.

pakoito · 2019-04-15T08:27:15Z

Update Applicative documentation with a small section about Apply.

Why not a new docs page?

pedrovgs · 2019-04-15T08:38:42Z

I asked for this in the PR description @pakoito

Should we create a new page for the Apply typeclass is it ok if we just update the Applicative page?

As the usage of Apply is not so common and most of the people will use Applicative I thought adding a section inside applicative's page should be enough. You can see how Cat's documentation uses this approach.

Let me know if you'd prefer a new page or just a section inside applicative and I'll update the PR with it.

pedrovgs · 2019-04-19T11:59:51Z

Hi there! As discussed in this draft PR I've updated this PR with the extensions needed, the documentation updated and a new section added to the applicative documentation about the Apply typeclass 😃 So I'm moving this PR to the final stage. I'd like to thank you all for your help and comments. Please, let me know if you think there is anything else you'd like to add or review 😃

modules/core/arrow-core-extensions/src/main/kotlin/arrow/core/extensions/try.kt

modules/core/arrow-test/src/main/kotlin/arrow/test/laws/ApplicativeLaws.kt

modules/core/arrow-typeclasses/src/main/kotlin/arrow/typeclasses/Applicative.kt

raulraja · 2019-04-19T13:18:17Z

modules/docs/arrow-docs/docs/docs/patterns/glossary/README.md


 ```kotlin:ank
-import arrow.core.extensions.option.applicative.map
+import arrow.core.extensions.option.apply.map


have we found collision issues with the stdlib apply function when requesting the apply() instance explicitly from a companion? Seems like this could be potentially confusing to users.

I hope not. I know the name can be confusing for some users but as most of the people will consume it as is in this example I guess this won't be an issue.

@raulraja there is no issue with apply. It's called invoke in Kotlin. Scala mix-up :D

@nomisRev I think @raulraja is talking about this one:

package kotlin /** * Calls the specified function [block] with `this` value as its receiver and returns `this` value. */ @kotlin.internal.InlineOnly public inline fun <T> T.apply(block: T.() -> Unit): T { contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) } block() return this }

raulraja

Awesome stuff!

raulraja

While this can go as is it's missing something very important. Kdocs. The docs in markdown are deprecated and we want all examples and playgrounds ready as part of the Kdocs. See Functor for an example on how that is done. Also relevant https://github.com/arrow-kt/arrow/blob/master/modules/docs/arrow-docs/README.md

We want to abandon md based docs in favor of kdocs.

modules/docs/arrow-docs/docs/docs/arrow/typeclasses/applicative/README.md

…nd Raúl

pedrovgs · 2019-04-22T07:33:34Z

Thanks for your comments @raulraja @pakoito applied the changes you suggested and updated this PR 😃

raulraja · 2019-04-22T09:46:27Z

Awesome stuff, thanks @pedrovgs ! 👏

pedrovgs · 2019-04-22T10:30:37Z

Thank you all for your comments and feedback. I'll keep contributing to this awesome project when possible 😃

Refactor Applicative to be based on Apply typeclass and update the la…

c270322

…ws associated

pedrovgs requested review from 1Jajen1, JorgeCastilloPrz, pakoito, raulraja and truizlop April 12, 2019 18:13

Add try extension so we can get code building. We still have to discu…

595c260

…ss how many extensions we will add

raulraja reviewed Apr 12, 2019

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pakoito reviewed Apr 12, 2019

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1Jajen1 reviewed Apr 12, 2019

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pedrovgs added 5 commits April 19, 2019 11:00

Recover applicative laws

80e7e08

Update branch with master and format code

980d72f

Add extensions we had pending

c600ac4

Finish extensions and tests

8cb9094

Add missing extensions and update documentation

9f83430

pedrovgs marked this pull request as ready for review April 19, 2019 11:59

raulraja reviewed Apr 19, 2019

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raulraja approved these changes Apr 19, 2019

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Fix formatting issues and update documentation as suggested by Paco a…

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…nd Raúl

raulraja merged commit 89ec278 into arrow-kt:master Apr 22, 2019

raulraja mentioned this pull request Sep 10, 2019

Release 0.10.0 #1590

Merged


		object ApplyLaws {

		fun <F, A> laws(AP: Applicative<F>, gen: Gen<A>, EQ: Eq<Kind<F, A>>): List<Law> =

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📌 References

🎩 What is the goal?

How is it being implemented?

How can it be tested?

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