Contravariant & Invariant #975

Cotel · 2018-08-04T08:09:50Z

Adding Contravariant & Invariant typeclasses and instances.

Cotel · 2018-08-04T08:13:03Z

modules/core/arrow-data/src/test/kotlin/arrow/data/MonoidTest.kt

+
+    init {
+        testLaws(
+            InvariantLaws.laws(Monoid.invariant<Int>(), { Int.monoid() }, EQ)


@pakoito It seems like our experiment success at compiling but tests are failing 🤔

pakoito · 2018-08-04T11:09:43Z

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

+  fun G(): Invariant<G>
+
+  override fun <A, B> Kind<Nested<F, G>, A>.imap(f: (A) -> B, g: (B) -> A): Kind<Nested<F, G>, B> =
+    F().run { unnest().imap({ ga -> G().run { ga.imap(f, g) } }, { gb -> G().run { gb.imap(g, f) } }) }.nest()


There's a chance this doesn't work on Android on the D8 compiler due to a problem with scopes and run. Can you please move each of the lambdas in imap to a separate line?

pakoito · 2018-08-04T11:39:51Z

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

+        F().run {
+            val fl: (Kind<G, A>) -> Kind<G, B> = { ga -> G().run { ga.contramap(g) } }
+            val fr: (Kind<G, B>) -> Kind<G, A> = { gb -> G().run { gb.contramap(f) } }
+            unnest().imap(fl, fr)


Sorry, my bad. Take these out of the F() run block I mean.

Cotel · 2018-08-04T14:28:35Z

I've run all the tests locally and it seems like Monoid is passing Invariant Laws now!

So now I'm going to add an Invariant instance for Semigroup and that should be it for Invariant.

EDIT: When I add the typealias SemigroupOf<A> = arrow.Kind<ForSemigroup, A> boilerplate, Monoid can no longer extend MonoidOf<A> because as it already extends Semigroup<A> there is an inconsistent type at F of Kind (ForSemigroup, ForMonoid).

Can we fix this? Or should we just offer one instance of Invariant for Monoid or Semigroup?

pakoito · 2018-08-04T15:23:11Z

modules/core/arrow-data/src/test/kotlin/arrow/data/MonoidTest.kt

@@ -11,7 +11,7 @@ import org.junit.runner.RunWith
 class MonoidTest : UnitSpec() {

    val EQ: Eq<MonoidOf<Int>> = Eq.invoke { a, b ->
-        a.fix() == b.fix()
+        a.fix().run { 3.combine(1) } == b.fix().run { 3.combine(1) }


You figured out why it didn't work :D Well done!

codecov · 2018-08-04T15:30:13Z

Codecov Report

Merging #975 into master will increase coverage by 0.15%.
The diff coverage is 51.3%.

@@             Coverage Diff              @@
##             master     #975      +/-   ##
============================================
+ Coverage      47.8%   47.96%   +0.15%     
- Complexity      695      702       +7     
============================================
  Files           320      325       +5     
  Lines          8099     8200     +101     
  Branches        753      753              
============================================
+ Hits           3872     3933      +61     
- Misses         3943     3984      +41     
+ Partials        284      283       -1

Impacted Files	Coverage Δ	Complexity Δ
...ses/src/main/kotlin/arrow/typeclasses/Bifunctor.kt	`0% <0%> (ø)`	`0 <0> (ø)`	⬇️
...-data/src/main/kotlin/arrow/instances/coproduct.kt	`46.66% <0%> (-3.34%)`	`0 <0> (ø)`
...nces-core/src/main/kotlin/arrow/instances/const.kt	`52.38% <0%> (-5.52%)`	`0 <0> (ø)`
...arrow-data/src/main/kotlin/arrow/data/Coproduct.kt	`57.14% <0%> (-2.86%)`	`7 <0> (ø)`
...-data/src/main/kotlin/arrow/instances/cokleisli.kt	`0% <0%> (ø)`	`0 <0> (ø)`	⬇️
...asses/src/main/kotlin/arrow/typeclasses/Functor.kt	`33.33% <100%> (+8.33%)`	`0 <0> (ø)`	⬇️
...arrow-core/src/main/kotlin/arrow/core/Function1.kt	`88.23% <100%> (+0.73%)`	`6 <1> (+1)`	⬆️
...src/main/kotlin/arrow/typeclasses/Contravariant.kt	`25% <25%> (ø)`	`0 <0> (?)`
...es/src/main/kotlin/arrow/typeclasses/Cocomposed.kt	`28.57% <28.57%> (ø)`	`0 <0> (?)`
...sses/src/main/kotlin/arrow/typeclasses/Composed.kt	`61.47% <45.28%> (-7.28%)`	`0 <0> (ø)`
... and 17 more

Continue to review full report at Codecov.

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pakoito · 2018-08-04T16:28:42Z

Can we fix this?

Try making ForMonoid inherit ForSemigroup, as the F in Kind is covariant. Subtyping sucks :) Test thoroughly.

Cotel · 2018-08-04T16:29:06Z

I'm also having some troubles to create a Function1ContravariantInstance. I've defined a contramap function in Function1:

class Function1<I, out O>(val f: (I) -> O) {
  fun <B> contramap(f: (B) -> I): Function1<B, O> = (this.f compose f).k()
}

But now I can't use it when I am defining the ContravariantInstance:

interface Function1ContravariantInstance<I> : Contravariant<Function1PartialOf<I>> {
  override fun <A, B> Kind<Function1PartialOf<I>, A>.contramap(f: (B) -> A): Function1<I, B> =
    fix().contramap(f)
}

The editor tells me that contramap will be called recursively. But even if I change it to contramapp to distinguish between scopes, the signature is incorrect. I would need it to be:

override fun <A, B> Kind<Function1PartialOf<I>, A>.contramap(f: (B) -> I): Function1<B, A>

pakoito · 2018-08-04T16:30:33Z

@Cotel Push it and I'll take a look

pakoito · 2018-08-04T16:42:30Z

@Cotel can't be done, same problem as the left functor from the other day

pakoito · 2018-08-05T13:30:41Z

modules/core/arrow-data/src/test/kotlin/arrow/instances/ComposedInstancesTest.kt


    testLaws(
+      InvariantLaws.laws(ComposedInvariant(Option.functor(), NonEmptyList.functor()), cf, EQ_OPTION_NEL),
+      InvariantLaws.laws(ComposedInvariantCovariant(Option.functor(), NonEmptyList.functor()), cf, EQ_OPTION_NEL),
+      InvariantLaws.laws(ComposedInvariantContravariant(Option.functor(), Function1.contravariant<Int>()), cf2, EQ_OPTION_FN1),


TestLaws deduplicates tests based on name. That means only one of these will be run. Split 2 of them into a new call to testLaws.

Cotel · 2018-08-06T06:25:59Z

I've been looking for Invariant and Contravariant instances in cats but, besides the typeclasses stuff, these are all the instances we can make.

The only thing that remains is to test the laws for ComposedContravariant, ComposedContravariantCovariant and ComposedCovariantContravariant but these are really unintuitive for me, I am not able to create the required lambda for the law or the Eq instance 😢

Cotel · 2018-08-24T08:03:02Z

I would like to finish this asap and start something related with comonadic UIs but I was stuck and I'm quite busy right now. I feel bad for keeping this PR open for so long with no activity 😅

pakoito · 2018-08-24T16:10:11Z

I'm traveling, let's talk in September :D

…isli

Cotel · 2018-09-08T18:31:45Z

Hey! I might have some spare time tomorrow and I would like to close this and start working on another issue or something 😁

Cotel · 2018-09-09T09:07:46Z

I'm sorry but I really don't know what is failing here. I suppose that the laws which are failing are ComposedInvariantContravariant and the cause are probably the lambda and EQ instance for the test.

val cf2: (Int) -> Kind<Nested<ForOption, Conested<ForFunction1, Int>>, Int> = { x: Int ->
  Some({ y: Int -> x + y }.k().conest()).nest()                                          
}

val EQ_OPTION_FN1: Eq<NestedType<ForOption, Conested<ForFunction1, Int>, Int>> = Eq { a, b ->
  a.unnest().fix() == b.unnest().fix()                                                       
}

Any ideas? 😅

pakoito · 2018-09-09T11:29:13Z

I may take a look directly in the afternoon. Meanwhile, use the extfun Eq.logged and run the tests with -i to at least see the values.

Cotel · 2018-09-11T17:33:21Z

God bless that Eq.logged function 😍.

pakoito · 2018-09-11T22:32:51Z

@Cotel It's saved my ass more times that I'm willing to admit hahaha

Gotta give a last check and we'll go green ;)

pakoito · 2018-09-11T22:34:46Z

modules/core/arrow-data/src/test/kotlin/arrow/instances/ComposedInstancesTest.kt

@@ -36,7 +36,15 @@ class ComposedInstancesTest : UnitSpec() {
      }

    val EQ_OPTION_FN1: Eq<NestedType<ForOption, Conested<ForFunction1, Int>, Int>> = Eq { a, b ->
-      a.unnest().fix() == b.unnest().fix()
+      a.unnest().fix().fold(


Cheezus fucking chrixt

pakoito · 2018-09-11T22:36:22Z

modules/core/arrow-data/src/test/kotlin/arrow/data/ConstTest.kt

@@ -16,6 +13,7 @@ class ConstTest : UnitSpec() {
  init {
    ForConst(Int.monoid()) extensions {
      testLaws(
+        InvariantLaws.laws(this, { Const(it) }, Eq.any()),


InvariantLaws should be part of FunctorLaws, so it's tested by ApplicativeLaws!

pakoito · 2018-09-11T22:37:09Z

modules/core/arrow-data/src/test/kotlin/arrow/instances/ComposedInstancesTest.kt


    testLaws(
+      InvariantLaws.laws(ComposedInvariant(Option.functor(), NonEmptyList.functor()), cf, EQ_OPTION_NEL),


Same as above, InvariantLaws should be part of FunctorLaws.

FuntorLaws are being used at line 74. Should I remove that line too?

pakoito · 2018-09-11T22:37:59Z

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

+object ContravariantLaws {
+
+    inline fun <F> laws(CF: Contravariant<F>, noinline cf: (Int) -> Kind<F, Int>, EQ: Eq<Kind<F, Int>>): List<Law> =
+        listOf(


In here ContravariantLaws imply InvariantLaws also pass, so replace this line with:

InvariantLaws.laws(...) + listOf(

You'll need to do the same in FunctorLaws.

pakoito · 2018-09-11T22:38:57Z

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

@@ -211,20 +348,20 @@ interface ComposedBifoldable<F, G> : Bifoldable<Nested<F, G>> {

  fun G(): Bifoldable<G>

-  override fun <A, B, C> BiunnestedType<F, G, A, B>.bifoldLeft(c: C, f: (C, A) -> C, g: (C, B) -> C): C = F().run {
+  override fun <A, B, C> BinestedType<F, G, A, B>.bifoldLeft(c: C, f: (C, A) -> C, g: (C, B) -> C): C = F().run {


Are we sure of this change, or was it part of the merge?

I've checked the history and this is part from our changes, related to fa50cf1

Right, right, they're correct, there's a couple of fixes to other that were wrong.

Cool, only the test left then.

pakoito

One last fix: the FunctorLaws and ContravariantLaws require also passing the InvariantLaws. With that change we're ready to go!!

pakoito · 2018-09-12T20:07:53Z

🎉

Cotel added the work in progress label Aug 4, 2018

Cotel self-assigned this Aug 4, 2018

Cotel requested a review from pakoito August 4, 2018 08:09

Cotel commented Aug 4, 2018

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pakoito reviewed Aug 4, 2018

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Cotel force-pushed the cotel-contravariance branch from 3d5b50e to e467b97 Compare August 4, 2018 11:42

pakoito reviewed Aug 4, 2018

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Cotel force-pushed the cotel-contravariance branch from 96612ab to 5c422d8 Compare August 4, 2018 16:33

pakoito reviewed Aug 5, 2018

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Cotel and others added 11 commits September 3, 2018 20:48

Adding Invariant & Contravariant typeclasses

6ee0802

Adding Invariant and Contravariant laws

e3fd111

Adding invariant and contravariant Const instances

0bf6a42

Monoid invariant instance experiment

7a7929e

Moving lambdas out to separate lines

f3c9640

Adding Invariant Laws for Const and fixing monoid Invariant laws

5c1a67a

WIP Function1 Contravariant Instance

f778843

Invent Cocomposed and Conested. I should get a PhD for this crap.

452dee8

Function1 ContravariantLaw working thanks to Paco's PhD

63f79d5

Changing contramapL to contramapC

4831ac9

LeftFunctor for Bifunctor

a8520b4

pakoito and others added 6 commits September 3, 2018 20:54

More fixes to Composed

fa50cf1

More fixes to Composed

9071d1e

Instances of Contravariant for Kleisli and Cokleisli and laws for Kle…

5d6ad08

…isli

Adding Invariant composed laws and bifunctor composed

a4f8c08

Adding Contravariant instance for coproduct

5b78e8d

Moving laws to different testlaws

3fa098c

Cotel force-pushed the cotel-contravariance branch from 941c721 to 3fa098c Compare September 3, 2018 18:55

Merge branch 'master' into cotel-contravariance

a72c26c

Fixing composed instances tests

e0643bc

pakoito reviewed Sep 11, 2018

View reviewed changes

pakoito approved these changes Sep 11, 2018

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Cotel added 2 commits September 12, 2018 19:03

Moving InvariantLaws to FunctorLaws and ContravariantLaws

c99d58f

Merge branch 'master' into cotel-contravariance

7a8474a

pakoito merged commit 0a0bec2 into master Sep 12, 2018

pakoito deleted the cotel-contravariance branch September 12, 2018 20:08

Cotel removed the work in progress label Sep 17, 2018

raulraja mentioned this pull request Nov 2, 2018

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Contravariant & Invariant #975

Contravariant & Invariant #975

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		testLaws(
		InvariantLaws.laws(ComposedInvariant(Option.functor(), NonEmptyList.functor()), cf, EQ_OPTION_NEL),

Contravariant & Invariant #975

Contravariant & Invariant #975

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