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Add semiflatTap and leftSemiflatTap functions to EitherT #3316

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Feb 27, 2020
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Add semiflatTap and leftSemiflatTap functions to EitherT #3316

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cdb165c
Add semiflatTap and leftSemiflatTap functions to EitherT
matwojcik Feb 24, 2020
74cd724
Replaces Monad[F] with F
matwojcik Feb 24, 2020
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6 changes: 6 additions & 0 deletions core/src/main/scala/cats/data/EitherT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -447,6 +447,12 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
}
})

def semiflatTap[C](f: B => F[C])(implicit F: Monad[F]): EitherT[F, A, B] =
semiflatMap(b => F.as(f(b), b))

def leftSemiflatTap[C](f: A => F[C])(implicit F: Monad[F]): EitherT[F, A, B] =
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What do we think about using Unit here to signify that this resulting type is going to be discarded? Might be a bit more cumbersome on the usage side, but it's just an extra .void

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@travisbrown travisbrown Feb 24, 2020 •
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We already have a flatTap on FlatMap that doesn't require Unit, and tap, tapWith, etc. on Kleisli don't constrain the output type to be Unit. My personal preference would be consistency—I think that plus slightly easier use outweighs a little extra expressivity in the types.

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@LukaJCB I first thought about it, but then saw that FlatMap[F].flatTap takes A => F[B] instead of A => F[Unit], so I wanted to be consistent with existing signature.

leftSemiflatMap(a => F.as(f(a), a))

def compare(that: EitherT[F, A, B])(implicit o: Order[F[Either[A, B]]]): Int =
o.compare(value, that.value)

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42 changes: 39 additions & 3 deletions tests/src/test/scala/cats/tests/EitherTSuite.scala
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Original file line number Diff line number Diff line change
@@ -1,13 +1,12 @@
package cats
package tests

import cats.Bifunctor
import cats.data.EitherT

import cats.data.{EitherT, State}
import cats.laws.discipline._
import cats.laws.discipline.arbitrary._
import cats.laws.discipline.SemigroupalTests.Isomorphisms
import cats.kernel.laws.discipline.{EqTests, MonoidTests, OrderTests, PartialOrderTests, SemigroupTests}

import scala.util.{Failure, Success, Try}

class EitherTSuite extends CatsSuite {
Expand Down Expand Up @@ -541,6 +540,43 @@ class EitherTSuite extends CatsSuite {
}
}

test("semiflatTap does not change the return value") {
type TestEffect[A] = State[List[Int], A]
forAll { (eithert: EitherT[TestEffect, String, Int], f: Int => TestEffect[Int], initial: List[Int]) =>
eithert.semiflatTap(v => f(v)).value.runA(initial) should ===(eithert.value.runA(initial))
}
}

test("semiflatTap runs the effect") {
type TestEffect[A] = State[List[Int], A]
forAll { (eithert: EitherT[TestEffect, String, Int], f: Int => TestEffect[Int], initial: List[Int]) =>
eithert.semiflatTap(v => f(v)).value.runS(initial) should ===(eithert.semiflatMap(f).value.runS(initial))
}
}

test("leftSemiflatTap does not change the return value") {
type TestEffect[A] = State[List[Int], A]
forAll { (eithert: EitherT[TestEffect, String, Int], f: String => TestEffect[Int], initial: List[Int]) =>
eithert.leftSemiflatTap(v => f(v)).value.runA(initial) should ===(eithert.value.runA(initial))
}
}

test("leftSemiflatTap runs the effect") {
type TestEffect[A] = State[List[Int], A]
forAll { (eithert: EitherT[TestEffect, String, Int], f: String => TestEffect[Int], initial: List[Int]) =>
eithert.leftSemiflatTap(v => f(v)).value.runS(initial) should ===(eithert.leftSemiflatMap(f).value.runS(initial))
}
}

test("leftSemiflatTap consistent with swap and the semiflatTap") {
type TestEffect[A] = State[List[Int], A]
forAll { (eithert: EitherT[TestEffect, String, Int], f: String => TestEffect[Int], initial: List[Int]) =>
eithert.leftSemiflatTap(v => f(v)).value.runA(initial) should ===(
eithert.swap.semiflatTap(v => f(v)).swap.value.runA(initial)
)
}
}

test("biSemiflatMap consistent with leftSemiflatMap and semiFlatmap") {
forAll { (eithert: EitherT[List, String, Int], fa: String => List[Int], fb: Int => List[String]) =>
eithert.biSemiflatMap(fa, fb) should ===(eithert.leftSemiflatMap(fa).semiflatMap(fb))
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