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Adding biSemiflatMap to EitherT (#2269) #2274

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merged 4 commits into from
Jun 11, 2018
Merged

Adding biSemiflatMap to EitherT (#2269) #2274

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76fc39a
Adding biSemiflatMap to EitherT
Jun 1, 2018
596ad6e
Expanding leftSemiflatMap and semiflatMap
Jun 7, 2018
7a40d95
Added documentation
Jun 7, 2018
7015a82
Added EitherT types for 2.10 and 2.11
Jun 8, 2018
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3 changes: 3 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 @@ -120,6 +120,9 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
case r@Right(_) => F.pure(r.leftCast)
})

def biSemiflatMap[C, D](fa: A => F[C], fb: B => F[D])(implicit F: Monad[F]): EitherT[F, C, D] =
leftSemiflatMap(fa).semiflatMap(fb)
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Maybe it would be better to implement this in a single walk over F?

If F is e.g. List, this would traverse the list twice, and we should probably not do that ;)

Other than that, I think it's worth mentioning this PR to one of the maintainers, since there hasn't been any activity from them on this or the original issue.

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Fair point @kubukoz , so I would refactor it to be:

def biSemiflatMap[C, D](fa: A => F[C], fb: B => F[D])(implicit F: Monad[F]): EitherT[F, C, D] =
    EitherT(F.flatMap(value) {
      case Left(a) => F.map(fa(a)) { c => Left(c) }
      case Right(b) => F.map(fb(b)) { d => Right(d) }
    })

In this case it will flatMap and map it once, instead of flatMap it twice.


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

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18 changes: 18 additions & 0 deletions tests/src/test/scala/cats/tests/EitherTSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -471,4 +471,22 @@ class EitherTSuite extends CatsSuite {
}
}

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))
}
}

test("biSemiflatMap consistent with leftSemiflatMap") {
forAll { (eithert: EitherT[List, String, Int], fa: String => List[Int]) =>
eithert.biSemiflatMap(fa, List(_)) should ===(eithert.leftSemiflatMap(a => fa(a)))
}
}

test("biSemiflatMap consistent with semiflatMap") {
forAll { (eithert: EitherT[List, String, Int], fb: Int => List[String]) =>
eithert.biSemiflatMap(List(_), fb) should ===(eithert.semiflatMap(b => fb(b)))
}
}

}