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Implement EitherT#leftFlatMap and EitherT#leftSemiflatMap #1790

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Sep 29, 2017
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Implement EitherT#leftFlatMap and EitherT#leftSemiflatMap #1790

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cd38406
Implement EitherT#leftFlatMap and EitherT#leftSemiflatMap
vendethiel Aug 6, 2017
9c27cf3
Add additional tests for EitherT
Sep 26, 2017
edef908
Rename tests
Sep 29, 2017
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12 changes: 12 additions & 0 deletions core/src/main/scala/cats/data/EitherT.scala
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Expand Up @@ -96,6 +96,18 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {

def leftMap[C](f: A => C)(implicit F: Functor[F]): EitherT[F, C, B] = bimap(f, identity)

def leftFlatMap[BB >: B, D](f: A => EitherT[F, D, BB])(implicit F: Monad[F]): EitherT[F, D, BB] =
EitherT(F.flatMap(value) {
case Left(a) => f(a).value
case r@Right(_) => F.pure(r.leftCast)
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Tiny nitpick, but can we change this to case r: Right => ... since we don't actually want to extract anything?

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While in practice I don't think it matters, the use of : implies type casing which is generally used in contexts where parametricity is violated: https://typelevel.org/blog/2014/11/10/why_is_adt_pattern_matching_allowed.html

I could have sworn there were some cases where : was allowed and ADT patmat wasn't but I can't remember.

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Very interesting article, thanks a lot! :)
I always thought that scalac transforms case t: Type into an isInstanceOf check whereas case Type(_) would be turned into Type.unapply(...), but that article shows that my assumption was false.
It seems then, there's no real benefit to my suggestion, so disregard.

})

def leftSemiflatMap[D](f: A => F[D])(implicit F: Monad[F]): EitherT[F, D, B] =
EitherT(F.flatMap(value) {
case Left(a) => F.map(f(a)) { d => Left(d) }
case r@Right(_) => F.pure(r.leftCast)
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Ditto, as above :)

})

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/EitherTTests.scala
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Expand Up @@ -434,4 +434,22 @@ class EitherTTests extends CatsSuite {
} yield s1 ++ s2
}

test("leftFlatMap") {
forAll { (eithert: EitherT[List, String, Int], f: String => String) =>
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Could we name this one "leftFlatMap consistent with leftMap"?

eithert.leftFlatMap(v => EitherT.left[Int](List(f(v)))) should ===(eithert.leftMap(f))
}
forAll { (eithert: EitherT[List, String, Int], f: String => EitherT[List, String, Int]) =>
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How about naming this one "leftFlatMap consistent with swap and then flatMap"?

eithert.leftFlatMap(f) should ===(eithert.swap.flatMap(a => f(a).swap).swap)
}
}

test("leftSemiflatMap") {
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Same as above for these two tests :)

forAll { (eithert: EitherT[List, String, Int], f: String => String) =>
eithert.leftSemiflatMap(v => List(f(v))) should ===(eithert.leftMap(f))
}
forAll { (eithert: EitherT[List, String, Int], f: String => List[String]) =>
eithert.leftSemiflatMap(f) should ===(eithert.swap.semiflatMap(a => f(a)).swap)
}
}

}