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Add WriterT Bifunctor #673

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Nov 19, 2015
Merged

Add WriterT Bifunctor #673

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31d86c3
Add WriterT Bifunctor
ceedubs Nov 18, 2015
0edfe13
Use fully-qualified import for Bifunctor
ceedubs Nov 19, 2015
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11 changes: 11 additions & 0 deletions core/src/main/scala/cats/data/WriterT.scala
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Original file line number Diff line number Diff line change
@@ -1,6 +1,8 @@
package cats
package data

import cats.functor.Bifunctor

final case class WriterT[F[_], L, V](run: F[(L, V)]) {
def written(implicit functorF: Functor[F]): F[L] =
functorF.map(run)(_._1)
Expand Down Expand Up @@ -31,6 +33,9 @@ final case class WriterT[F[_], L, V](run: F[(L, V)]) {
def mapBoth[M, U](f: (L, V) => (M, U))(implicit functorF: Functor[F]): WriterT[F, M, U] =
WriterT { functorF.map(run)(f.tupled) }

def bimap[M, U](f: L => M, g: V => U)(implicit functorF: Functor[F]): WriterT[F, M, U] =
mapBoth((l, v) => (f(l), g(v)))

def mapWritten[M](f: L => M)(implicit functorF: Functor[F]): WriterT[F, M, V] =
mapBoth((l, v) => (f(l), v))

Expand All @@ -51,6 +56,12 @@ private[data] sealed abstract class WriterTInstances extends WriterTInstances0 {
// on an algebra release that includes https://github.com/non/algebra/pull/82
implicit def writerTIdEq[L, V](implicit E: Eq[(L, V)]): Eq[WriterT[Id, L, V]] =
writerTEq[Id, L, V]

implicit def writerTBifunctor[F[_]:Functor]: Bifunctor[WriterT[F, ?, ?]] =
new Bifunctor[WriterT[F, ?, ?]] {
def bimap[A, B, C, D](fab: WriterT[F, A, B])(f: A => C, g: B => D): WriterT[F, C, D] =
fab.bimap(f, g)
}
}

private[data] sealed abstract class WriterTInstances0 extends WriterTInstances1 {
Expand Down
4 changes: 4 additions & 0 deletions tests/src/test/scala/cats/tests/WriterTTests.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@ package cats
package tests

import cats.data.{Writer, WriterT}
import cats.functor.Bifunctor
import cats.laws.discipline._
import cats.laws.discipline.eq._
import cats.laws.discipline.arbitrary._
Expand Down Expand Up @@ -56,6 +57,9 @@ class WriterTTests extends CatsSuite {
Functor[Writer[ListWrapper[Int], ?]]

Functor[Logged]

checkAll("WriterT[ListWrapper, ?, ?]", BifunctorTests[WriterT[ListWrapper, ?, ?]].bifunctor[Int, Int, Int, Int, Int, Int])
checkAll("Bifunctor[WriterT[ListWrapper, ?, ?]]", SerializableTests.serializable(Bifunctor[WriterT[ListWrapper, ?, ?]]))
}

// We have varying instances available depending on `F` and `L`.
Expand Down