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Add Foldable, Traverse and Comonad instances to WriterT #2182

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merged 16 commits into from
Mar 12, 2018
Merged

Add Foldable, Traverse and Comonad instances to WriterT #2182

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0f3b16c
Added foldLeft and foldRight to WriterT and Foldable instance
barambani Mar 3, 2018
4d1d7f7
Added tests for Foldable instance
barambani Mar 3, 2018
241f213
Added traverse. Fixed priority of the implicit search for isntances
barambani Mar 4, 2018
e68849d
Added traverse implementation to WriterT
barambani Mar 4, 2018
6729608
Added test for WriterT Traverse
barambani Mar 4, 2018
2a7f5f2
Refactored names
barambani Mar 5, 2018
454cc8f
Changed priority of Traverse instance
barambani Mar 5, 2018
4cf8304
Added comonad instance for WrtiterT and Const. Added tests
barambani Mar 7, 2018
0c2c002
Completed Comonad tests using tuple instead of Const
barambani Mar 7, 2018
e63a7e3
Refactored extract derivation to avoid implicit search
barambani Mar 7, 2018
fba0f7a
Added instances and reolution tests for Id. Removed unneeded type
barambani Mar 9, 2018
2e4b2b5
Minor refactoring
barambani Mar 9, 2018
fd077c3
Attempt to increase coverage
barambani Mar 9, 2018
a40e9f7
Removed unneeded serialization test
barambani Mar 9, 2018
b3ab664
Refactored traverse derivation
barambani Mar 12, 2018
152ee7a
Fixed improper inheritance of WriterTFunctor
barambani Mar 12, 2018
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5 changes: 3 additions & 2 deletions core/src/main/scala/cats/data/WriterT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -72,7 +72,7 @@ final case class WriterT[F[_], L, V](run: F[(L, V)]) {

def traverse[G[_], V1](f: V => G[V1])(implicit F: Traverse[F], G: Applicative[G]): G[WriterT[F, L, V1]] =
G.map(
F.traverse(run)(lv => G.product(G.pure(lv._1), f(lv._2)))
F.traverse(run)(lv => G.tupleLeft(f(lv._2), lv._1))
)(WriterT.apply)
}

Expand Down Expand Up @@ -457,10 +457,11 @@ private[data] sealed trait WriterTFoldable[F[_], L] extends Foldable[WriterT[F,
def foldRight[A, B](fa: WriterT[F, L, A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] = fa.foldRight(lb)(f)
}

private[data] sealed trait WriterTTraverse[F[_], L] extends Traverse[WriterT[F, L, ?]] with WriterTFoldable[F, L] {
private[data] sealed trait WriterTTraverse[F[_], L] extends Traverse[WriterT[F, L, ?]] with WriterTFoldable[F, L] with WriterTFunctor[F, L] {

override implicit def F0: Traverse[F]

override def map[A, B](fa: WriterT[F, L, A])(f: A => B): WriterT[F, L, B] = super[WriterTFunctor].map(fa)(f)
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I think we can omit this line if we defined map in WriterTFunctor as override def map.

See for example EitherTFunctor and EitherTMonad (although EitherTTraverse doesn't extends EitherTFunctor at the moment and neither does OptionTTraverse OptionTFunctor).

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Very true, apologies. I'm fixing it. I rushed it in. I have to stop reading those intellij errors. Do you think I should change also EitherTTraverse and OptionTTraverse accordingly ?

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I think there are probably more cases than just EitherTTraverse and OptionTTraverse which can be improved, I would leave those for another PR.

def traverse[G[_]: Applicative, A, B](fa: WriterT[F, L, A])(f: A => G[B]): G[WriterT[F, L, B]] = fa.traverse(f)
}

Expand Down