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Adding sequenceM method to Traverse #1394

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Oct 24, 2016
Merged

Adding sequenceM method to Traverse #1394

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7f18a41
Adding sequenceM method to Traverse
Sep 24, 2016
b7aa488
renaming sequenceM to traverseM
Sep 30, 2016
af2c3b7
Changing traverseM to flatTraverse and sequenceA to flatSequence
Oct 11, 2016
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18 changes: 18 additions & 0 deletions core/src/main/scala/cats/Traverse.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,24 @@ import simulacrum.typeclass
def sequence[G[_]: Applicative, A](fga: F[G[A]]): G[F[A]] =
traverse(fga)(ga => ga)

/**
* Thread all the G effects through the F structure and flatten to invert the
* structure from F[G[F[A]]] to G[F[A]].
*
* Example:
* {{{
* scala> import cats.implicits._
* scala> val x: List[Option[List[Int]]] = List(Some(List(1, 2)), Some(List(3)))
* scala> val y: List[Option[List[Int]]] = List(None, Some(List(3)))
* scala> x.sequenceM
* res0: Option[List[Int]] = Some(List(1, 2, 3))
* scala> y.sequenceM
* res1: Option[List[Int]] = None
* }}}
*/
def sequenceM[G[_], A](fgfa: F[G[F[A]]])(implicit G: Applicative[G], F: FlatMap[F]): G[F[A]] =
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sequenceA? since applicative

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@kailuowang kailuowang Sep 30, 2016 •
edited
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Well this one is corresponding traverseM, maybe both should be renamed? (if we ended up doing that we need to add the "breaking change" label. ) Also, maybe implementing this with traverseM would make the connection more obvious?

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What about flatTraverse or flatSequence, taking the convention from Scala's flatMap? I realize that this isn't well established in other languages, but both traverseM and sequenceM aren't as self explanatory

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There was a similar conversation when I added traverseM in #1020.

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Sorry for the delayed response. I like flatTraverse and flatSequence, they make sense to me.

G.map(sequence(fgfa))(F.flatten)

/**
* Behaves just like sequence, but uses [[Unapply]] to find the
* Applicative instance for G.
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