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Add an If-Then-Else operation to the Apply. #2609

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merged 8 commits into from
Jan 12, 2019
Merged

Add an If-Then-Else operation to the Apply. #2609

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b52471e
Add an If-Then-Else operation to the Apply.
diesalbla Nov 13, 2018
75a7d4d
Avoid adding methods to the type class.
diesalbla Nov 14, 2018
8480c5a
Merge branch 'master' into applicative_if
diesalbla Nov 22, 2018
fde754d
Add traits for Binary Compatibility.
diesalbla Nov 22, 2018
062d7fe
Merge master into applicative_if
diesalbla Nov 29, 2018
926423b
Attend code review comments.
diesalbla Nov 29, 2018
1d627c5
Merge branch 'master' into applicative_if
kailuowang Jan 8, 2019
2517183
Merge branch 'master' into applicative_if
kailuowang Jan 12, 2019
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1 change: 1 addition & 0 deletions core/src/main/scala/cats/syntax/all.scala
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Expand Up @@ -81,6 +81,7 @@ trait AllSyntaxBinCompat3 extends UnorderedFoldableSyntax with Function1Syntax

trait AllSyntaxBinCompat4
extends TraverseFilterSyntaxBinCompat0
with ApplySyntaxBinCompat0
with ParallelApplySyntax
with FoldableSyntaxBinCompat0
with ReducibleSyntaxBinCompat0
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41 changes: 41 additions & 0 deletions core/src/main/scala/cats/syntax/apply.scala
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Expand Up @@ -14,6 +14,47 @@ trait ApplySyntax extends TupleSemigroupalSyntax {
new ApplyOps(fa)
}

trait ApplySyntaxBinCompat0 {
implicit final def catsSyntaxIfApplyOps[F[_]](fa: F[Boolean]): IfApplyOps[F] =
new IfApplyOps[F](fa)
}

final class IfApplyOps[F[_]](private val fcond: F[Boolean]) extends AnyVal {

/**
* An `if-then-else` lifted into the `F` context.
* This function combines the effects of the `fcond` condition and of the two branches,
* in the order in which they are given.
*
* The value of the result is, depending on the value of the condition,
* the value of the first argument, or the value of the second argument.
*
* Example:
* {{{
* scala> import cats.implicits._
*
* scala> val b1: Option[Boolean] = Some(true)
* scala> val asInt1: Option[Int] = b1.ifA(Some(1), Some(0))
* scala> asInt1.get
* res0: Int = 1
*
* scala> val b2: Option[Boolean] = Some(false)
* scala> val asInt2: Option[Int] = b2.ifA(Some(1), Some(0))
* scala> asInt2.get
* res1: Int = 0
*
* scala> val b3: Option[Boolean] = Some(true)
* scala> val asInt3: Option[Int] = b3.ifA(Some(1), None)
* asInt2: Option[Int] = None
*
* }}}
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Unfortunately we can't add any methods to type classses without breaking binary compatibility. So for now, we have to settle adding to a new syntax trait. Sorry I am on my phone and couldn't point you to an example.

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@kailuowang If I understand you correctly, we can not add the method to the trait Apply[F[_]] (the type-class) but can only leave it as part of a syntactic extension implicit class?

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sorry for the delayed response. Yes, that's the idea. You already updated the PR, we also need to create a new ApplySyntaxBinCompat trait to include an implicit conversion from F[Boolean] to IfAOps, then there are a couple of more steps,

  1. here we need to add a new trait AllSyntaxBinCompat4 to extends the new ApplySyntaxBinCompat,
    let the AllSyntaxBinCompat at the top extend this AllSyntaxBinCompat4
  2. [here] https://github.com/typelevel/cats/blob/master/core/src/main/scala/cats/implicits.scala we also need to let the implicits object extends this new AllSyntaxBinCompat4
  3. then we can probably add a doctest here that import cats.implicits._ to validate that it's all hooked up.

Sorry about the gymnastics you have to go through to overcome the Scala 2.11 binary compatibility restrictions.

*/
def ifA[A](ifTrue: F[A], ifFalse: F[A])(implicit F: Apply[F]): F[A] = {
def ite(b: Boolean)(ifTrue: A, ifFalse: A) = if (b) ifTrue else ifFalse
F.ap2(F.map(fcond)(ite))(ifTrue, ifFalse)
}
}

final class ApplyOps[F[_], A](private val fa: F[A]) extends AnyVal {

/** Alias for [[Apply.productR]]. */
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