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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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8 changes: 8 additions & 0 deletions core/src/main/scala/cats/Apply.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -194,6 +194,14 @@ trait Apply[F[_]] extends Functor[F] with InvariantSemigroupal[F] with ApplyArit
def map2Eval[A, B, Z](fa: F[A], fb: Eval[F[B]])(f: (A, B) => Z): Eval[F[Z]] =
fb.map(fb => map2(fa, fb)(f))

/**
* `if-then-else` lifted into the applicative
*/
def ifA[A](cond: F[Boolean])(ifTrue: F[A], ifFalse: F[A]): F[A] = {
def ite(cond: Boolean)(t: A, f: A) = if (cond) t else f
ap2(map(cond)(ite))(ifTrue, ifFalse)
}

/**
* Compose an `Apply[F]` and an `Apply[G]` into an `Apply[λ[α => F[G[α]]]]`.
*
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28 changes: 28 additions & 0 deletions core/src/main/scala/cats/syntax/apply.scala
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Expand Up @@ -14,6 +14,34 @@ trait ApplySyntax extends TupleSemigroupalSyntax {
new ApplyOps(fa)
}

final class IfAOps[F[_]](val fa: F[Boolean]) extends AnyVal {

/**
* A conditional lifted into the `F` context.
*
* 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] = F.ifA(fa)(ifTrue, ifFalse)
}

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

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