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Add EitherT.cond #1481

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Dec 16, 2016
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Add EitherT.cond #1481

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22 changes: 22 additions & 0 deletions core/src/main/scala/cats/data/EitherT.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -293,6 +293,28 @@ private[data] trait EitherTFunctions {
def apply[E, A](opt: Option[A], ifNone: => E)(implicit F: Applicative[F]): EitherT[F, E, A] =
EitherT(F.pure(Either.fromOption(opt, ifNone)))
}

/** If the condition is satisfied, return the given `A` in `Right`
* lifted into the specified `Applicative`, otherwise, return the
* given `E` in `Left` lifted into the specified `Applicative`.
*
* {{{
* scala> import cats.Id
* scala> import cats.data.EitherT
* scala> val userInput = "hello world"
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I think we need to import cats.data.EitherT and (for the Applicative[Future]) cats.implicits._, scala.concurrent.Future and an ExecutionContext. Newlines also need a | at the start of the line (see for example the toNested example).

It seems that this cond example wasn't tested by travis (if I haven't missed it), maybe because the format wasn't right ?

The easiest way I found to quickly test a doctest example is to paste it in a fresh console (otherwise it is easy to forget some of the imports).

Apart of the example 👍

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I had the doc example code running correctly, but the travis build was spitting out errors that the imports weren't needed. I actually went back and removed the imports just for the sake of appeasing Travis!

I'll push a new commit to see what happens. If it all goes well, I can squash before a merge.

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I pushed a new commit, but when I run the tests locally I get:

> test:compileIncremental
[info] Compiling 26 Scala sources to /Users/andy/git/cats/core/.jvm/target/scala-2.12/test-classes...
[error] /Users/andy/git/cats/core/.jvm/target/scala-2.12/src_managed/test/cats/data/EitherTDoctest.scala:217: Unused import
[error]     import cats.data.EitherT
[error]                      ^
[error] /Users/andy/git/cats/core/.jvm/target/scala-2.12/src_managed/test/cats/data/EitherTDoctest.scala:219: Unused import
[error]     import cats.implicits._
[error]                           ^
[error] /Users/andy/git/cats/core/.jvm/target/scala-2.12/src_managed/test/cats/data/EitherTDoctest.scala:221: Unused import
[error]     import scala.concurrent.Future
[error]                             ^
[error] /Users/andy/git/cats/core/.jvm/target/scala-2.12/src_managed/test/cats/data/EitherTDoctest.scala:223: Unused import
[error]     import scala.concurrent.ExecutionContext.Implicits.global
[error]                                                        ^
[error] four errors found
[error] (coreJVM/test:compileIncremental) Compilation failed
[error] Total time: 12 s, completed Dec 6, 2016 9:09:21 AM
>

* scala> EitherT.cond[Id](
* | userInput.forall(_.isDigit) && userInput.size == 10,
* | userInput,
* | "The input does not look like a phone number")
* res0: EitherT[Id, String, String] = EitherT(Left(The input does not look like a phone number))
* }}}
*/
final def cond[F[_]]: CondPartiallyApplied[F] = new CondPartiallyApplied

final class CondPartiallyApplied[F[_]] private[EitherTFunctions] {
def apply[E, A](test: Boolean, right: => A, left: => E)(implicit F: Applicative[F]): EitherT[F, E, A] =
EitherT(F.pure(Either.cond(test, right, left)))
}
}

private[data] abstract class EitherTInstances extends EitherTInstances1 {
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6 changes: 6 additions & 0 deletions tests/src/test/scala/cats/tests/EitherTTests.scala
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Expand Up @@ -156,6 +156,12 @@ class EitherTTests extends CatsSuite {
}
}

test("cond") {
forAll { (cond: Boolean, s: String, i: Int) =>
Either.cond(cond, s, i) should === (EitherT.cond[Id](cond, s, i).value)
}
}

test("isLeft negation of isRight") {
forAll { (eithert: EitherT[List, String, Int]) =>
eithert.isLeft should === (eithert.isRight.map(! _))
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