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Add existsM and forallM to Foldable #1784

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Aug 7, 2017
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Add existsM and forallM to Foldable #1784

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6ff5f12
Add `existsM` and `forallM` to Foldable
aryairani Aug 4, 2017
9800e4a
update Foldable.existsM/forallM to short-circuit
aryairani Aug 6, 2017
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50 changes: 50 additions & 0 deletions core/src/main/scala/cats/Foldable.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -326,6 +326,56 @@ import simulacrum.typeclass
if (p(a)) lb else Eval.False
}.value

/**
* Check whether at least one element satisfies the predicate.
*
* If there are no elements, the result is `false`.
*
* All effects are sequenced, i.e. later effects aren't skipped even if a `true` is encountered.
*
* For example:
*
* {{{
* scala> import cats.implicits._
* scala> val F = Foldable[List]
* scala> F.existsM(List(1,2,3,4))(n => Option(n <= 4))
* res0: Option[Boolean] = Some(true)
*
* scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else Option(false))
* res1: Option[Boolean] = Some(true)
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might be better to have a negative test case here, e.g.

   * scala> F.existsM(List(1,2,3,4))(n => if (n > 4) Option(true) else Option(false))
   * res1: Option[Boolean] = Some(false)

*
* scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else None)
* res2: Option[Boolean] = None
* }}}
*/
def existsM[G[_], A](fa: F[A])(p: A => G[Boolean])(implicit G: Monad[G]): G[Boolean] =
foldM(fa, false)((z, a) => G.map(p(a))(b => b || z))

/**
* Check whether all elements satisfy the predicate.
*
* If there are no elements, the result is `true`.
*
* All effects are sequenced, i.e. later effects aren't skipped even if a `false` is encountered.
*
* For example:
*
* {{{
* scala> import cats.implicits._
* scala> val F = Foldable[List]
* scala> F.forallM(List(1,2,3,4))(n => Option(n <= 4))
* res0: Option[Boolean] = Some(true)
*
* scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) Option(true) else Option(false))
* res1: Option[Boolean] = Some(false)
*
* scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) Option(false) else None)
* res2: Option[Boolean] = None
* }}}
*/
def forallM[G[_], A](fa: F[A])(p: A => G[Boolean])(implicit G: Monad[G]): G[Boolean] =
foldM(fa, true)((z, a) => G.map(p(a))(b => b && z))

/**
* Convert F[A] to a List[A].
*/
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4 changes: 3 additions & 1 deletion tests/src/test/scala/cats/tests/FoldableTests.scala
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Expand Up @@ -35,11 +35,13 @@ abstract class FoldableCheck[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
}
}

test(s"Foldable[$name].find/exists/forall/filter_/dropWhile_") {
test(s"Foldable[$name].find/exists/forall/existsM/forallM/filter_/dropWhile_") {
forAll { (fa: F[Int], n: Int) =>
fa.find(_ > n) should === (iterator(fa).find(_ > n))
fa.exists(_ > n) should === (iterator(fa).exists(_ > n))
fa.forall(_ > n) should === (iterator(fa).forall(_ > n))
fa.existsM(k => Option(k > n)) should === (Option(iterator(fa).exists(_ > n)))
fa.forallM(k => Option(k > n)) should === (Option(iterator(fa).forall(_ > n)))
fa.filter_(_ > n) should === (iterator(fa).filter(_ > n).toList)
fa.dropWhile_(_ > n) should === (iterator(fa).dropWhile(_ > n).toList)
fa.takeWhile_(_ > n) should === (iterator(fa).takeWhile(_ > n).toList)
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