90 lines
2.6 KiB
Scala
90 lines
2.6 KiB
Scala
package green.thisfieldwas.embracingnondeterminism.stdlib
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import green.thisfieldwas.embracingnondeterminism.data.{Monoid, MonoidLaws, SemigroupLaws}
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import green.thisfieldwas.embracingnondeterminism.util._
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import org.scalacheck.Arbitrary.arbitrary
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import org.scalacheck.{Arbitrary, Gen}
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/** Proves that Scala's List conforms to the following typeclasses:
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* - Functor
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* - Monad
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* - Semigroup
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* - Monoid
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*/
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class ListSpec extends Laws with SemigroupLaws with MonoidLaws {
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property("List.map() preserves identity functions") {
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forAll(arbitrary[List[Int]]) { fa =>
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fa.map(identity) mustBe identity(fa)
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}
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}
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property("List.map() preserves function composition") {
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forAll(for {
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fa <- arbitrary[List[Double]]
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f <- arbitrary[Double => String]
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g <- arbitrary[String => Int]
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} yield (fa, f, g)) { case (fa, f, g) =>
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fa.map(g compose f) mustBe fa.map(f).map(g)
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}
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}
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property(s"List.flatMap() preserves left identity") {
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forAll(for {
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a <- arbitrary[Int]
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h <- arbitrary[Int => List[String]]
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} yield (a, h)) { case (a, h) =>
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val leftIdentity = (x: Int) => List(x).flatMap(h)
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leftIdentity(a) mustBe h(a)
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}
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}
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property(s"List.flatMap() preserves right identity") {
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forAll(for {
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a <- arbitrary[Int]
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h <- arbitrary[Int => List[String]]
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} yield (a, h)) { case (a, h) =>
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val rightIdentity = h(_: Int).flatMap(List(_))
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rightIdentity(a) mustBe h(a)
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}
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}
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property(s"List.flatMap() is associative") {
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forAll(for {
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a <- arbitrary[Double]
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f <- arbitrary[Double => List[String]]
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g <- arbitrary[String => List[Int]]
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h <- arbitrary[Int => List[Boolean]]
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} yield (a, f, g, h)) { case (a, f, g, h) =>
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val assocLeft = f(_: Double).flatMap(g).flatMap(h)
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val assocRight = f(_: Double).flatMap(g(_).flatMap(h))
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assocLeft(a) mustBe assocRight(a)
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}
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}
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implicit def arbitraryList[A: Arbitrary]: Arbitrary[List[A]] = Arbitrary {
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for {
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length <- Gen.sized(Gen.choose(0, _))
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list <- Gen.listOfN(length, arbitrary[A])
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} yield list
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}
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/** List forms a Semigroup under concatenation and a Monoid with an identity
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* value of an empty List().
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*
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* @tparam A
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* Any type held by the List.
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* @return
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* The Semigroup instance for List.
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*/
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implicit def listMonoid[A: Arbitrary]: Monoid[List[A]] = new Monoid[List[A]] {
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override def empty: List[A] = List()
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override def combine(left: List[A], right: List[A]): List[A] = left ++ right
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}
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checkSemigroupLaws[List[Int]]()
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checkMonoidLaws[List[Int]]()
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}
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