From my talk at the conference Scala.IO at Paris in October 2014

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7. 6TCFKVKQPCNVGUVUNKOKVCVKQPU
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10. 5ECNC%JGEM
ō #2TQRGTV[$CUGF6GUVKPIHTCOGYQTM
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ō #2+HQTRTQRGTV[FGHKPKVKQP
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ō TGITGUUKQPEJGEMKPI
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ō VGUVKPIYKVJNGUUEQFG
ō KPXGUVKICVGRTQRGTV[HCKNWTGU

12. (KTUVGZCORNG
val propMax = Prop.forAll { (x: Int, y: Int) =
val z = math.max(x, y)
(z == x || z == y) (z = x z = y)
}
propMax.check
+ OK, passed 100 tests.

13. (KTUVGZCORNG
7PKXGTUCN3WCPVKHKECVKQP∀
val propMax = Prop.forAll { (x: Int, y: Int) =
val z = math.max(x, y)
(z == x || z == y) (z = x z = y)
}
propMax.check
+ OK, passed 100 tests.

14. (KTUVGZCORNG
#DUVTCEVFGHKPKVKQPQH+PRWVU
7PKXGTUCN3WCPVKHKECVKQP∀
val propMax = Prop.forAll { (x: Int, y: Int) =
val z = math.max(x, y)
(z == x || z == y) (z = x z = y)
}
propMax.check
+ OK, passed 100 tests.

15. (KTUVGZCORNG
#DUVTCEVFGHKPKVKQPQH+PRWVU
val propMax = Prop.forAll { (x: Int, y: Int) =
val z = math.max(x, y)
(z == x || z == y) (z = x z = y)
}
propMax.check
+ OK, passed 100 tests.
%QORNGVGFGHKPKVKQPQHOCZ
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16. (KTUVGZCORNG
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val propMax = Prop.forAll { (x: Int, y: Int) =
val z = math.max(x, y)
(z == x || z == y) (z = x z = y)
}
propMax.check
+ OK, passed 100 tests.
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class Gen[+T] {
def apply(prms: Gen.Params): Option[T]
}
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19. 7UKPICIGPGTCVQT
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Gen.choose(0, 100).sample
res0: Option[Int] = Some(89)
Prop.forAll(Gen.choose(0, 100)) ( _ = 100 ).check
+ OK, passed 100 tests.
)GPGTCVQTKURCUUGFGZRNKEKVN[

20. 5ECNC%JGEM)GP
5ECNC%JGEMFGHKPGUCNCTIGUGVQH)GPGTCVQTU
scala Gen.
alphaChar fail nonEmptyContainerOf sequence
alphaLowerChar freqTuple nonEmptyListOf size
alphaNumChar frequency nonEmptyMap sized
alphaStr identifier numChar someOf
alphaUpperChar isInstanceOf numStr toString
asInstanceOf listOf oneOf uuid
choose listOf1 option value
chooseNum listOfN parameterized wrap
const lzy pick zip
containerOf mapOf posNum
containerOf1 mapOfN resize
containerOfN negNum resultOf

21. Custom Generators
)GPKUC/QPCFCPFECPDGWUGFKPHQTEQORTGJGPUKQP
val personGenerator = for {
name - Gen.alphaStr
age - Gen.choose(0, 150)
} yield Person(name, age)
Prop.forAll(personGenerator :| Person) { p =
p.toString ?= sPerson(${p.name},${p.age})
}
case class Person(name: String, age: Int)

22. 1VJGTUCORNGUQH%WUVQO
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for {
s - Arbitrary.arbitrary[String]
index - Gen.choose(0, s.length)
} yield (s, index)

23. 9GKIJVGFIGPGTCVQTYKVJ
HTGSWGPE[
val even = Gen.posNum[Int].suchThat(_ % 2 == 0)
val odd = Gen.posNum[Int].suchThat(_ % 2 == 1)
val number = Gen.frequency((2, even), (4, odd), (1, 0))
Prop.forAll(number) { i =
val label =
if (i == 0) zero
else if (i % 2 == 0) even
else odd
Prop.collect(label)(true)
}.check(1000)
+ OK, passed 1000 tests.
Collected test data:
57% odd
29% even
14% zero

24. 9GKIJVGFIGPGTCVQTYKVJ
HTGSWGPE[
GHKPGYGKIJVHQTGCEJ)GPGTCVQT
val even = Gen.posNum[Int].suchThat(_ % 2 == 0)
val odd = Gen.posNum[Int].suchThat(_ % 2 == 1)
val number = Gen.frequency((2, even), (4, odd), (1, 0))
Prop.forAll(number) { i =
val label =
if (i == 0) zero
else if (i % 2 == 0) even
else odd
Prop.collect(label)(true)
}.check(1000)
+ OK, passed 1000 tests.
Collected test data:
57% odd
29% even
14% zero

25. 9GKIJVGFIGPGTCVQTYKVJ
HTGSWGPE[
GHKPGYGKIJVHQTGCEJ)GPGTCVQT
val even = Gen.posNum[Int].suchThat(_ % 2 == 0)
val odd = Gen.posNum[Int].suchThat(_ % 2 == 1)
val number = Gen.frequency((2, even), (4, odd), (1, 0))
Prop.forAll(number) { i =
val label =
if (i == 0) zero
else if (i % 2 == 0) even
else odd
Prop.collect(label)(true)
}.check(1000)
+ OK, passed 1000 tests.
Collected test data:
57% odd
29% even
14% zero
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26. )GPGTCVGCNKUV
sealed trait Coin
case object Head extends Coin
case object Tail extends Coin
#TDKVTCT[UKGFNKUVQHCoin
val coin = Gen.oneOf(Head, Tail)
(KZGFUKGFNKUVQHCoin
val coins = Gen.listOf(coin)
val threeCoins = Gen.listOfN(3, coin)
scala coins.sample
res1: Option[List[Coin]] = Some(List(Head, Tail, Head, Tail, …))
scala threeCoins.sample
res2: Option[List[Coin]] = Some(List(Head, Head, Tail))

27. 5EJGFWNG
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28. 2TQITCORTQRGTVKGU
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30. 5CORNG2TQRGTV[
Prop.forAll { xs: List[Int] = xs.reverse.reverse == xs }

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Prop.forAll { xs: List[Int] = xs.reverse.reverse == xs }

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Prop.forAll { xs: List[Int] = xs.reverse.reverse == xs }
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import org.scalacheck.Prop.BooleanOperators
Prop.forAll { x: Int =
(x = 0) == {
val root = math.sqrt(x)
math.round(root * root) == x
}
}

34. 6GUVECUGTGUVTKEVKQPUECWVKQP
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5ECNC%JGEMDGCDNGVQRTQFWEGXCNWGU
val p = Prop.forAll { n: Int =
(n = 0 n % 2 == 0 n % 3 == 0) ==
n = 0
}
scala p.check
! Gave up after only 52 passed tests. 262 tests were discarded.

35. 6GUVECUGTGUVTKEVKQPUECWVKQP
$GECTGHWNVQPQVDGVQQTGUVTKEVKXGKH[QWYCPV
5ECNC%JGEMDGCDNGVQRTQFWEGXCNWGU
val p = Prop.forAll { n: Int =
(n = 0 n % 2 == 0 n % 3 == 0) ==
n = 0
}
scala p.check
! Gave up after only 52 passed tests. 262 tests were discarded.
7UGKPUVGCFCEWUVQOIGPGTCVQT
val genNum = Gen.choose(0, Int.MaxValue)
.retryUntil(_ % 2 == 0)
.retryUntil(_ % 3 == 0)
val p = Prop.forAll(genNum) { x: Int = x = 0 }
scala p.check
+ OK, passed 100 tests.

36. 6GUVECUGUKORNKHKECVKQP
val p = Prop.forAll(
Gen.choose(0, Int.MaxValue) :| x,
Gen.choose(0, Int.MaxValue) :| y
) { case (x, y) =
add(x, y) = x add(x, y) = y
}
scala p.check
! Falsified after 0 passed tests.
x: 195667048
x_ORIGINAL: 1350546201
y: 1951816600
y_ORIGINAL: 2004457204

37. 6GUVECUGUKORNKHKECVKQP
val p = Prop.forAll(
Gen.choose(0, Int.MaxValue) :| x,
Gen.choose(0, Int.MaxValue) :| y
) { case (x, y) =
add(x, y) = x add(x, y) = y
}
scala p.check
! Falsified after 0 passed tests.
x: 195667048
x_ORIGINAL: 1350546201
y: 1951816600
y_ORIGINAL: 2004457204
195667048 + 1951816600 == Int.MaxValue + 1

38. 6GUVECUGUKORNKHKECVKQP
val p = Prop.forAll(
Gen.choose(0, Int.MaxValue) :| x,
Gen.choose(0, Int.MaxValue) :| y
) { case (x, y) =
add(x, y) = x add(x, y) = y
}
scala p.check
! Falsified after 0 passed tests.
x: 195667048
x_ORIGINAL: 1350546201
y: 1951816600
y_ORIGINAL: 2004457204
195667048 + 1951816600 == Int.MaxValue + 1
VGUVECUGUKORNKHKECVKQPOGEJCPKUOJCUHQWPFVJG
UKORNGUVVGUVECUGVJCVOCMGVJGVGUVHCKN

39. 6GUVECUGUKORNKHKECVKQP
val genListPosNum = Gen.listOf(Gen.choose(1, Int.MaxValue))
val p = Prop.forAll(genListPosNum :| positive numbers) { xs: List[Int] =
xs.sorted.foldLeft(Option(0)) {
case (mp, x) = mp.flatMap(p = if (p x) Some(x) else None)
}.isDefined
}
scala p.check
! Falsified after 6 passed tests.
positive numbers: List(1, 1)
positive numbers_ORIGINAL: List(1901315974, 963110019, 991102462)

40. 6GUVECUGUKORNKHKECVKQP
val genListPosNum = Gen.listOf(Gen.choose(1, Int.MaxValue))
val p = Prop.forAll(genListPosNum :| positive numbers) { xs: List[Int] =
xs.sorted.foldLeft(Option(0)) {
case (mp, x) = mp.flatMap(p = if (p x) Some(x) else None)
}.isDefined
}
scala p.check
! Falsified after 6 passed tests.
positive numbers: List(1, 1)
positive numbers_ORIGINAL: List(1901315974, 963110019, 991102462)

41. 6GUVECUGUKORNKHKECVKQP
val genListPosNum = Gen.listOf(Gen.choose(1, Int.MaxValue))
val p = Prop.forAll(genListPosNum :| positive numbers) { xs: List[Int] =
xs.sorted.foldLeft(Option(0)) {
case (mp, x) = mp.flatMap(p = if (p x) Some(x) else None)
}.isDefined
}
scala p.check
! Falsified after 6 passed tests.
positive numbers: List(1, 1)
positive numbers_ORIGINAL: List(1901315974, 963110019, 991102462)

42. +PVGITCVKQPYKVJ5RGEU
class SampleTest extends Specification with ScalaCheck {
Integer should {
have a max method in {
Prop.forAll { (x: Int, y: Int) =
val z = math.max(x, y)
(z == x || z == y) (z = x z = y)
}
}
}

43. 5EJGFWNG
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GHKPKPICRTQITCORTQRGTVKGU
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6JGECUGQH/QPQKF

44. #NIGDTCKEFGUKIP
ō #NIGDTCKEFGUKIPKUWUGFVQFGHKPGNKDTCTKGUQH
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ō +PCNIGDTCKEFGUKIPYGHQEWUQPHWPEVKQPU
QRGTCVKPIQXGTFCVCV[RGUCPFNCYUVJCVFGHKPG
VJGUGHWPEVKQPU
ō 6JGCNIGDTCQHCNKDTCT[FQGUPņVFGRGPFQPVJG
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45. #NIGDTCKEFGUKIP
ō 5GXGTCNV[RGENCUUGUGZKUVUYKVJCRTQRGTN[FGHKPGF
CNIGDTC
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ō /QUVRQRWNCTV[RGENCUUGUKPENWFG/QPCFU
/QPQKFU(WPEVQT#RRNKECVKXGUŎ
ō 6JGHWPEVKQPCNRTQITCOOKPINKDTCT[5ECNCFGHKPGU
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46. 5EJGFWNG
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+PVTQFWEKPI#NIGDTCKEGUKIP
6JGECUGQH/QPQKF

47. 6JGECUGQH/QPQKF
ō #6[RG#KUC/QPQKFKHVJGTGKU
ō #PCUUQEKCVKXGENQUGFDKPCT[QRGTCVKQP
ō #PKFGPVKV[GNGOGPV
QTGTQ
ō #UCPGZCORNG+PVKUC/QPQKFYKVJ
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QRGTCVKQPCPFCUKFGPVKV[GNGOGPV

48. GHKPKVKQPQHC/QPQKF
trait Monoid[A] {
def op(a1: A, a2: A): A
def zero: A
object Laws {
import scalaio.scalacheck.Eq
import Eq._
def monoidIsAssociative(in: Gen[(A, A, A)])(implicit eq: Eq[A]): Prop =
Prop.forAll(in) { case (x, y, z) =
op(op(x, y), z) === op(x, op(y, z))
}
def monoidHaveZero(in: Gen[A])(implicit eq: Eq[A]): Prop =
Prop.forAll(in) { a = (op(a, zero) === a) (op(zero, a) === a)}
}
}

49. 'ZCORNGUQH/QPQKF
val stringMonoid = new Monoid[String] {
override def zero =
override def op(a1: String, a2: String) = a1 + a2
}
def listMonoid[A] = new Monoid[List[A]] {
override def op(a1: List[A], a2: List[A]) = a1 ++ a2
override def zero = List.empty[A]
}
val intAdditionMonoid = new Monoid[Int] {
override def op(a1: Int, a2: Int) = a1 + a2
override def zero = 0
}
val intMultiplicationMonoid = new Monoid[Int] {
override def op(a1: Int, a2: Int) = a1 * a2
override def zero = 1
}
def endoMonoid[A] = new Monoid[A = A] {
override def op(a1: A = A, a2: A = A) = { a: A = a1(a2(a)) }
override def zero = identity
}

50. %QODKPCVQTUHQT/QPQKFU
def productMonoid[A, B](a: Monoid[A], b: Monoid[B]): Monoid[(A, B)] = new Monoid[(A, B)] {
override def op(a1: (A, B), a2: (A, B)) = (a.op(a1._1, a2._1), b.op(a1._2, a2._2))
override def zero = (a.zero, b.zero)
}
def mapMergeMonoid[K, V](V: Monoid[V]): Monoid[Map[K, V]] = new Monoid[Map[K, V]] {
override def op(a1: Map[K, V], a2: Map[K, V]) = a1.map {
case (k, v) = (k, V.op(v, a2.get(k) getOrElse V.zero))
}
override def zero = Map()
}
9GECPETGCVGPGY/QPQKFUHTQOGZKUVKPIKPUVCPEGU

51. %QODKPCVQTUHQT/QPQKFU
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RTGVV[YGNNUWKVGFVQRGTHQTOKPRCTCNNGNQTKP
FKUVTKDWVGFYC[
def parReduce[A](as: List[A])
(implicit M: Monoid[A]): A = ???
def parMapReduce[A, B](as: List[A], f: A = B)
(implicit M: Monoid[B]): B = ???

52. 6GUVKPI/QPQKF.CYU
String Monoid should {
be associative in {
val genTriple = arbitrary[(String, String, String)]
Monoids.stringMonoid.Laws.monoidIsAssociative(genTriple)
}
have a zero in {
Monoids.stringMonoid.Laws.monoidHaveZero(arbitrary[String])
}
}

53. 'SV[RGENCUUFGHKPKVKQP
trait Eq[A] {
def equals(a1: A, a2: A): Prop
}
object Eq {
implicit def orderedEq[A: Ordering]: Eq[A] = new Eq[A] {
override def equals(a1: A, a2: A) = implicitly[Ordering[A]].equiv(a1, a2)
}
implicit def functionEq[A: Arbitrary, B]: Eq[A = B] = new Eq[A = B] {
override def equals(f1: (A) = B, f2: (A) = B) =
Prop.forAll(arbitrary[A]) { a = f1(a) == f2(a) }
}
implicit def listEq[A]: Eq[List[A]] = new Eq[List[A]] {
override def equals(a1: List[A], a2: List[A]) = a1 == a2
}
implicit class EqOps[A: Eq](a1: A) {
def ===(a2: A): Prop = implicitly[Eq[A]].equals(a1, a2)
}
}

54. 6QIQHWTVJGT
Functional Programming in Scala
Paul Chiusano and Rúnar Bjarnason
Manning
ScalaCheck
The Definitive Guide
Richard Nilson
artima

55. 3WGUVKQPU!