Improve docs for Arbitrary

Author: ashawley

src/main/scala/org/scalacheck/Arbitrary.scala

@@ -17,48 +17,77 @@ import scala.concurrent.duration.{Duration, FiniteDuration}
 import util.Buildable
 import util.SerializableCanBuildFroms._
 
-
-sealed abstract class Arbitrary[T] extends Serializable {
-  def arbitrary: Gen[T]
-}
-
-/** Defines implicit [[org.scalacheck.Arbitrary]] instances for common types.
- *  <p>
- *  ScalaCheck
- *  uses implicit [[org.scalacheck.Arbitrary]] instances when creating properties
- *  out of functions with the `Prop.property` method, and when
- *  the `Arbitrary.arbitrary` method is used. For example, the
- *  following code requires that there exists an implicit
- *  `Arbitrary[MyClass]` instance:
- *  </p>
+/**
+ * Define an arbitrary generator for properties
+ *
+ * The [[Arbitrary]] module defines implicit generator instances for
+ * common types.
+ * 
+ * The implicit definitions of [[Arbitrary]] provide type-directed
+ * [[Gen]]s so they are available for properties, generators, or other
+ * definitions of [[Arbitrary]].
+ *
+ * ScalaCheck expects an implicit [[Arbitrary]] instance is in scope
+ * for [[Prop]]s that are defined with functions, like [[Prop$.forAll[T1,P](g1*
+ * Prop.forAll]] and so on.
+ *
+ * For instance, the definition for `Arbitrary[Boolean]` is used by
+ * `Prop.forAll` to automatically provide a `Gen[Boolean]` when one
+ * of the parameters is a `Boolean`:
  *
  *  {{{
- *    val myProp = Prop.forAll { myClass: MyClass =>
- *      ...
+ *    Prop.forAll { (b: Boolean) =>
+ *      b || !b
  *    }
+ *  }}}
  *
- *    val myGen = Arbitrary.arbitrary[MyClass]
+ * Thanks to `Arbitrary`, you don't need to provide an explicit
+ * `Gen` instance to `Prop.forAll`.  For instance, this is
+ * unnecessary:
+ *
+ *  {{{
+ *    val genBool: Gen[Boolean] = Gen.oneOf(true,false)
+ *    Prop.forAll(genBool) { (b: Boolean) =>
+ *      b || !b
+ *    }
  *  }}}
  *
- *  <p>
- *  The required implicit definition could look like this:
- *  </p>
+ * Since an arbitrary `Gen` for `Boolean` is defined in `Arbitrary`,
+ * it can be summoned with `Arbitrary.arbitrary` in cases where you
+ * need to provide one explicitly:
  *
  *  {{{
- *    implicit val arbMyClass: Arbitrary[MyClass] = Arbitrary(...)
+ *    val genBool: Gen[Boolean] = Arbitrary.arbitrary[Boolean]
+ *    val genSmallInt: Gen[Int] = Gen.choose(0, 9)
+ *    Prop.forAll(genBool, genSmallInt) { (b: Boolean, i: Int) =>
+ *      i < 10 && b || !b
+ *    }
  *  }}}
  *
- *  <p>
- *  The factory method `Arbitrary(...)` takes a generator of type
- *  `Gen[T]` and returns an instance of `Arbitrary[T]`.
- *  </p>
+ * For a user-defined `MyClass`, writing the following requires that
+ * there exists an implicit `Arbitrary[MyClass]` instance:
  *
- *  <p>
- *  The `Arbitrary` module defines implicit [[org.scalacheck.Arbitrary]]
- *  instances for common types, for convenient use in your properties and
- *  generators.
- *  </p>
+ *  {{{
+ *    Prop.forAll { (myClass: MyClass) =>
+ *      ...
+ *    }
+ *  }}}
+ *
+ * The implicit definition of `Arbitrary[MyClass]` would look like:
+ *
+ *  {{{
+ *    implicit val arbMyClass: Arbitrary[MyClass] = Arbitrary {
+ *      ...
+ *    }
+ *  }}}
+ *
+ *  The factory method `Arbitrary(...)` expects a generator of type
+ *  `Gen[MyClass]` then it will return an instance of `Arbitrary[MyClass]`.
  */
+sealed abstract class Arbitrary[T] extends Serializable {
+  def arbitrary: Gen[T]
+}
+
 object Arbitrary extends ArbitraryLowPriority with ArbitraryArities with time.JavaTimeArbitrary {
 
   /** Arbitrary instance of the Function0 type. */