Example for using flatmap (as in #79)

example_flatmap_test.go

@@ -0,0 +1,50 @@
+package gopter_test
+
+import (
+	"reflect"
+
+	"github.com/leanovate/gopter"
+	"github.com/leanovate/gopter/gen"
+	"github.com/leanovate/gopter/prop"
+)
+
+func Example_flatmap() {
+
+	type IntPair struct {
+		Fst int
+		Snd int
+	}
+
+	// Generate a pair of integers, such that the first
+	// is in the range of 10-20 and the second in the
+	// in the range of 2k-50, depending on the value of
+	// the first.
+	genIntPair := func() gopter.Gen {
+		return gen.IntRange(10, 20).FlatMap(func(v interface{}) gopter.Gen {
+			k := v.(int)
+			return gen.IntRange(2*k, 50).Map(func(m int) IntPair {
+				return IntPair{Fst: k, Snd: m}
+			})
+		},
+			reflect.TypeOf(int(0)))
+	}
+
+	parameters := gopter.DefaultTestParameters()
+	parameters.Rng.Seed(1234) // Just for this example to generate reproducible results
+	properties := gopter.NewProperties(parameters)
+
+	properties.Property("Generate a dependent pair of integers", prop.ForAll(
+		func(p IntPair) bool {
+			a := p.Fst
+			b := p.Snd
+			return a*2 <= b
+		},
+		genIntPair(),
+	))
+
+	// When using testing.T you might just use: properties.TestingRun(t)
+	properties.Run(gopter.ConsoleReporter(false))
+
+	// Output:
+	// + Generate a dependent pair of integers: OK, passed 100 tests.
+}