fix: better tests and doc

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

v2/function/flip_test.go

@@ -22,15 +22,265 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+// TestFlip tests the Flip function with various scenarios
 func TestFlip(t *testing.T) {
+	t.Run("flips string concatenation", func(t *testing.T) {
+		// Create a curried function that formats strings
+		format := Curry2(func(a, b string) string {
+			return fmt.Sprintf("%s:%s", a, b)
+		})
 
-	x := Curry2(func(a, b string) string {
-		return fmt.Sprintf("%s:%s", a, b)
+		// Original order: a then b
+		assert.Equal(t, "a:b", format("a")("b"))
+		assert.Equal(t, "hello:world", format("hello")("world"))
+
+		// Flipped order: b then a
+		flipped := Flip(format)
+		assert.Equal(t, "b:a", flipped("a")("b"))
+		assert.Equal(t, "world:hello", flipped("hello")("world"))
 	})
 
-	assert.Equal(t, "a:b", x("a")("b"))
+	t.Run("flips numeric operations", func(t *testing.T) {
+		// Curried subtraction: subtract(a)(b) = a - b
+		subtract := Curry2(func(a, b int) int {
+			return a - b
+		})
 
-	y := Flip(x)
+		// Original: 10 - 3 = 7
+		assert.Equal(t, 7, subtract(10)(3))
 
-	assert.Equal(t, "b:a", y("a")("b"))
+		// Flipped: 3 - 10 = -7
+		flipped := Flip(subtract)
+		assert.Equal(t, -7, flipped(10)(3))
+	})
+
+	t.Run("flips division", func(t *testing.T) {
+		// Curried division: divide(a)(b) = a / b
+		divide := Curry2(func(a, b float64) float64 {
+			return a / b
+		})
+
+		// Original: 10 / 2 = 5.0
+		assert.Equal(t, 5.0, divide(10)(2))
+
+		// Flipped: 2 / 10 = 0.2
+		flipped := Flip(divide)
+		assert.Equal(t, 0.2, flipped(10)(2))
+	})
+
+	t.Run("flips with partial application", func(t *testing.T) {
+		// Curried append-like operation
+		prepend := Curry2(func(prefix, text string) string {
+			return prefix + text
+		})
+
+		// Create specialized functions with original order
+		addHello := prepend("Hello, ")
+		assert.Equal(t, "Hello, World", addHello("World"))
+		assert.Equal(t, "Hello, Go", addHello("Go"))
+
+		// Flip and create specialized functions with reversed order
+		flipped := Flip(prepend)
+		addToWorld := flipped("World")
+		assert.Equal(t, "Hello, World", addToWorld("Hello, "))
+		assert.Equal(t, "Goodbye, World", addToWorld("Goodbye, "))
+	})
+
+	t.Run("flips with different types", func(t *testing.T) {
+		// Curried function with different input types
+		repeat := Curry2(func(s string, n int) string {
+			result := ""
+			for i := 0; i < n; i++ {
+				result += s
+			}
+			return result
+		})
+
+		// Original: repeat("x")(3) = "xxx"
+		assert.Equal(t, "xxx", repeat("x")(3))
+		assert.Equal(t, "abab", repeat("ab")(2))
+
+		// Flipped: repeat(3)("x") = "xxx"
+		flipped := Flip(repeat)
+		assert.Equal(t, "xxx", flipped(3)("x"))
+		assert.Equal(t, "abab", flipped(2)("ab"))
+	})
+
+	t.Run("double flip returns to original", func(t *testing.T) {
+		// Flipping twice should return to original behavior
+		original := Curry2(func(a, b string) string {
+			return a + "-" + b
+		})
+
+		flipped := Flip(original)
+		doubleFlipped := Flip(flipped)
+
+		// Original and double-flipped should behave the same
+		assert.Equal(t, original("a")("b"), doubleFlipped("a")("b"))
+		assert.Equal(t, "a-b", doubleFlipped("a")("b"))
+	})
+
+	t.Run("flips with complex types", func(t *testing.T) {
+		type Person struct {
+			Name string
+			Age  int
+		}
+
+		// Curried function creating a person
+		makePerson := Curry2(func(name string, age int) Person {
+			return Person{Name: name, Age: age}
+		})
+
+		// Original order: name then age
+		alice := makePerson("Alice")(30)
+		assert.Equal(t, "Alice", alice.Name)
+		assert.Equal(t, 30, alice.Age)
+
+		// Flipped order: age then name
+		flipped := Flip(makePerson)
+		bob := flipped(25)("Bob")
+		assert.Equal(t, "Bob", bob.Name)
+		assert.Equal(t, 25, bob.Age)
+	})
+
+	t.Run("flips map operations", func(t *testing.T) {
+		// Curried map getter: get(map)(key)
+		get := Curry2(func(m map[string]int, key string) int {
+			return m[key]
+		})
+
+		data := map[string]int{"a": 1, "b": 2, "c": 3}
+
+		// Original: provide map first, then key
+		getValue := get(data)
+		assert.Equal(t, 1, getValue("a"))
+		assert.Equal(t, 2, getValue("b"))
+
+		// Flipped: provide key first, then map
+		flipped := Flip(get)
+		getA := flipped("a")
+		assert.Equal(t, 1, getA(data))
+
+		data2 := map[string]int{"a": 10, "b": 20}
+		assert.Equal(t, 10, getA(data2))
+	})
+
+	t.Run("flips boolean operations", func(t *testing.T) {
+		// Curried logical operation
+		implies := Curry2(func(a, b bool) bool {
+			return !a || b
+		})
+
+		// Test truth table for implication
+		assert.True(t, implies(true)(true))   // T → T = T
+		assert.False(t, implies(true)(false)) // T → F = F
+		assert.True(t, implies(false)(true))  // F → T = T
+		assert.True(t, implies(false)(false)) // F → F = T
+
+		// Flipped version (reverse implication)
+		flipped := Flip(implies)
+		assert.True(t, flipped(true)(true))   // T ← T = T
+		assert.True(t, flipped(true)(false))  // T ← F = T
+		assert.False(t, flipped(false)(true)) // F ← T = F
+		assert.True(t, flipped(false)(false)) // F ← F = T
+	})
+
+	t.Run("flips with slice operations", func(t *testing.T) {
+		// Curried slice append
+		appendTo := Curry2(func(slice []int, elem int) []int {
+			return append(slice, elem)
+		})
+
+		nums := []int{1, 2, 3}
+
+		// Original: provide slice first, then element
+		add4 := appendTo(nums)
+		result1 := add4(4)
+		assert.Equal(t, []int{1, 2, 3, 4}, result1)
+
+		// Flipped: provide element first, then slice
+		flipped := Flip(appendTo)
+		appendFive := flipped(5)
+		result2 := appendFive(nums)
+		assert.Equal(t, []int{1, 2, 3, 5}, result2)
+	})
+}
+
+// TestFlipProperties tests mathematical properties of Flip
+func TestFlipProperties(t *testing.T) {
+	t.Run("flip is involutive (flip . flip = id)", func(t *testing.T) {
+		// Flipping twice should give back the original function behavior
+		original := Curry2(func(a, b int) int {
+			return a*10 + b
+		})
+
+		flipped := Flip(original)
+		doubleFlipped := Flip(flipped)
+
+		// Test with multiple inputs
+		testCases := []struct{ a, b int }{
+			{1, 2},
+			{5, 7},
+			{0, 0},
+			{-1, 3},
+		}
+
+		for _, tc := range testCases {
+			assert.Equal(t,
+				original(tc.a)(tc.b),
+				doubleFlipped(tc.a)(tc.b),
+				"flip(flip(f)) should equal f for inputs (%d, %d)", tc.a, tc.b)
+		}
+	})
+
+	t.Run("flip preserves function composition", func(t *testing.T) {
+		// If we have f: A → B → C and g: C → D
+		// then g ∘ f(a)(b) = g(f(a)(b))
+		// and g ∘ flip(f)(b)(a) = g(flip(f)(b)(a))
+
+		f := Curry2(func(a, b int) int {
+			return a + b
+		})
+
+		g := func(n int) int {
+			return n * 2
+		}
+
+		flippedF := Flip(f)
+
+		// Compose g with f
+		composed1 := func(a, b int) int {
+			return g(f(a)(b))
+		}
+
+		// Compose g with flipped f
+		composed2 := func(a, b int) int {
+			return g(flippedF(b)(a))
+		}
+
+		// Both should give the same result
+		assert.Equal(t, composed1(3, 5), composed2(3, 5))
+		assert.Equal(t, 16, composed1(3, 5)) // (3 + 5) * 2 = 16
+	})
+}
+
+// BenchmarkFlip benchmarks the Flip function
+func BenchmarkFlip(b *testing.B) {
+	add := Curry2(func(a, b int) int {
+		return a + b
+	})
+
+	flipped := Flip(add)
+
+	b.Run("original", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_ = add(i)(i + 1)
+		}
+	})
+
+	b.Run("flipped", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_ = flipped(i)(i + 1)
+		}
+	})
 }