fn: add slice utilities

In the year of our lord 2024 we should not be writing for loops
for standard operations. Here we introduce named slice operations
not found in the golang slices package. Note all these functions
are pure.

fn/slice_test.go

@@ -0,0 +1,138 @@
+package fn
+
+import (
+	"slices"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func even(a int) bool { return a%2 == 0 }
+func odd(a int) bool  { return a%2 != 0 }
+
+func TestAll(t *testing.T) {
+	x := []int{0, 2, 4, 6, 8}
+	require.True(t, All(even, x))
+	require.False(t, All(odd, x))
+
+	y := []int{1, 3, 5, 7, 9}
+	require.False(t, All(even, y))
+	require.True(t, All(odd, y))
+
+	z := []int{0, 2, 4, 6, 9}
+	require.False(t, All(even, z))
+	require.False(t, All(odd, z))
+}
+
+func TestAny(t *testing.T) {
+	x := []int{1, 3, 5, 7, 9}
+	require.False(t, Any(even, x))
+	require.True(t, Any(odd, x))
+
+	y := []int{0, 3, 5, 7, 9}
+	require.True(t, Any(even, y))
+	require.True(t, Any(odd, y))
+
+	z := []int{0, 2, 4, 6, 8}
+	require.True(t, Any(even, z))
+	require.False(t, Any(odd, z))
+}
+
+func TestMap(t *testing.T) {
+	inc := func(i int) int { return i + 1 }
+
+	x := []int{0, 2, 4, 6, 8}
+
+	y := Map(inc, x)
+
+	z := []int{1, 3, 5, 7, 9}
+
+	require.True(t, slices.Equal(y, z))
+}
+
+func TestFilter(t *testing.T) {
+	x := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+
+	y := Filter(even, x)
+
+	require.True(t, All(even, y))
+
+	z := Filter(odd, y)
+
+	require.Zero(t, len(z))
+}
+
+func TestFoldl(t *testing.T) {
+	seed := []int{}
+	stupid := func(s []int, a int) []int { return append(s, a) }
+
+	x := []int{0, 1, 2, 3, 4}
+
+	r := Foldl(stupid, seed, x)
+
+	require.True(t, slices.Equal(x, r))
+}
+
+func TestFoldr(t *testing.T) {
+	seed := []int{}
+	stupid := func(a int, s []int) []int { return append(s, a) }
+
+	x := []int{0, 1, 2, 3, 4}
+
+	z := Foldr(stupid, seed, x)
+
+	slices.Reverse[[]int](x)
+
+	require.True(t, slices.Equal(x, z))
+}
+
+func TestFind(t *testing.T) {
+	x := []int{10, 11, 12, 13, 14, 15}
+
+	div3 := func(a int) bool { return a%3 == 0 }
+	div8 := func(a int) bool { return a%8 == 0 }
+
+	require.Equal(t, Find(div3, x), Some(12))
+
+	require.Equal(t, Find(div8, x), None[int]())
+}
+
+func TestFlatten(t *testing.T) {
+	x := [][]int{{0}, {1}, {2}}
+
+	y := Flatten(x)
+
+	require.True(t, slices.Equal(y, []int{0, 1, 2}))
+}
+
+func TestReplicate(t *testing.T) {
+	require.True(t, slices.Equal([]int{1, 1, 1, 1, 1}, Replicate(5, 1)))
+}
+
+func TestSpan(t *testing.T) {
+	x := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	lt5 := func(a int) bool { return a < 5 }
+
+	low, high := Span(lt5, x)
+
+	require.True(t, slices.Equal(low, []int{0, 1, 2, 3, 4}))
+	require.True(t, slices.Equal(high, []int{5, 6, 7, 8, 9}))
+}
+
+func TestSplitAt(t *testing.T) {
+	x := []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	fst, snd := SplitAt(5, x)
+
+	require.True(t, slices.Equal(fst, []int{0, 1, 2, 3, 4}))
+	require.True(t, slices.Equal(snd, []int{5, 6, 7, 8, 9}))
+}
+
+func TestZipWith(t *testing.T) {
+	eq := func(a, b int) bool { return a == b }
+	x := []int{0, 1, 2, 3, 4}
+	y := Replicate(5, 1)
+	z := ZipWith(eq, x, y)
+	require.True(t, slices.Equal(
+		z, []bool{false, true, false, false, false},
+	))
+}