feat: add slice.HeadTail

README.md

@@ -491,6 +491,9 @@ Some utilities that are convenient when working with slices.
 - `slice.Take[A any](as []A, n int) []A` - Take returns at most n elements.
 - `slice.Drop[A any](as []A, n int) []A` - Drop removes initial n elements. 
 - `slice.Reverse[A any](as []A) (res []A)` - Reverse creates a new slice with elements reversed.
+- `slice.HeadTail[A any](as []A) (A, []A)` - HeadTail returns head, tail. Panics with ErrHeadOfEmptySlice when slice is empty.
+- `slice.Head[A any](as []A) (a A)` - Head returns head of the slice. Panics with `ErrHeadOfEmptySlice` when slice is empty.
+- `slice.Tail[A any](as []A) []A` - Tail returns tail of the slice.
 
 We can convert a slice to a set:
 

slice/slice.go

@@ -2,6 +2,8 @@
 package slice
 
 import (
+	"errors"
+
 	"github.com/primetalk/goio/fun"
 	"github.com/primetalk/goio/maps"
 	"github.com/primetalk/goio/option"
@@ -298,3 +300,29 @@ func Union[A comparable](as []A, as2 []A) (res []A) {
 	sr := ToSet(as)
 	return append(as, FilterNot(as2, set.Contains(sr))...)
 }
+
+// ErrHeadOfEmptySlice is thrown when there's an attempt to take head of an empty slice.
+var ErrHeadOfEmptySlice = errors.New("head of empty slice")
+
+// HeadTail returns head, tail.
+// panics with ErrHeadOfEmptySlice when slice is empty.
+func HeadTail[A any](as []A) (A, []A) {
+	switch len(as) {
+	case 0:
+		panic(ErrHeadOfEmptySlice)
+	default:
+		return as[0], as[1:]
+	}
+}
+
+// Head returns head of the slice.
+// panics with ErrHeadOfEmptySlice when slice is empty.
+func Head[A any](as []A) (a A) {
+	a, _ = HeadTail(as)
+	return
+}
+
+// Tail returns tail of the slice.
+func Tail[A any](as []A) []A {
+	return Drop(as, 1)
+}

slice/slice_test.go

@@ -140,3 +140,28 @@ func TestUnion(t *testing.T) {
 	znats15 := slice.Range(0, 15)
 	assert.ElementsMatch(t, znats15, slice.Union(znats10, znats515))
 }
+
+func TestHeadTail(t *testing.T) {
+	znats5 := slice.Range(0, 5)
+	z, nats5 := slice.HeadTail(znats5)
+	assert.Equal(t, 0, z)
+	assert.ElementsMatch(t, slice.Range(1, 5), nats5)
+}
+
+func TestHeadTailPanic(t *testing.T) {
+	defer func() {
+		if r := recover(); r != slice.ErrHeadOfEmptySlice {
+			t.Errorf("expected panic ErrHeadOfEmptySlice")
+		}
+	}()
+	znats5 := slice.Range(0, 0)
+	slice.HeadTail(znats5)
+}
+
+func TestHeadAndTail(t *testing.T) {
+	znats5 := slice.Range(0, 5)
+	z := slice.Head(znats5)
+	nats5 := slice.Tail(znats5)
+	assert.Equal(t, 0, z)
+	assert.ElementsMatch(t, slice.Range(1, 5), nats5)
+}