Add distinct() function to lists extension.

ext/lists.go

@@ -40,6 +40,22 @@ var comparableTypes = []*cel.Type{
 // Lists returns a cel.EnvOption to configure extended functions for list manipulation.
 // As a general note, all indices are zero-based.
 //
+// # Distinct
+//
+// Introduced in version: 2
+//
+// Returns the distinct elements of a list. If the element type is not comparable
+// or the element types are not the same, the function will produce an error.
+//
+//	<list(T)>.distinct() -> <list(T)>
+//	T in {int, uint, double, bool, duration, timestamp, string, bytes}
+//
+// Examples:
+//
+//	[1, 2, 2, 3, 3, 3].distinct() // return [1, 2, 3]
+//	["b", "b", "c", "a", "c"].distinct() // return ["b", "c", "a"]
+//	[1, "b", 2, "b"].distinct() // return [1, "b", 2]
+//
 // # Range
 //
 // Introduced in version: 2
@@ -269,6 +285,19 @@ func (lib listsLib) CompileOptions() []cel.EnvOption {
 				}),
 			),
 		))
+		opts = append(opts, cel.Function("distinct",
+			cel.MemberOverload("list_distinct",
+				[]*cel.Type{listType}, listType,
+				cel.FunctionBinding(func(args ...ref.Val) ref.Val {
+					list := args[0].(traits.Lister)
+					result, err := distinctList(list)
+					if err != nil {
+						return types.WrapErr(err)
+					}
+					return result
+				}),
+			),
+		))
 	}
 
 	return opts
@@ -371,6 +400,24 @@ func sortList(list traits.Lister) (ref.Val, error) {
 	return types.DefaultTypeAdapter.NativeToValue(sorted), nil
 }
 
+func distinctList(list traits.Lister) (ref.Val, error) {
+	listLength := list.Size().(types.Int)
+	if listLength == 0 {
+		return list, nil
+	}
+	uniqueList := make([]ref.Val, 0, listLength)
+	seen := make(map[ref.Val]bool, listLength)
+	for i := types.IntZero; i < listLength; i++ {
+		val := list.Get(i)
+		if !seen[val] {
+			uniqueList = append(uniqueList, val)
+			seen[val] = true
+		}
+	}
+
+	return types.DefaultTypeAdapter.NativeToValue(uniqueList), nil
+}
+
 func templatedOverloads(types []*cel.Type, template func(t *cel.Type) cel.FunctionOpt) []cel.FunctionOpt {
 	overloads := make([]cel.FunctionOpt, len(types))
 	for i, t := range types {

ext/lists_test.go

@@ -50,9 +50,16 @@ func TestLists(t *testing.T) {
 		{expr: `[1,2,[],[],[3,4]].flatten() == [1,2,3,4]`},
 		{expr: `[1,[2,[3,4]]].flatten(2) == [1,2,3,4]`},
 		{expr: `[1,[2,[3,[4]]]].flatten(-1) == [1,2,3,4]`, err: "level must be non-negative"},
+		{expr: `[].sort() == []`},
+		{expr: `[1].sort() == [1]`},
 		{expr: `[4, 3, 2, 1].sort() == [1, 2, 3, 4]`},
 		{expr: `["d", "a", "b", "c"].sort() == ["a", "b", "c", "d"]`},
 		{expr: `["d", 3, 2, "c"].sort() == ["a", "b", "c", "d"]`, err: "list elements must have the same type"},
+		{expr: `[].distinct() == []`},
+		{expr: `[1].distinct() == [1]`},
+		{expr: `[-2, 5, -2, 1, 1, 5, -2, 1].distinct() == [-2, 5, 1]`},
+		{expr: `['c', 'a', 'a', 'b', 'a', 'b', 'c', 'c'].distinct() == ['c', 'a', 'b']`},
+		{expr: `[1, 2.0, "c", 3, "c", 1].distinct() == [1, 2.0, "c", 3]`},
 	}
 
 	env := testListsEnv(t)