diff --git a/builtin/builtin.go b/builtin/builtin.go
index cc6f197c..1b1f7ff9 100644
--- a/builtin/builtin.go
+++ b/builtin/builtin.go
@@ -103,6 +103,20 @@ var Builtins = []*Function{
 		Predicate: true,
 		Types:     types(new(func([]any, func(any, any) any, any) any)),
 	},
+	{
+		Name: "flatten",
+		Func: func(args ...any) (any, error) {
+			if len(args) != 1 {
+				return nil, fmt.Errorf("invalid number of arguments for flatten (expected 1, got %d)", len(args))
+			}
+			res, err := flatten(args[0])
+
+			return any(res), err
+		},
+		Validate: func(args []reflect.Type) (reflect.Type, error) {
+			return validateAggregateFunc("flatten", args)
+		},
+	},
 	{
 		Name: "len",
 		Fast: Len,
diff --git a/builtin/builtin_test.go b/builtin/builtin_test.go
index 97f24989..a61c026f 100644
--- a/builtin/builtin_test.go
+++ b/builtin/builtin_test.go
@@ -152,6 +152,10 @@ func TestBuiltin(t *testing.T) {
 		{`reduce([], 5, 0)`, 0},
 		{`concat(ArrayOfString, ArrayOfInt)`, []any{"foo", "bar", "baz", 1, 2, 3}},
 		{`concat(PtrArrayWithNil, [nil])`, []any{42, nil}},
+		{`flatten([[1, 2], [3, 4]])`, []any{1, 2, 3, 4}},
+		{`flatten([[1, 2], [3, 4], "5"])`, []any{1, 2, 3, 4, "5"}},
+		{`flatten([1, 2, 3, 4])`, []any{1, 2, 3, 4}},
+		{`flatten([1,[2,[3,4]]])`, []any{1, 2, 3, 4}},
 	}
 
 	for _, test := range tests {
@@ -236,6 +240,8 @@ func TestBuiltin_errors(t *testing.T) {
 		{`now(nil)`, "invalid number of arguments (expected 0, got 1)"},
 		{`date(nil)`, "interface {} is nil, not string (1:1)"},
 		{`timezone(nil)`, "cannot use nil as argument (type string) to call timezone (1:10)"},
+		{`flatten(nil)`, "invalid argument for flatten (type %!s(<nil>)) (1:1)"},
+		{`flatten([1, 2], [3, 4])`, "invalid number of arguments for flatten (expected 1, got 2) (1:1)"},
 	}
 	for _, test := range errorTests {
 		t.Run(test.input, func(t *testing.T) {
diff --git a/builtin/lib.go b/builtin/lib.go
index e3cd61b9..88f75cd0 100644
--- a/builtin/lib.go
+++ b/builtin/lib.go
@@ -359,3 +359,25 @@ func median(args ...any) ([]float64, error) {
 	}
 	return values, nil
 }
+
+func flatten(args ...any) ([]any, error) {
+	var values []any
+
+	for _, arg := range args {
+		rv := reflect.ValueOf(deref.Deref(arg))
+		switch rv.Kind() {
+		case reflect.Array, reflect.Slice:
+			size := rv.Len()
+			for i := 0; i < size; i++ {
+				elems, err := flatten(rv.Index(i).Interface())
+				if err != nil {
+					return nil, err
+				}
+				values = append(values, elems...)
+			}
+		default:
+			values = append(values, arg)
+		}
+	}
+	return values, nil
+}
diff --git a/checker/checker_test.go b/checker/checker_test.go
index fad56eb2..be66db61 100644
--- a/checker/checker_test.go
+++ b/checker/checker_test.go
@@ -100,6 +100,7 @@ func TestCheck(t *testing.T) {
 		{"count(1..30, {# % 3 == 0}) > 0"},
 		{"map(1..3, {#}) == [1,2,3]"},
 		{"map(1..3, #index) == [0,1,2]"},
+		{"map(1..3, [#index]) == [[0],[1],[2]]"},
 		{"map(filter(ArrayOfFoo, {.Bar.Baz != ''}), {.Bar}) == []"},
 		{"filter(Any, {.AnyMethod()})[0] == ''"},
 		{"Time == Time"},
diff --git a/docgen/docgen.go b/docgen/docgen.go
index 1844f23b..66209e20 100644
--- a/docgen/docgen.go
+++ b/docgen/docgen.go
@@ -38,16 +38,17 @@ type Type struct {
 var (
 	Operators = []string{"matches", "contains", "startsWith", "endsWith"}
 	Builtins  = map[Identifier]*Type{
-		"true":   {Kind: "bool"},
-		"false":  {Kind: "bool"},
-		"len":    {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}}, Return: &Type{Kind: "int"}},
-		"all":    {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
-		"none":   {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
-		"any":    {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
-		"one":    {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
-		"filter": {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "array", Type: &Type{Kind: "any"}}},
-		"map":    {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "array", Type: &Type{Kind: "any"}}},
-		"count":  {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "int"}},
+		"true":    {Kind: "bool"},
+		"false":   {Kind: "bool"},
+		"len":     {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}}, Return: &Type{Kind: "int"}},
+		"all":     {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
+		"none":    {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
+		"any":     {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
+		"one":     {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "bool"}},
+		"filter":  {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "array", Type: &Type{Kind: "any"}}},
+		"map":     {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "array", Type: &Type{Kind: "any"}}},
+		"count":   {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}, {Kind: "func"}}, Return: &Type{Kind: "int"}},
+		"flatten": {Kind: "func", Arguments: []*Type{{Kind: "array", Type: &Type{Kind: "any"}}}, Return: &Type{Kind: "array", Type: &Type{Kind: "any"}}},
 
 		"trim":       {Kind: "func", Arguments: []*Type{{Name: "string", Kind: "string"}, {Name: "cutstr", Kind: "string"}}, Return: &Type{Name: "string", Kind: "string"}},
 		"trimPrefix": {Kind: "func", Arguments: []*Type{{Name: "string", Kind: "string"}, {Name: "cutstr", Kind: "string"}}, Return: &Type{Name: "string", Kind: "string"}},