First and last (#1067)

* Add first and last functions to the lists extension.

This adds .first() and .last() to the list extension library which can
return the first and last elements from a list.

The results are returned as Optional values.

---------

Signed-off-by: Kevin McDermott <[email protected]>

cel/cel_test.go

@@ -2743,6 +2743,10 @@ func TestOptionalValuesEval(t *testing.T) {
 				RepeatedString: []string{"greetings", "world"},
 			},
 		},
+		{expr: `[].first()`, out: types.OptionalNone},
+		{expr: `['a','b','c'].first()`, out: types.OptionalOf(types.String("a"))},
+		{expr: `[].last()`, out: types.OptionalNone},
+		{expr: `[1, 2, 3].last()`, out: types.OptionalOf(types.Int(3))},
 	}
 
 	for i, tst := range tests {

cel/library.go

@@ -260,6 +260,27 @@ func (stdLibrary) ProgramOptions() []ProgramOption {
 // be expressed with `optMap`.
 //
 //	msg.?elements.optFlatMap(e, e[?0]) // return the first element if present.
+
+// # First
+//
+// Introduced in version: 2
+//
+// Returns an optional with the first value from the right hand list, or
+// optional.None.
+//
+// [1, 2, 3].first().value() == 1
+
+// # Last
+//
+// Introduced in version: 2
+//
+// Returns an optional with the last value from the right hand list, or
+// optional.None.
+//
+// [1, 2, 3].last().value() == 3
+//
+// This is syntactic sugar for msg.elements[msg.elements.size()-1].
+
 func OptionalTypes(opts ...OptionalTypesOption) EnvOption {
 	lib := &optionalLib{version: math.MaxUint32}
 	for _, opt := range opts {
@@ -375,6 +396,39 @@ func (lib *optionalLib) CompileOptions() []EnvOption {
 	if lib.version >= 1 {
 		opts = append(opts, Macros(ReceiverMacro(optFlatMapMacro, 2, optFlatMap)))
 	}
+
+	if lib.version >= 2 {
+		opts = append(opts, Function("last",
+			MemberOverload("list_last", []*Type{listTypeV}, optionalTypeV,
+				UnaryBinding(func(v ref.Val) ref.Val {
+					list := v.(traits.Lister)
+					sz := list.Size().Value().(int64)
+
+					if sz == 0 {
+						return types.OptionalNone
+					}
+
+					return types.OptionalOf(list.Get(types.Int(sz - 1)))
+				}),
+			),
+		))
+
+		opts = append(opts, Function("first",
+			MemberOverload("list_first", []*Type{listTypeV}, optionalTypeV,
+				UnaryBinding(func(v ref.Val) ref.Val {
+					list := v.(traits.Lister)
+					sz := list.Size().Value().(int64)
+
+					if sz == 0 {
+						return types.OptionalNone
+					}
+
+					return types.OptionalOf(list.Get(types.Int(0)))
+				}),
+			),
+		))
+	}
+
 	return opts
 }
 

ext/README.md

@@ -500,6 +500,36 @@ Examples:
 	].sortBy(e, e.score).map(e, e.name)
 	== ["bar", "foo", "baz"]
 
+### Last
+
+**Introduced in the OptionalTypes library version 2**
+
+Returns an optional with the last value from the list or `optional.None` if the
+list is empty.
+
+    <list(T)>.list() -> <Optional(T)>
+
+Examples:
+
+    [1, 2, 3].list().value() == 3
+    [].last().orValue('test') == 'test'
+
+This is syntactic sugar for list[list.size()-1].
+
+### First
+
+**Introduced in the OptionalTypes library version 2**
+
+Returns an optional with the first value from the list or `optional.None` if the
+list is empty.
+
+    <list(T)>.first() -> <Optional(T)>
+
+Examples:
+
+     [1, 2, 3].first().value() == 1
+     [].first().orValue('test') == 'test'
+
 ## Sets
 
 Sets provides set relationship tests.