Add first and last functions.

docs/cel_expressions.md

@@ -442,6 +442,35 @@ which can be accessed by indexing.
      <pre>{"testing":"value"}.marshalJSON() == "{\"testing\": \"value\"}"</pre>
     </td>
   </tr>
+  <tr>
+    <th>
+     first()
+    </th>
+    <td>
+     <pre>&lt;jsonArray&gt;.first() -> &lt;jsonObject&gt;</pre>
+    </td>
+    <td>
+     Returns the first element in the array.
+    </td>
+    <td>
+     <pre>[1, 2, 3, 4, 5].first() == 1</pre>
+    </td>
+  </tr>
+  <tr>
+    <th>
+     last()
+    </th>
+    <td>
+     <pre>&lt;jsonArray&gt;.last() -> &lt;jsonObject&gt;</pre>
+    </td>
+    <td>
+     Returns the last element in the array.
+    </td>
+    <td>
+     <pre>[1, 2, 3, 4, 5].last() == 5</pre>
+    </td>
+  </tr>
+
 </table>
 
 ## Troubleshooting CEL expressions

pkg/interceptors/cel/cel_test.go

@@ -440,6 +440,10 @@ func TestExpressionEvaluation(t *testing.T) {
 				},
 			},
 		},
+		"numbers": []int64{
+			1, 2, 3, 4, 5,
+		},
+		"emptyList": []int64{},
 	}
 
 	refParts := strings.Split(testRef, "/")
@@ -621,6 +625,47 @@ func TestExpressionEvaluation(t *testing.T) {
 			expr: "body.jsonArray.join(', ')",
 			want: types.String("one, two"),
 		},
+		{
+			name: "last element in array",
+			expr: "body.testURL.parseURL().path.split('/').last()",
+			want: types.String("to"),
+		},
+		{
+			name: "last element in empty array",
+			expr: `body.emptyList.last()`,
+			want: types.NullValue,
+		},
+		{
+			name: "last element in numeric array",
+			expr: `body.numbers.last()`,
+			want: types.Int(5),
+		},
+		{
+			name: "last element in literal array",
+			expr: "[1, 2, 3, 4, 5].last()",
+			want: types.Int(5),
+		},
+		{
+			name: "first element in literal array",
+			expr: "[1, 2, 3, 4, 5].first()",
+			want: types.Int(1),
+		},
+		{
+			name: "first element in array",
+			// Splitting gets an empty route element so this filters it out.
+			expr: "body.testURL.parseURL().path.split('/').filter(t, t.size() > 0).first()",
+			want: types.String("path"),
+		},
+		{
+			name: "first element in empty array",
+			expr: `body.emptyList.first()`,
+			want: types.NullValue,
+		},
+		{
+			name: "first element in numeric array",
+			expr: `body.numbers.first()`,
+			want: types.Int(1),
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(rt *testing.T) {

pkg/interceptors/cel/triggers.go

@@ -27,6 +27,7 @@ import (
 	"github.com/google/cel-go/cel"
 	"github.com/google/cel-go/common/types"
 	"github.com/google/cel-go/common/types/ref"
+	"github.com/google/cel-go/common/types/traits"
 	"github.com/google/cel-go/interpreter/functions"
 	"github.com/tektoncd/triggers/pkg/interceptors"
 	"sigs.k8s.io/yaml"
@@ -187,6 +188,12 @@ func (t triggersLib) CompileOptions() []cel.EnvOption {
 				cel.UnaryBinding(marshalJSON)),
 			cel.MemberOverload("marshalJSON_list", []*cel.Type{listStrDyn}, cel.StringType,
 				cel.UnaryBinding(marshalJSON))),
+		cel.Function("last",
+			cel.MemberOverload("last_list", []*cel.Type{listStrDyn}, cel.DynType,
+				cel.UnaryBinding(listLast))),
+		cel.Function("first",
+			cel.MemberOverload("first_list", []*cel.Type{listStrDyn}, cel.DynType,
+				cel.UnaryBinding(listFirst))),
 	}
 }
 
@@ -324,6 +331,28 @@ func marshalJSON(val ref.Val) ref.Val {
 	return types.String(marshaledVal)
 }
 
+func listLast(val ref.Val) ref.Val {
+	l := val.(traits.Lister)
+	sz := l.Size().Value().(int64)
+
+	if sz == 0 {
+		return types.NullValue
+	}
+
+	return l.Get(types.Int(sz - 1))
+}
+
+func listFirst(val ref.Val) ref.Val {
+	l := val.(traits.Lister)
+	sz := l.Size().Value().(int64)
+
+	if sz == 0 {
+		return types.NullValue
+	}
+
+	return l.Get(types.Int(0))
+}
+
 func max(x, y types.Int) types.Int {
 	switch x.Compare(y) {
 	case types.IntNegOne: