Add nil keyword

ast/ast.go

@@ -20,6 +20,14 @@ func (p Position) String() string {
 	return fmt.Sprintf("%s: line %d, col %d", p.Name, p.Line, p.Col)
 }
 
+type Nil struct {
+	Position Position
+}
+
+func (n Nil) Pos() Position {
+	return n.Position
+}
+
 type Tag struct {
 	Name     Ident
 	Params   []Node

expr.go

@@ -54,6 +54,17 @@ func (t *Template) performOp(a, b reflect.Value, op ast.Operator) (any, error) {
 		default:
 			return nil, ast.PosError(op, "the in operator can only be used on strings, arrays, and slices (got %s and %s)", a.Type(), b.Type())
 		}
+	} else if !b.IsValid() {
+		if op.Value != "==" {
+			return nil, ast.PosError(op, "invalid operator for nil value (expected ==, got %s)", op.Value)
+		}
+
+		switch a.Kind() {
+		case reflect.Chan, reflect.Slice, reflect.Map, reflect.Func, reflect.Interface, reflect.Pointer:
+			return a.IsNil(), nil
+		default:
+			return nil, ast.PosError(op, "values of type %s cannot be compared against nil", a.Type())
+		}
 	} else if b.CanConvert(a.Type()) {
 		b = b.Convert(a.Type())
 	} else {

parser/salix.peg

@@ -129,7 +129,7 @@ ParamList = '(' params:(Expr ( ',' _ Expr )* )? ')' {
     return out, nil
 }
 
-Value = not:"!"? node:(MethodCall / FieldAccess / Index / String / RawString / Float / Integer / Bool / FuncCall / VariableOr / Ident / ParenExpr / Array / Map) {
+Value = not:"!"? node:(Nil / MethodCall / FieldAccess / Index / String / RawString / Float / Integer / Bool / FuncCall / VariableOr / Ident / ParenExpr / Array / Map) {
     return ast.Value{
         Node: node.(ast.Node),
         Not:  not != nil,
@@ -288,6 +288,10 @@ ArithmeticOp = ('+' / '-' / '/' / '*' / '%') {
     }, nil
 }
 
+Nil = "nil" {
+    return ast.Nil{Position: getPos(c)}, nil
+}
+
 Text = . [^#]* { return ast.Text{Data: c.text, Position: getPos(c)}, nil }
 
 _ "whitespace" ← [ \t\r\n]*

salix.go

@@ -239,6 +239,8 @@ func (t *Template) getValue(node ast.Node, local map[string]any) (any, error) {
 		return t.convertArray(node, local)
 	case ast.Assignment:
 		return node, t.handleAssignment(node, local)
+	case ast.Nil:
+		return nil, nil
 	default:
 		return nil, nil
 	}