Support nil literals

boot/stage15/cl.elna

@@ -10,6 +10,8 @@
 (* - Record declarations are supported. Access is done with generated procedures,
    record_name_get_field and record_name_set_field. Record size can be queried with
    record_name_size(). *)
+(* - Program modules start with the "program;" keyword. *)
+(* - nil literal. *)
 
 type
 	ElnaLexerAction = (none, accumulate, skip, single, eof, finalize, composite, key_id, integer, delimited);
@@ -153,7 +155,8 @@ type
 		named_type_expression,
 		type_declaration,
 		module_declaration,
-		record_type_expression
+		record_type_expression,
+		null
 	);
 	InfoKind = (type_info, parameter_info, temporary_info, procedure_info);
 	TypeKind = (primitive, enumeration, _record);
@@ -1300,6 +1303,10 @@ begin
 	this^ := kind
 end;
 
+proc _nil_node_size();
+	return 4
+end;
+
 proc _integer_literal_node_size();
 	return 12
 end;
@@ -1350,6 +1357,19 @@ begin
 	return result
 end;
 
+proc _elna_parser_nil();
+var
+	result: Word;
+	literal_size: Word;
+begin
+	literal_size := _nil_node_size();
+	result := _allocate(literal_size);
+
+	_elna_lexer_skip_token();
+	_node_set_kind(result, NodeKind.null);
+	return result
+end;
+
 proc _elna_tac_integer_literal(integer_literal_node: Word);
 var
 	integer_token: Word;
@@ -1362,6 +1382,11 @@ begin
 	return _elna_tac_load_immediate(ElnaTacRegister.t0, integer_token, integer_length)
 end;
 
+proc _elna_tac_nil(nil_node: Word);
+begin
+	return _elna_tac_load_immediate(ElnaTacRegister.t0, 0, 0)
+end;
+
 proc _character_literal_node_size();
 	return 12
 end;
@@ -1587,6 +1612,8 @@ begin
 		parser_node := _elna_parser_integer_literal()
 	elsif token_kind = ElnaLexerKind.string then
 		parser_node := _elna_parser_string_literal()
+	elsif token_kind = ElnaLexerKind.null then
+		parser_node := _elna_parser_nil()
 	elsif token_kind = ElnaLexerKind.identifier then
 		parser_node := _elna_parser_variable_expression()
 	end;
@@ -1658,6 +1685,8 @@ begin
 		instruction := _elna_tac_string_literal(parser_node)
 	elsif node_kind = NodeKind.integer_literal then
 		instruction := _elna_tac_integer_literal(parser_node)
+	elsif node_kind = NodeKind.null then
+		instruction := _elna_tac_nil(parser_node)
 	else
 		instruction := _elna_tac_variable_expression(parser_node, symbol_table);
 		is_address^ := 1