diff --git a/lang/ast/expr/arith.c b/lang/ast/expr/arith.c
index 179529d..0934921 100644
--- a/lang/ast/expr/arith.c
+++ b/lang/ast/expr/arith.c
@@ -7,6 +7,7 @@
 #include <ivy/lang/lex.h>
 #include <ivy/lang/operator.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 static void print_operand(struct ivy_ast_node *node)
 {
@@ -448,6 +449,33 @@ struct token_parse_result arith_parse_operand(
 		return PARSE_RESULT(IVY_ERR_BAD_SYNTAX, 0);
 	}
 
+	if (state->s_sub_type == EXPR_SUBTYPE_COMPLEX_MSG) {
+		/* this is an unlabeled complex message arg */
+		if (state->s_prev_token != IVY_SYM_COMMA
+		    && state->s_prev_token != IVY_SYM_LEFT_PAREN) {
+			return PARSE_RESULT(IVY_ERR_BAD_SYNTAX, 0);
+		}
+
+		struct ivy_token *empty_label = malloc(sizeof *empty_label);
+		if (!empty_label) {
+			return PARSE_RESULT(IVY_ERR_NO_MEMORY, 0);
+		}
+		memset(empty_label, 0x0, sizeof *empty_label);
+		empty_label->t_type = IVY_TOK_LABEL;
+
+		b_queue_push_back(&state->s_labels, &empty_label->t_entry);
+
+		struct expr_parser_state *arg_expr
+			= (struct expr_parser_state *)parser_push_state(
+				ctx, IVY_AST_EXPR, 0);
+		expr_copy_terminators(state, arg_expr);
+		expr_add_terminator(arg_expr, IVY_SYM_COMMA);
+		arg_expr->s_sub_type = EXPR_SUBTYPE_COMPLEX_ARG;
+		arg_expr->s_subexpr_depth = state->s_subexpr_depth + 1;
+
+		return PARSE_RESULT(IVY_OK, PARSE_REPEAT_TOKEN);
+	}
+
 	if (state->s_prev_component == EXPR_CMP_OPERAND) {
 		return PARSE_RESULT(IVY_ERR_BAD_SYNTAX, 0);
 	}
@@ -623,11 +651,14 @@ struct token_parse_result arith_parse_left_paren(
 			= (struct expr_parser_state *)parser_push_state(
 				ctx, IVY_AST_EXPR, 0);
 
+		expr_add_terminator(msg_expr, IVY_SYM_COMMA);
 		msg_expr->s_msg = (struct ivy_ast_msg_node *)msg;
 		msg_expr->s_sub_type = EXPR_SUBTYPE_COMPLEX_MSG;
 		msg_expr->s_type = EXPR_TYPE_ARITH;
 		msg_expr->s_subexpr_depth = state->s_subexpr_depth + 1;
 
+		msg_expr->s_prev_token = state->s_prev_token = IVY_SYM_LEFT_PAREN;
+
 		return PARSE_RESULT(IVY_OK, 0);
 	}
 
@@ -648,6 +679,8 @@ not_complex_msg:
 		msg_expr->s_type = EXPR_TYPE_ARITH;
 		msg_expr->s_subexpr_depth = state->s_subexpr_depth + 1;
 
+		msg_expr->s_prev_token = state->s_prev_token = IVY_SYM_LEFT_PAREN;
+
 		return PARSE_RESULT(IVY_OK, 0);
 	}
 
@@ -974,6 +1007,7 @@ struct token_parse_result arith_parse_semicolon(
 		break;
 	case EXPR_SUBTYPE_KEYWORD_MSG:
 	case EXPR_SUBTYPE_CASCADE:
+	case EXPR_SUBTYPE_COMPLEX_ARG:
 	case EXPR_SUBTYPE_NONE:
 		break;
 	default:
@@ -1217,6 +1251,12 @@ struct token_parse_result arith_parse_comma(
 	struct expr_parser_state *state
 		= parser_get_state(ctx, struct expr_parser_state);
 
+	if (state->s_sub_type == EXPR_SUBTYPE_COMPLEX_MSG) {
+		/* this is a comma separator between two complex message args */
+		state->s_prev_token = IVY_SYM_COMMA;
+		return PARSE_RESULT(IVY_OK, 0);
+	}
+
 	if (expr_terminates_at_token(state, IVY_SYM_COMMA)) {
 		struct token_parse_result result
 			= expr_finalise_and_return(ctx, state);
@@ -1348,8 +1388,23 @@ struct token_parse_result arith_parse_label(
 		state = msg_expr;
 	}
 
-	if (state->s_sub_type == EXPR_SUBTYPE_KEYWORD_MSG
-	    || state->s_sub_type == EXPR_SUBTYPE_COMPLEX_MSG) {
+	if (state->s_sub_type == EXPR_SUBTYPE_COMPLEX_MSG) {
+		/* this label denotes the start of a complex message arg */
+		if (state->s_prev_token != IVY_SYM_COMMA
+		    && state->s_prev_token != IVY_SYM_LEFT_PAREN) {
+			return PARSE_RESULT(IVY_ERR_BAD_SYNTAX, 0);
+		}
+
+		b_queue_push_back(&state->s_labels, &tok->t_entry);
+
+		struct expr_parser_state *arg_expr
+			= (struct expr_parser_state *)parser_push_state(
+				ctx, IVY_AST_EXPR, 0);
+		expr_copy_terminators(state, arg_expr);
+		expr_add_terminator(arg_expr, IVY_SYM_COMMA);
+		arg_expr->s_sub_type = EXPR_SUBTYPE_COMPLEX_ARG;
+		arg_expr->s_subexpr_depth = state->s_subexpr_depth + 1;
+	} else if (state->s_sub_type == EXPR_SUBTYPE_KEYWORD_MSG) {
 		/* we may have just finished parsing a keyword-message argument,
 		 * and this label marks the start of a new one. store the label
 		 * and create a new argument parsing context. */
diff --git a/lang/ast/expr/expr.h b/lang/ast/expr/expr.h
index c8c725e..b71861a 100644
--- a/lang/ast/expr/expr.h
+++ b/lang/ast/expr/expr.h
@@ -33,6 +33,8 @@ enum expr_subtype {
 	EXPR_SUBTYPE_KEYWORD_ARG,
 	/* complex messages */
 	EXPR_SUBTYPE_COMPLEX_MSG,
+	/* expression delimited by commas */
+	EXPR_SUBTYPE_COMPLEX_ARG,
 	/* message cascade operation */
 	EXPR_SUBTYPE_CASCADE,
 };