assert: Include assert message in error message if there is one

flamingo/grammar/assert.h

@@ -9,7 +9,10 @@
 #include "../val.h"
 
 static int parse_assert(flamingo_t* flamingo, TSNode node) {
-	assert(ts_node_child_count(node) == 2);
+	size_t const child_count = ts_node_named_child_count(node);
+	assert(child_count == 1 || child_count == 2);
+
+	// Get test expression.
 
 	TSNode const test_node = ts_node_child_by_field_name(node, "test", 4);
 	char const* const type = ts_node_type(test_node);
@@ -18,6 +21,21 @@ static int parse_assert(flamingo_t* flamingo, TSNode node) {
 		return error(flamingo, "expected expression for test, got %s", type);
 	}
 
+	// Get message.
+
+	TSNode const msg_node = ts_node_child_by_field_name(node, "msg", 3);
+	bool const has_msg = !ts_node_is_null(msg_node);
+
+	if (has_msg) {
+		char const* const msg_type = ts_node_type(msg_node);
+
+		if (strcmp(msg_type, "expression") != 0) {
+			return error(flamingo, "expected expression for message, got %s", msg_type);
+		}
+	}
+
+	// Evaluate the test expression.
+
 	flamingo_val_t* val = NULL;
 
 	if (parse_expr(flamingo, test_node, &val, NULL) < 0) {
@@ -28,16 +46,52 @@ static int parse_assert(flamingo_t* flamingo, TSNode node) {
 		return error(flamingo, "expected boolean value for test, got %s", val_type_str(val));
 	}
 
+	// If the test succeeded, exit now.
+
+	if (val->boolean.boolean) {
+		val_decref(val);
+		return 0;
+	}
+
+	// Otherwise, start by getting a string for the test expression.
+
 	size_t const start = ts_node_start_byte(test_node);
 	size_t const end = ts_node_end_byte(test_node);
 
 	char const* const test_str = flamingo->src + start;
 	size_t const test_size = end - start;
 
-	if (!val->boolean.boolean) {
-		return error(flamingo, "assertion test '%.*s' failed", (int) test_size, test_str);
+	// If we have a message, evaluate it and include it in the error message.
+
+	if (has_msg) {
+		flamingo_val_t* msg_val = NULL;
+
+		if (parse_expr(flamingo, msg_node, &msg_val, NULL) < 0) {
+			val_decref(val);
+			return error(flamingo, "assertion test '%.*s' failed, and failed to parse message expression in doing so", (int) test_size, test_str);
+		}
+
+		char* msg_repr = NULL;
+
+		if (repr(flamingo, msg_val, &msg_repr) < 0) {
+			val_decref(val);
+			val_decref(msg_val);
+
+			return error(flamingo, "assertion test '%.*s' failed, and failed to represent message expression in doing so", (int) test_size, test_str);
+		}
+
+		val_decref(val);
+		val_decref(msg_val);
+
+		// Finally, actually error.
+
+		int const rv = error(flamingo, "assertion test '%.*s' failed: %s", (int) test_size, test_str, msg_repr);
+		free(msg_repr);
+
+		return rv;
 	}
 
-	val_decref(val);
-	return 0;
+	// Otherwise, just spit out something generic.
+
+	return error(flamingo, "assertion test '%.*s' failed", (int) test_size, test_str);
 }