lang: replace stack-based parser with a recursive parser

lang/parse/fstring.c

@@ -0,0 +1,64 @@
+#include "ctx.h"
+
+#include <ivy/lang/ast.h>
+
+bool parse_fstring(struct ivy_parser *parser, struct ivy_ast_node **out)
+{
+	if (!parse_str_start(parser)) {
+		parse_trace(parser, "expected STR_START");
+		return false;
+	}
+
+	struct ivy_ast_fstring_node *fstring = create_node(IVY_AST_FSTRING);
+	bool ok = true;
+
+	while (1) {
+		if (parse_str_end(parser)) {
+			break;
+		}
+
+		struct ivy_token *tok = NULL;
+		if (parse_string(parser, &tok)) {
+			struct ivy_ast_string_node *string
+				= create_node(IVY_AST_STRING);
+			string->n_value = tok;
+			fx_queue_push_back(
+				&fstring->n_value, &string->n_base.n_entry);
+			continue;
+		}
+
+		if (parse_symbol(parser, IVY_SYM_LEFT_BRACE)) {
+			struct ivy_ast_node *expr = NULL;
+			if (!parse_expr(parser, EXPR_PARSE_NORMAL, &expr)) {
+				parse_trace(
+					parser,
+					"invalid expression in fstring");
+				ok = false;
+				break;
+			}
+
+			if (!parse_symbol(parser, IVY_SYM_RIGHT_BRACE)) {
+				parse_trace(
+					parser,
+					"expected `}` after expression in "
+					"fstring");
+				ok = false;
+				break;
+			}
+
+			fx_queue_push_back(&fstring->n_value, &expr->n_entry);
+			continue;
+		}
+
+		parse_trace(parser, "unexpected token in fstring");
+		ok = false;
+		break;
+	}
+
+	if (!ok) {
+		return ok;
+	}
+
+	*out = generic_node(fstring);
+	return ok;
+}