From b287b1d9b61474f5d914cbc3b813f1d95e93d8d3 Mon Sep 17 00:00:00 2001
From: Tangent Wantwight <tangent128@gmail.com>
Date: Fri, 4 Aug 2023 17:43:11 -0400
Subject: [PATCH] WIP hack combinator to try and improve error messages.

---
 peg.js | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/peg.js b/peg.js
index 95a8231..d3c1171 100644
--- a/peg.js
+++ b/peg.js
@@ -183,3 +183,26 @@ Peg.End = () => {
   }).expects("<eof>");
   return end;
 };
+
+/**
+ * Creates a pattern that never succeeds, but reports how far its wrapped pattern could match before failing.
+ *
+ * This is a hack, meant to improve error messages after an AtLeast(). Maybe this can be removed by patterns returning both success and failure
+ *
+ * Never consumes input, and fails with zero length if the pattern succeeds.
+ * @param {Peg.Pattern<unknown>} pattern
+ * @return {Peg.Pattern<never>}
+ */
+Peg.Hint = function (pattern) {
+  return /** @type {Peg.Pattern<never>} */ (
+    Peg.WrapPattern(function (source, index) {
+      const match = pattern(source, index);
+      if (match[0]) {
+        console.log("oops match", match);
+        return [false, index, pattern.expectLabel];
+      } else {
+        return match;
+      }
+    })
+  ).expects(pattern.expectLabel);
+};