From 039bc03163f9233db7502408de59a22fd8c9c14e Mon Sep 17 00:00:00 2001
From: Adithya Kumar <adithya@composewell.com>
Date: Tue, 30 Jul 2024 16:50:51 +0530
Subject: [PATCH] Fix ParserK.toParser

---
 core/src/Streamly/Internal/Data/ParserK/Type.hs | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/core/src/Streamly/Internal/Data/ParserK/Type.hs b/core/src/Streamly/Internal/Data/ParserK/Type.hs
index 1d93e6efa5..f33b112d34 100644
--- a/core/src/Streamly/Internal/Data/ParserK/Type.hs
+++ b/core/src/Streamly/Internal/Data/ParserK/Type.hs
@@ -774,14 +774,14 @@ toParser parser = ParserD.Parser step initial extract
         r <- cont None
         case r of
             -- This is extract so no input has been given, therefore, the
-            -- translation here is (1 + n) rather than n.
-            -- XXX How should we visualize (1 + n)?
-            -- XXX Go forward by 1 element on extract?
-            Done n b ->  assert (n <= 0) (return $ ParserD.SDone (1 + n) b)
+            -- translation here is (n - 1) rather than n.
+            -- In ParserD @n@ means go back by @n@ including the current input
+            -- Since there is no current input we reduce moving by 1.
+            Done n b ->  assert (n <= 0) (return $ ParserD.SDone (n - 1) b)
             Error _ e -> return $ ParserD.Error e
             Partial _ cont1 -> extract cont1
             Continue n cont1 ->
-                assert (n <= 0) (return $ ParserD.SContinue (1 + n) cont1)
+                assert (n <= 0) (return $ ParserD.SContinue (n - 1) cont1)
 
 {-# RULES "fromParser/toParser fusion" [2]
     forall s. toParser (adapt s) = s #-}