Make ‘try’ truly backtrack parser state

Close #142.

This seemingly has no effect on performance.

CHANGELOG.md

@@ -23,6 +23,10 @@
   `hpesc-megaparsec` package, which also improved the latter (that package
   is the recommended way to test Megaparsec parsers).
 
+* The `try` combinator now truly backtracks parser state when its argument
+  parser fails (either consuming input or not). Most users will never notice
+  the difference though. See #142.
+
 * Further documentation improvements.
 
 ## Megaparsec 5.0.1

Text/Megaparsec/Prim.hs

@@ -536,8 +536,8 @@ class (ErrorComponent e, Stream s, A.Alternative m, MonadPlus m)
   hidden :: m a -> m a
   hidden = label ""
 
-  -- | The parser @try p@ behaves like parser @p@, except that it
-  -- pretends that it hasn't consumed any input when an error occurs.
+  -- | The parser @try p@ behaves like parser @p@, except that it backtracks
+  -- parser state when @p@ fails (either consuming input or not).
   --
   -- This combinator is used whenever arbitrary look ahead is needed. Since
   -- it pretends that it hasn't consumed any input when @p@ fails, the
@@ -708,7 +708,8 @@ pLabel l p = ParsecT $ \s cok cerr eok eerr ->
 
 pTry :: ParsecT e s m a -> ParsecT e s m a
 pTry p = ParsecT $ \s cok _ eok eerr ->
-  unParser p s cok eerr eok eerr
+  let eerr' err _ = eerr err s
+  in unParser p s cok eerr' eok eerr'
 {-# INLINE pTry #-}
 
 pLookAhead :: ParsecT e s m a -> ParsecT e s m a

tests/Text/Megaparsec/PrimSpec.hs

@@ -598,13 +598,12 @@ spec = do
             grs' p s (`succeedsLeaving` "")
       context "when inner parser fails consuming" $
         it "backtracks, it appears as if the parser has not consumed anything" $
-        -- TODO also check that it backtracks state in general as well
           property $ \a b c -> b /= c ==> do
             let p :: MonadParsec Dec String m => m Char
                 p = try (char a *> char b)
                 s = [a,c]
             grs  p s (`shouldFailWith` err (posN (1 :: Int) s) (utok c <> etok b))
-            grs' p s (`failsLeaving` [c]) -- FIXME it should leave entire input
+            grs' p s (`failsLeaving` s)
       context "when inner parser succeeds without consuming" $
         it "try has no effect" $
           property $ \a -> do
@@ -612,10 +611,12 @@ spec = do
                 p = try (return a)
             grs p "" (`shouldParse` a)
       context "when inner parser fails without consuming" $
-        it "try has no effect" $ do
-          let p :: MonadParsec Dec String m => m Char
-              p = try empty
-          grs p "" (`shouldFailWith` err posI mempty)
+        it "try backtracks parser state anyway" $
+          property $ \w -> do
+            let p :: MonadParsec Dec String m => m Char
+                p = try (setTabWidth w *> empty)
+            grs  p "" (`shouldFailWith` err posI mempty)
+            grs' p "" ((`shouldBe` defaultTabWidth) . stateTabWidth . fst)
 
     describe "lookAhead" $ do
       context "when inner parser succeeds consuming" $