Allow ‘many’ run parsers that do not consume input

CHANGELOG.md

@@ -2,6 +2,15 @@
 
 * Added `MonadParsec` instance for `RWST`.
 
+* Allowed `many` to run parsers that do not consume input. Previously this
+  signalled an `error` which was ugly. Of course, in most cases giving
+  `many` a parser that do not consume input will lead to non-termination
+  bugs, but there are legal cases when this should be allowed. The test
+  suite now contains an example of this. Non-termination issues is something
+  inherited from the power Megaparsec gives (with more power comes more
+  responsibility), so that `error` case in `many` really does not solve the
+  problem, it was just a little ah-hoc guard we got from Parsec's past.
+
 ## Megaparsec 5.1.2
 
 * Stopped using property tests with `dbg` helper to avoid flood of debugging

Text/Megaparsec/Prim.hs

@@ -385,14 +385,9 @@ manyAcc p = ParsecT $ \s cok cerr eok _ ->
         unParser p s'
         (seq xs $ walk $ x:xs)       -- consumed-OK
         cerr                         -- consumed-error
-        manyErr                      -- empty-OK
+        (seq xs $ walk $ x:xs)       -- empty-OK
         (errToHints $ cok (x:xs) s') -- empty-error
-  in unParser p s (walk []) cerr manyErr (errToHints $ eok [] s)
-
-manyErr :: a
-manyErr = error $
-  "Text.Megaparsec.Prim.many: combinator 'many' is applied to a parser"
-  ++ " that may consume no input."
+  in unParser p s (walk []) cerr (walk []) (errToHints $ eok [] s)
 
 instance (ErrorComponent e, Stream s)
     => Monad (ParsecT e s m) where

tests/Text/Megaparsec/PrimSpec.hs

@@ -336,6 +336,16 @@ spec = do
           let p = many (char 'a') *> many (char 'b') *> eof
           prs p "c" `shouldFailWith` err posI
             (utok 'c' <> etok 'a' <> etok 'b' <> eeof)
+      context "when the argument parser succeeds without consuming" $
+        it "is run nevertheless" $
+          property $ \n' -> do
+            let n = getSmall (getNonNegative n') :: Integer
+                p = void . many $ do
+                  x <- S.get
+                  if x < n then S.modify (+ 1) else empty
+                v :: S.State Integer (Either (ParseError Char Dec) ())
+                v = runParserT p "" ""
+            S.execState v 0 `shouldBe` n
 
     describe "some" $ do
       context "when stream begins with things argument of some parses" $