Refactor implementation of many and arguments.

Clean up recursion in the implementation of `many`, so that AltP is
inside BindP, instead of the other way around.

Options/Applicative/Builder.hs

@@ -251,7 +251,7 @@ argument p (Mod _ d g) = mkParser d g (ArgReader p)
 -- command line, all following arguments are included in the result, even if
 -- they start with @'-'@.
 arguments :: (String -> Maybe a) -> Mod ArgumentFields [a] -> Parser [a]
-arguments p m = args1 <|> pure (fromMaybe [] def)
+arguments p m = set_default <$> args
   where
     Mod _ (DefaultProp def sdef) g = m
     show_def = sdef <*> def
@@ -262,10 +262,13 @@ arguments p m = args1 <|> pure (fromMaybe [] def)
     props = mkProps mempty g
     props' = (mkProps mempty g) { propShowDefault = show_def }
 
-    args1 = ((Just <$> arg') <|> (ddash *> pure Nothing)) `BindP` \x -> case x of
-      Nothing -> many arg
-      Just a -> fmap (a:) args
-    args = args1 <|> pure []
+    args = optional arg_or_ddash `BindP` \mx -> case mx of
+      Nothing       -> pure []
+      Just Nothing  -> many arg
+      Just (Just x) -> (x:) <$> args
+    arg_or_ddash = (Just <$> arg') <|> (ddash *> pure Nothing)
+    set_default [] = fromMaybe [] def
+    set_default xs = xs
 
     arg = liftOpt (Option (ArgReader p) props)
     arg' = liftOpt (Option (ArgReader p') props')

Options/Applicative/Types.hs

@@ -115,7 +115,9 @@ instance Applicative Parser where
 instance Alternative Parser where
   empty = NilP Nothing
   (<|>) = AltP
-  many p = some p <|> pure []
+  many p = optional p `BindP` \mx -> case mx of
+    Nothing -> pure []
+    Just x  -> (x:) <$> many p
   some p = p `BindP` (\r -> (r:) <$> many p)
 
 -- | Result after a parse error.