Fix position when an error occurs in `sat`.

src/GenericParser/Parser.purs

@@ -121,8 +121,10 @@ try p = Parser p'
                      _       -> failureError position error
 
 sat :: forall e. (Char -> Boolean) -> Parser e Char
-sat p = do x <- item
-           if p x then pure x else empty
+sat p = do
+           pos <- current_position
+           x <- item
+           if p x then pure x else Parser \input -> failure pos
 
 digit :: forall e. Parser e Char
 digit = sat isDigit