[ fixed #46 ] give proper error when no lexing rules

The new error

  file.x contains no lexer rules

can be tested with file.x containing

  :- <0> { }

The previous error was

  (Array.!): undefined array element

(I did not find a testsuite testing for error messages Alex gives, thus,
no test case with this commit.)

Author: andreasabel

src/Main.hs

@@ -113,7 +113,11 @@ parseScript file prg =
         Left (Nothing, err) ->
                 die (file ++ ": " ++ err ++ "\n")
 
-        Right script -> return script
+        Right script@(_, _, scanner, _) -> do
+          -- issue 46: give proper error when lexer definition is empty
+          when (null $ scannerTokens scanner) $
+            dieAlex $ file ++ " contains no lexer rules\n"
+          return script
 
 alex :: [CLIFlags]
      -> FilePath