Hi, the stangest bug I've ever experienced has manifested in my code and I badly need some help finding it. I'm currently writing a small lexer generator using template haskell. It is supposed to fit in nicely with Happy and is therefore of the "threaded monadic" type. However it does not work, or at lest not when the number of regexps get too big and I compile the code. Using it with ghci is never a problem but when compiling (no matter which optimisation level I use) it often hangs in the middle of lexing (it is not the generation of code that is the problem, but the generated code itself). Given the very same input it seems to hang (eating 100% cpu but without doing anything visible) at very different stages. Sometimes it outputs almost all tokens and sometimes it doesn't read more that one or two. This is strange behaviour from a pure function but may be due to lazyness. To add more strangeness to it all the following code does not work (the function lexer is the TH generated function, imports omitted): main = do [arg] <- getArgs str <- readFile arg putStrLn $ show $ lexer cont str 0 0 cont t s c l = case t of Token.To_Error -> error "" Token.To_Eof -> [] _ -> t : lexer cont s c l However if you change "case t of" to the more esoteric "case unsafePerformIO (putStrLn (show t) >> return t) of " it works like a charm. The same if you plug it into Happy. Using it with normal parameters it does not work, but generating it with -d does the trick. Something strange with lazyness comes into mind but I can't think of what. The second thing I can think of is the compiler infering strange types somehow. All TH generated code is untyped, but that should not give any trouble? What can it be? Is it some error in ghc, perhaps something connected with template haskell (I do have a slightly different version that generates a .hs file instead and that has not failed me yet)? Do anybody know what's wrong or can give me some pointers to how I can find out? I do not appreciate reading external core or so since the problem only occurs for large lexers (which currently generates a few thousand lines since it doesn't use tables but functions). It's the very same with both ghc 6.0 and yesterdays CVS version and -dcore-lint does not say anything. I should perhaps also say that "hangs" means that it hasn't produced any output after it has run for about one hour at ~100% cpu. That seems to me as quite a bit too much time for one to suspect that it just takes a lot of time to produce output... /Hampus - very confused ps. If you need more information (such as intenals of "lexer") just say so, i think this letter is big enough now :) ds. -- Homepage: http://www.dtek.chalmers.se/~d00ram E-mail: d00ram@dtek.chalmers.se "Det är aldrig försent att ge upp"