Curious problem... Suppose MyMonad is a Monad other than the IO Monad. Consider the following piece of code:
> f :: MyMonad () > f = action1 Now suppose also that ioToMyMonad converts an IO Monad to a MyMonad Monad: > ioToMyMonad :: IO a -> MyMonad a > ioToMyMonad = return . unsafePerformIO Suppose also that action1 needs to perform some IO operation (called IO_op), which returns IO (). In other words, action1 would be: > action1 :: MyMonad () > action1 = do > -- MyMonad operations > ioToMyMonad $ IO_op > -- other MyMonad operations When I run f (via runMyMonad or something like that), I noticed that the io operation (IO_op) is not executed. I guess that this happens because of laziness properties, since the value returned by IO_op is useless to MyMonad. I have then two questions: 1. Does this really happen because of laziness properties? 2. Which is the best way to force IO_op to be performed? I tried the following approach: Step1: Change the type of IO_op from "IO ()" to "IO Bool" and make it returns True. Step2: Change action1 to > action1 :: MyMonad Bool > action1 = do > -- MyMonad operations > b <- ioToMyMonad $ IO_op > -- other MyMonad operations > return b Step3: Change the type of f to MyMonad Bool Step4: Finally, use this boolean somewhere after calling "runMyMonad", e. g., printing it on the screen. I'm sure this is not the best approach, so I'd really appreciate any suggestions. [I'm sending this mail to the HOpenGL list also because runMyMonad is being called inside the display callback, so perhaps it may have something to do with the problem] Thanks a lot, -- Andre _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
