On Fri, Sep 19, 2008 at 2:51 AM, <[EMAIL PROTECTED]> wrote:
>
> Lennart Augustsson wrote
>
>> main = do
>> name:_ <- getArgs
>> file <- readFile name
>> print $ length $ lines file
>
> Given the stance against top-level mutable variables, I have not
> expected to see this Lazy IO code. After all, what could be more against
> the spirit of Haskell than a `pure' function with observable side
> effects. With Lazy IO, one indeed has to choose between correctness
> and performance. The appearance of such code is especially strange
> after the evidence of deadlocks with Lazy IO, presented on this list
> less than a month ago. Let alone unpredictable resource usage and
> reliance on finalizers to close files (forgetting that GHC does not
> guarantee that finalizers will be run at all).
>
> Is there an alternative?
>
> -- Counting the lines in a file
> import IterateeM
>
> count_nl = liftI $ IE_cont (step 0)
> where
> step acc (Chunk str) = liftI $ IE_cont (step $! acc + count str)
> step acc stream = liftI $ IE_done acc stream
> count [] = 0
> count ('\n':str) = succ $! count str
> count (_:str) = count str
>
> main = do
> name:_ <- getArgs
> IE_done counter _ <- unIM $ enum_file name >. enum_eof ==<< count_nl
> print counter
>
>
> The function count_nl could have been in the library, but I'm a
> minimalist. It is written in a declarative rather than imperative
> style, and one easily sees what it does. The above code as well as the
> IterateeM library is Haskell98. It does not use any unsafe Haskell
> functions whatsoever.
Is the IterateeM library available on-line anywhere? I'm familiar
enough with your earlier work on enumerators that I can guess what
most of what that code is doing, but I'd like a better idea of what
==<< does.
--
Dave Menendez <[EMAIL PROTECTED]>
<http://www.eyrie.org/~zednenem/>
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