October 2009 - Beginners

Re: (ab)use compiler optimization
by Martin Hofmann 09 Oct '09

09 Oct '09

Thanks apfelmus this worked fine.

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(ab)use compiler optimization
by Martin Hofmann 09 Oct '09

09 Oct '09

Hello, To analyse, test, and debug my system I use a WriterT for logging. The log file created can be quite large and a lot of 'Log' is created in memory, too. I thought, that maybe I can (ab)use the compiler to switch logging on and off as needed before compiling. I tried to either always return an empty Log when running the WriterT, or just always ignore logging depending on a constant and hoped GHC to optimize the logging away. _NOLOGGING = True runWT :: (Monad m) => (WriterT Log m a) -> m (a, Log) runWT m | _NOLOGGING = runWriterT m empty >>= \(a,_) -> return (a,emptyLog ) | otherwise = runWriterT m emptyLog logging :: Message -> WriterT Log m () logging msg = if _NOLOGGING then return () else tell msg However, according to the heap profile there is still a lot of 'Log' in the memory. So, is this possible this way at all and if not, is there another (apart from removing all logging by hand). Thanks

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Compiling C that calls Haskell with GCC
by John Velman 05 Oct '09

05 Oct '09

More specifically, I want to link a haskell .o file into a project I create on OS X using Xcode, but the answer to the gcc question should do it. I've followed the simple "calling haskell from C" example from HaskellWiki, and it works as advertised when compiled as advertised with gcc. However, I'm looking toward writing OS X programs with Xcode and the standard Mac interface, calling some functions written in Haskell. Consequently, I want to compile and link using the mac Xcode IDE, which uses gcc. Seems like the thing that GHC puts in must be include .h files and libraries. But which ones? I think I ran across the answer to this in my browsing, but can't find it again. Can someone point me in the right direction? Thanks, John Velman

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string instead of char
by John Moore 05 Oct '09

05 Oct '09

Hi, I am now writing a function that replaces vowels with the letter x. eg.put in a string "help" and out comes hxlp. I tried this: f x = case x of {'a' -> 'x';'e' -> 'x';'i' -> 'x';'o' -> 'x';'u' -> 'x';_ -> x} but this wont work on strings, only on the individual letters. Any direction would be very welcome. Yours john

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Advantage to installing Hugs with GHC?
by John Velman 05 Oct '09

05 Oct '09

My machine is an iMac running Leopard, 10.5.8. I've installed GHC from the haskell-platform-..(Leopard/x86) disk image. Ghci works fine as far as I can tell. Is there any particular advantage to also installing Hugs? Thanks, John Velman

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parse error
by John Moore 04 Oct '09

04 Oct '09

Hi, This is a function that returns true if divisible by 2. But it keeps telling me parse error on input | . What is wrong here. is_even n | mod n 2 == 0 | True Not sure if I need another statement for false John

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redirect stdout to /dev/null
by Hong Yang 04 Oct '09

04 Oct '09

On UNIX, we can easily redirect stdout to /dev/null in a Shell. How is this realized in Haskell on both UNIX and Windows? I used a library which gave me unwanted stdout display. I tried to find any function to hide the stdout output, but could not. Can someone help me out? Otherwise I have to modify the code in the library to comment out some print commands. Thanks, Hong

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main
by John Moore 03 Oct '09

03 Oct '09

Hi, I must be doing something wrong when I run a program it comes up on the screen as OK module loaded Main *Main> But it will not do what I program to do. What do I enter after *main> I tried putting in the arguments but ot just returns the arguments. For example: signum x | x < 0 = -1 | x == 0 = 0 | x > 0 = 1 Enter 5 it returns 5 What have I missed? Regards John

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$= operator
by Nathan Holden 03 Oct '09

03 Oct '09

I was reading Eleara examples out of Hackage, and I kept noticing lines like this: > t <- get GLFW.time > GLFW.time $= 0 Most of the rest of it made more-or-less sense (I mean, there IS a reason I'm still asking questions to beginners!) but I couldn't seem to find anything about it on my usual reference sources, and it's uses don't make that much sense to me.

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Help with Data.Time
by Hong Yang 02 Oct '09

02 Oct '09

I want to calculate the length of time between time ta and time tb. Time ta is a String looking like "N:NNAM" or "N:NNPM." I need tb to be 9:30AM on the current date. I think the flow should be 1) parse ta to utcTimeA :: UTCTime 2) get current date and form tb to utcTimeB :: UTCTime 3) call "diffUTCTime utcTimeA utcTimeB" So my program looks like as follows in a "do" expression: let utcTimeA = fromJust (parseTime defaultTimeLocale "%R%P" ta :: Maybe UTCTime) currentTime <- getCurrentTime let day = utctDay currentTime let utcTimeB = UTCTime day "09:30:00.0000000 UTC" let timeElapsed = diffUTCTime utcTimeA utcTimeB There are two difficulties I could not get around: 1) It seems that parseTime returns "Nothing". Why? 2) I knew "09:30:00.0000000 UTC" in the statement of "let utcTimeB ..." is incorrect, because it is a String instead of expected DiffTime type. After consulting Data.Time module (it just say "Data DiffTime"), I still cannot figure out how to get utcTimeB. Can someone help me? Thanks, Hong

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