- Haskell-Cafe - Haskell.org

How is laziness defined?
by TJ 05 Feb '07

05 Feb '07

I would think that with 100% laziness, nothing would happen until the Haskell program needed to output data to, e.g. the console. Quite obviously that's not it. So how is laziness defined in Haskell? I remember vaguely someone saying that pattern matching on a value forces it to be evaluated. Is that right? What else? This is one of the things that just boggles my mind everytime I try to wrap it around this thing called Haskell ;) Cheers, TJ

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List operation question
by Eric Olander 05 Feb '07

05 Feb '07

Hi, I'm still somewhat new to Haskell, so I'm wondering if there are better ways I could implement the following functions, especially shiftl: >> moves the first element to the end of the list shiftr :: [a] -> [a] shiftr [] = [] shiftr (x:y) = y ++ [x] >> moves the last element to the head of the list shiftl :: [a] -> [a] shiftl [] = [] shiftl x = [last x] ++ init x -Eric

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A function callable from C
by John Ky 05 Feb '07

05 Feb '07

Hi, The following code works: > type ServiceMainClosure = DWORD -> IO () > > foreign import ccall "wrapper" > mkServiceMainClosure :: ServiceMainClosure -> IO (FunPtr ServiceMainClosure) But the following doesn't: > type ServiceMainClosure = DWORD -> [String] -> IO () > > foreign import ccall "wrapper" > mkServiceMainClosure :: ServiceMainClosure -> IO (FunPtr ServiceMainClosure) System\Win32\Service.hsc:108:8: parse error on input `import' What can I do to get this to work? Thanks -John

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OT: any haskell-friendly / functional programming friendly comp sci programs? (for a 30s guy who did his undergrad in liberal arts)
by Thomas Hartman 05 Feb '07

05 Feb '07

haskellers, I'm contemplating returning to school after a decade as a worker bee, and almost that long as a worker bee doing computer consulting / miscelaneous tech stuff. Ideally I'd like to get a masters, but I don't know if that's feasible this late in the game. If it's not, I might settle for a lesser degree and worry about the masters later. Part of my desire to return to school is that after encountering functional programming over the past year or so (first through lisp, then haskell), I've found something that I'm really interested in, but that it feels the amount of learning I can do around a job just won't cut it to get to the level that I want. That's the personal reason; the practical reason is that I think it would be a good networking opportunity (hankering to start my own company, and not meeting the right kinds of people), and obviously increase my chance of getting better paying, but especially more *interesting* jobs. I have some savings, figure loans would cover the rest, and no wife or kids. So I'll make a rather open ended request for advice. Are there decent comp sci master's programs out there that will take someone who didn't do a hard science in undergrad, but has lots of work-related experience with programming? If not, what's the next best thing? Get a quick bachelor's? Spend six months cramming for the GREs and then try for a master's? Whether master's, bachelor's, or other, I am specifically interested in programs that are "functional" friendly. In other words, I don't want to just go and study algorithms in java for two years. Ideally, I'd like to go somewhere where I could really explore and get good at the functional languages, with haskell my current favorite but also open to others. (For what it's worth, most of my experience is in perl, but I take it seriously as a language and try not to write the kind of throwaway crap that mean people make fun of.) My background is that I have a liberal arts bachelors from an american ivy. Though I enjoyed the program very much, it probably wasn't the wisest financial decision, given the type of career I subsequently gravitated to. But hey, maybe that gives me a different kind of perspective that has its own kind of value. Since, then, there;s mainly work work work for me, with some time off bumming around europe. And, on my own time: learn learn learn. I'm now 31. I've been living in germany for a few years, mainly freelancing as a computer consultant, and am open to programs in europe -- probably either england or germany. Largely because programs out here seem to be significantly cheaper than programs in the states, and being a somewhat older student seems to be less unusual out here. However, truth be told, I have a hankering for my homeland, the good old USA. And if possible somewhere in california, where my extended family is based, or it not that new york, where I have a good network. (But open to other locations as well; cali and new york would just be my top choices.) Distance learning is okay if the program is really good, but doing the campus thing again, perhaps while working part or semi-full time, would be the ideal (though perhaps I'm pushing the age limit there?). This has gotten rather long, so I'll leave it at that. Sorry about going off topic, and thanks in advance for any advice. thomas

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Timezone Database Library
by Martin Percossi 05 Feb '07

05 Feb '07

Hello, is there a haskell library that provides facilities to read and use the tzfile format [1], or equivalent in Windows? TIA Martin [1] http://www.twinsun.com/tz/tz-link.htm

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How to use infinite lists to define the list of all negative integers
by Bahtijar Vogel 05 Feb '07

05 Feb '07

Hi, How am I supposed to use infinite lists to define the list of all negative integers?

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How to remove all numbers from a list that are divisible by an integer
by Miranda Kajtazi 05 Feb '07

05 Feb '07

Hi, Thanx to all for the previous hints how to solve and write the function to calculate the sum of a list of lists. Now I need to know how to remove all numbers that are divisable by an integer. Lets say I have a list [2,4,5,6,8] and when I divide them with number 2, my list should look like [5]...because all other numbers in this list are divisible with 2 therefore I remove or filter all of them. I tried to use some zipWith...filter and other predefined functions..but I can't really find the right solution :(

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Space leaks in large mutable data structures
by C Rodrigues 05 Feb '07

05 Feb '07

I'd like to hear what tips and techniques you guys have for avoiding space leaks. I understand the basic techniques to force evaluation of closures. What I'd like to know is how you avoid space leaks in large, long-lived, mutable data structures. These kind of data are particularly sensitive to laziness in my experience. (I'm using functional data structures and doing a lot of update operations.) If you use monads to enforce strictness, how do you maintain composability throughout the program? _________________________________________________________________ Invite your Hotmail contacts to join your friends list with Windows Live Spaces http://clk.atdmt.com/MSN/go/msnnkwsp0070000001msn/direct/01/?href=http://sp…

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how to calculate the sum of list of lists?
by Miranda Kajtazi 05 Feb '07

05 Feb '07

Help, How to calculate the sum of list of lists in Haskell? Please help me, Miranda

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mixing wxhaskell state and file io
by Martin DeMello 05 Feb '07

05 Feb '07

I'm having a lot of trouble mixing file io and wxhaskell's varCreate/Get/Set functions. I have functions readWords :: String -> IO WordMap wordGrid :: WordMap -> Layout And within my GUI code, the following compiles (ignores the variable, basically): words <- varCreate (do {w <- readWords "words"; return w}) wGrid <- do w <- readWords "words" return $ wordGrid w but I can't get the following (noncompiling code, but it shows what I'm trying to do) working: wGrid <- do w <- varGet words return $ wordGrid w Could someone give me a minimal example of reading in a list of words from a file, binding a wxHaskell variable to the list, and then mapping some GUI code over it? (Also, I'm making the base assumption that varSet and varGet are wxHaskell's analogue of the State monad - should I be looking at using StateT instead?) martin

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