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Draft chapters of "Real World Haskell" now publicly available
by Bryan O'Sullivan 08 Feb '08

08 Feb '08

John, Don and I are pleased to announce the beginning of the public beta programme for our upcoming book, "Real World Haskell". For further details, please see the following blog entry: http://www.realworldhaskell.org/blog/2008/01/21/finally-the-public-beta-pro… Thanks to all of the Haskell community members who have so far performed sterling service in commenting on our closed drafts. We look forward to your feedback! <b

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Re: [Haskell-cafe] FP and Quality
by PR Stanley 08 Feb '08

08 Feb '08

Thanks, keep the tips coming. I like the ones about the type safety and line counts. Cheers, Paul At 23:33 04/02/2008, you wrote: >Good luck with this - I'd love to see the outcome. > >My experience is that FP tends to result in a lot less code, so if >there are x >bugs per line of code, FP has less bugs per complete application. > >Talking about haskell, the typesystem dissalows whole classes of bugs. Things >simply do not compile if you stitch the innards together in the wrong order >(particuarly if you are agressive about using the most general types >possible). Since this accounts for perhaps 90% of where I do things wrong in >Java, I get a corresponding decrease in run-time bugs in haskell. However, >this is somewhat compensated for by the effort needed to get haskell programs >through the compiler in the first place - debug at compile or debug at >runtime is the tradeoff here. > >FP is easier to verify mechanically than imperative programming - more of the >logic is exposed directly. It's easier to do by-case proofs, even if they are >by-hand rather than mechanical. > >However, all of this is annecdotal. Good luck collecting real stats, or >peer-reviewed annecdotes. You may have luck looking at bug-fix times vs >number of developers for equivalent FP and Java apps/libs. Worth a shot, >given that the bug-trackers tend to be public. You could probably tie it back >to the size of the 'fix' patches. Get some nice graphs? > >Matthew > >On Monday 04 February 2008, you wrote: > > Hi folks > > I'm thinking of writing a little essay arguing the case for the > > advantages of FP for producing quality software. Can the list > > recommend any papers/articles which I can use as sources of my > > argument? I have access to the IEEE database too although earlier I > > couldn't find anything on the subject. > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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background question about IO monad
by Uwe Hollerbach 08 Feb '08

08 Feb '08

Hello, haskellers, I have a question for you about the IO monad. On one level, I seem to be "getting" it, at least I seem to be writing code that does what I want, but on another level I am almost certainly not at all clear on some concepts. In various tutorials around the web, I keep finding this notion that the IO monad is "one-way", that you can put stuff into it, but you can't take it out. (And, damn, some of the contortions I went through while trying to figure out how to do exceptions in my little scheme interpreter certainly bear that out! I was sure beating my head against liftIO et al for a fair while...) But let me post some code snippets here: > lispUTCTime [] = doIOAction (getClockTime) toS allErrs > where toS val = String (calendarTimeToString (toUTCTime val)) > lispUTCTime [IntNumber n] = > return (String (calendarTimeToString (toUTCTime (TOD n 0)))) This is a little function I added to the interpreter a couple of days ago: if you enter (UTCtime) with no arguments, it gets the current time and formats it as UTC: like so; this came from the first alternative above: lisp> (UTCtime) "Wed Feb 6 03:57:45 UTC 2008" and if you give an argument, you get that interpreted as a number of seconds since epoch, and that gets formatted as UTC; this is from the second alternative above: lisp> (UTCtime 1.203e9) "Thu Feb 14 14:40:00 UTC 2008" And here's the doIOAction routine: I wrote this, it's not some system-level routine. > doIOAction action ctor epred = > do ret <- liftIO (try action) > case ret of > Left err -> if epred err > then throwError (Default (show err)) > else return (Bool False) > Right val -> return (ctor val) OK, with all that as background, on one level I understand why I need the doIOAction routine in the first version of lispUTCTime: I'm calling getClockTime, that's an IO action, so I enter the IO monad, get the time, and return: all is cool. In the second version, all I'm doing is taking a number and interpreting it as a time, and writing that in a particular format; again, no problem. But after that, it sure seems to me as if I've taken data out of the IO monad... haven't I? Given that the second alternative never entered doIOAction and that after both are done I have a string of characters, prettily formatted to indicate a time, that's what it feels like to this unwashed C programmer. So, what's going on there? What's the difference between the two alternatives? I would appreciate any enlightenment you can send my way! regards, Uwe

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modeling ANSI C structs in Haskell?
by Galchin Vasili 08 Feb '08

08 Feb '08

Hello, I am reading through the FFI doc. Any suggestions on enabling Haskell programmers to model ANSI C structs that will be passed down to C run-time? Vasili

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Signature for non-empty filter
by Henning Thielemann 07 Feb '08

07 Feb '08

Is Haskell's type system including extensions strong enough for describing a function, that does not always return a trivial value? E.g. (filter (\x -> x==1 && x==2)) will always compute an empty list. Using an appropriate signature for the function it shall be rejected at compile time, because it is probably a mistake - probably (filter (\x -> x==1 || x==2) xs) was meant. I assume that the type must contain somehow an example input for which the function value is non-trivial. If Haskell's type system is not strong enough, what about dependently typed languages like Cayenne or Epigram? (I know, theorem provers have no problem with such types.)

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Two Lectureships in Nottingham
by Graham Hutton 07 Feb '08

07 Feb '08

Dear all, We are advertising two new Lectureships (Assistant Professorships) in Computer Science at the University of Nottingham in England. Applications in the area of functional programming are particularly welcome. Further details are available from: http://jobs.nottingham.ac.uk/vacancies.aspx?cat=160#j2797 The closing date for applications is 22nd Feburary 2008 Best wishes, Graham Hutton +-----------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | School of Computer Science | | University of Nottingham Web : www.cs.nott.ac.uk/~gmh | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB, UK Phone : +44 (0)115 951 4220 | +-----------------------------------------------------------------+ This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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FP and Quality
by PR Stanley 07 Feb '08

07 Feb '08

Hi folks I'm thinking of writing a little essay arguing the case for the advantages of FP for producing quality software. Can the list recommend any papers/articles which I can use as sources of my argument? I have access to the IEEE database too although earlier I couldn't find anything on the subject. Thanks, Paul

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Mutable arrays
by Jeff φ 07 Feb '08

07 Feb '08

Hello, I'm trying to write code that will take a mutable 2D array and normalize it by dividing all elements by the largest element. I managed to write code to do this, but it seems overly complex. I could write something much simpler in Clean or C++. Most likely, my code is complex because I don't have any experience with mutable arrays in Haskell. I couldn't find any tutorials on the Internet. I'd be grateful for suggestions on simplifying the following code. Thanks. {-# OPTIONS_GHC -fglasgow-exts -fbreak-on-exception #-} -- normalize_ary This takes a mutable array. Determines the largest -- element in the array (max_elem) and then divides every element by -- max_elem. normalize_ary :: (Num t1, Num t, Ix t, Ix t1, MArray a e t2, Ord e, Fractional e, Enum t, Enum t1) => a (t, t1) e -> t2 () normalize_ary ary = do -- The following two commented out lines of code show my first -- attempt at determining a value for max_elem. However, this -- produces a stack overflow. -- elem_ary <- getElems ary -- let max_elem = foldl1 max elem_ary max_elem <- calc_max_2d_elem ary max_elem `seq` map_in_place_2d_arr (\x -> x / max_elem) ary map_in_place_2d_arr :: (MArray a e t, Enum t2, Enum t1, Ix t1, Ix t2) => (e -> e) -> a (t1, t2) e -> t () map_in_place_2d_arr fn arr = ret where ret = do ((i1,j1),(i2,j2)) <- getBounds arr ( mapM_ (\i -> do v <- readArray arr i writeArray arr i (fn v) ) [(i,j) | i <- [i1..i2], j <- [j1..j2]]) calc_max_2d_elem :: (Ord t, MArray a t t1, Ix t2, Ix t3, Num t3, Num t2) => a (t3, t2) t -> t1 t calc_max_2d_elem arr = do m <- readArray arr (0,0) (_,(i_max, j_max)) <- getBounds arr let calc_max_loop arr m (i,j) | j == j_max = return m | otherwise = do e <- readArray arr (i,j) let m2 = max e m m2 `seq` calc_max_loop arr m2 nxt_idx where nxt_idx | i == i_max - 1 = (0,j+1) | otherwise = (i+1,j) calc_max_loop arr m (0,0)

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Why is this so inefficient?
by Jefferson Heard 06 Feb '08

06 Feb '08

I thought this was fairly straightforward, but where the marked line finishes in 0.31 seconds on my machine, the actual transpose takes more than 5 minutes. I know it must be possible to read data in haskell faster than this. I'm trying to read a 100MB comma delimited file. I've tried both CSV modules, and these take even longer to read the file. Is there some general best-practice for reading and parsing large amounts of data that I'm not aware of? I have tried, by the way, a couple of things, including putting a bang (!) before row in transposeRow and using foldr instead of foldl', but that didn't change anything other than force me to increase the stack size to 100M on the command line. I'm running in the profiler now, and I'll update this, but I thought I would check and see if my head was on remotely straight to begin with. -- Jeff --- module ColumnMajorCSV where import qualified Data.ByteString.Char8 as S import qualified Data.Map as M import qualified Data.List as L transposeRow cols row = zipWith (:) row cols transposeCSV :: [[S.ByteString]] -> M.Map String [S.ByteString] transposeCSV (header:rows) = M.fromList (zip (map S.unpack header) spreadsheet) where spreadsheet = L.foldl' transposeRow emptySheet rows emptySheet = take (length header) $ repeat [] dataFromFile :: String -> IO (M.Map String [S.ByteString]) dataFromFile filename = do f <- S.readFile filename print . length . map (S.split ',' $!) . S.lines $ f -- finishes in 0.31 seconds return . transposeCSV . map (S.split ',' $!) . S.lines $ f -- this takes 5 minutes and 6 seconds

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classes for data structures
by Matthew Pocock 06 Feb '08

06 Feb '08

Hi, I've been working a lot with maps, sets and lists. While the process of getting things out of them is abstracted by foldable, traversable and friends, the process of building one up is not. Would it be possible to have something like: class Buildable b where empty :: b a --makes an empty b with elements of type a insert :: a -> b a -> b a --inserts the new element into the buildable I'm not particularly wedded to the names. It's just that it would be very convenient sometimes to collect data items into e.g. a list or a set, without pushing in some klunky foralled insert function. I see that this is realted to MonadPlus, but the space of possible container classes is wider than that of monads e.g. set. Matthew

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