- Haskell-Cafe - Haskell.org

implementing a csv reader
by Tamas K Papp 24 Aug '06

24 Aug '06

Hi, Now that I have read the tutorials, I think that the best way to learn Haskell would be to use the language and write something simple yet useful. I noticed that Haskell lacks a module for reading/writing csv (comma separated value) files, so I thought I could implement that. Questions: 1. Please tell me if you know of a csv module, because then I would do something else. 2. I am looking for a parser, but I don't know Haskell parsers. Is Parsec a good choice? Thanks, Tamas

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question on traversing syntax tree
by Xiong Yingfei 24 Aug '06

24 Aug '06

I am writing a compiler using Haskell. After the compiler parses program, the program is stored into an syntax tree stucture defined blew: ...... data Exp = Plus Exp Term | Minus Exp Term | Term Term deriving Show data Term = Times Term Factor | Div Term Factor | Factor Factor deriving Show ...... This is just part of the definition. The full tree contains much more definition than this. Now I want to adjust the syntax-tree. However, I don't need to adjust all the data types, but a small subset of the syntax tree. e.g. I might adjust the "Times" data like the following, but not modify the rest of the syntax tree: transformTerm (Times t f) = Times t (FactorInt 100) However, in order to apply the modification like this, I have to write a series of function to traverse the tree until I get to the Term data type. e.g. I have to define: transformExp (Plus e t) = Plus (transformExp e) (transformTerm t) transformExp (Minus e t) = Minus (transformExp e)(transformTerm t) transformTerm (Term t) = ... This is tedious and error-prone. I want to know if there some means in Haskell to write a single "generic" function to traverse the syntax tree and only stop on the "Term" data type. Can anyone tell me something about it? Thanks a lot. -- Xiong, Yingfei (熊英飞) Ph.D. Student Institute of Software School of Electronics Engineering and Computer Science Peking University Beijing, 100871, PRC. Web: http://xiong.yingfei.googlepages.com

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Cabal question, adding data files
by Neil Mitchell 24 Aug '06

24 Aug '06

Hi, I have a cabal executable, which requires additional data files. How do I do this in Cabal? I have seen extra-source-files, but they are not added at install time to the destination directory, which doesn't help me. I noticed that Alex does this, but with a lot of Cabal calling in Setup.hs with hooks etc, which is not really ideal. Is there any easy way to do the "install these files along with the program, please" ? If anyone needs any concrete examples, take for example HsColour, which needs the files .hscolour and hscolour.css, but currently install doesn't put either of them anywhere useful. Thanks Neil

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Proposal to allow {} instead of () in contexts
by Brian Hulley 24 Aug '06

24 Aug '06

Hi - Disregarding my last proposal which involved the use of {} in types, I am wondering if anyone would agree with me that it would be a good idea to use {} instead of () when writing out the context ie: foo :: (Num a, Bar a) => a -> a would become: foo :: {Num a, Bar a} => a -> a and the same for every other situation in the language where a context appears. My reasons are twofold: 1) The context is supposed to be understood (afaiu) as a *set* of constraints not a *tuple* of constraints (ie what relevance does the ordering implied by the tuple notation have here?), so the using of set braces seems mathematically more appropriate 2) It would allow an editor to give the correct fontification for an incomplete type expression. At the moment, if you'd just typed: foo :: (Bar the editor would have no way of knowing if "Bar" is a classcon or a tycon, whereas with: foo :: {Bar the opening brace immediately informs the editor that the following text should be parsed as a context and so it could fontify Bar appropriately. For backwards compatibility the use of () could still be supported - braces don't occur in H98 in the positions I'm proposing to allow them so nothing would be broken. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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One thought: Num to 0 as ? to list?
by Marc Weber 24 Aug '06

24 Aug '06

Is there somethinig corresponding to Num concering lists? I mean there is + - /.. defined. + - are not type specific (Int, Double) neither is : [1,2] notation to Elements.. But what about different implementatins of lists? (linked lists? hash lists? array with index? In other words: why not overload (:) ? Then it would be possible to use "blah" for fast packed strings, too Then you could also implement implementations for C arrays and access them. (Sure you can do it anyway but not using syntactic sugar) One part of the answer is patterns? f 'a' = .. f _ = .. Marc

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extreme newbie: hugs prompt vs load module
by George Young 23 Aug '06

23 Aug '06

[linux, ghci 6.4.3.20060820, hugs May 2006] I have just started learning Haskell. I have hugs and ghci under linux, and I'm going through the Gentle Introduction to Haskell<http://www.haskell.org/tutorial>, so far through section 4, "case expressions and pattern matching". I'm a python programmer, with background in maclisp, scheme, T, C, C++, and a little J. I'm confused about what sort of things I can type at the interpreter prompt, and what things have to be loaded as a module. I keep trying to treat the prompt like a lisp or python REPL, which is obviously wrong. Can someone set me straight? Is there another tutorial that might be more appropriate for me? I am finding haskell quite appealing. I hope to start writing real (if small) applications to do some data analysis from our Postgres DB. Any hints? --George Young -- "Are the gods not just?" "Oh no, child. What would become of us if they were?" (C.S. Lewis)

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Re: [Haskell-cafe] extreme newbie: hugs prompt vs load module
by Hans van Thiel 23 Aug '06

23 Aug '06

Hi, I'm a beginner too and I'm quite taken with 'Yet Another Haskell Tutorial' by Hal Daume III, which is available from the Haskell site. It really starts from scratch and it covers almost everything, as far as I can see. Because I'm a lone learner, I got stuck several times, but there are several other short tutorials for beginners, with a slightly different slant, that helped. For monads, which I'm starting on now, I recommend 'All about Monads' by (Jeff?) Newbern. http://www.nomaware.com/monads/html/ But YAHT starts with a chapter on Hugs, GHC and how to use them. At the moment I'm reading the book again, from the beginning, and I'm seeing several things much better than I did the first time(s). Hope this helps, Hans van Thiel -- Don't worry, be happy, learn Haskell.

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Memoizing longest-common-subsequence
by Mark.Carroll＠Aetion.com 23 Aug '06

23 Aug '06

I wanted a longest common subsequence function and a bit of Googling failed to turn up a functional one, except for in a scary bit of darcs. So, I tried making a memoized functional version of the LCS delta algorithm on the problem's Wikipedia page. It's not the fastest, but it's simple and should be polynomial, growing with the product of input sequence lengths. I've not played with unboxing or strictness here yet, but with the help of Data.Array I have: longestCommonSubsequence xs ys = let lastIndex = (length xs, length ys) memo = array ((0, 0), lastIndex) [ ((xi, yi), deltaLCS (xi, x) (yi, y)) | (xi, x) <- zip [0..] (undefined : xs), (yi, y) <- zip [0..] (undefined : ys) ] deltaLCS (0, _) _ = (0, []) deltaLCS _ (0, _) = (0, []) deltaLCS (xl, x) (yl, y) = if (x == y) then let (xylShrunk, xysShrunk) = memo ! (xl-1, yl-1) in (xylShrunk + 1, x : xysShrunk) else let xaShrunk@(xlShrunk, _) = memo ! (xl-1, yl) yaShrunk@(ylShrunk, _) = memo ! (xl, yl-1) in if xlShrunk > ylShrunk then xaShrunk else yaShrunk in reverse (snd (memo ! lastIndex)) I haven't looked much at optimizing this further so there are probably yet some easy wins to be had - I'd be interested to see what they are. Still, I thought it worth sharing as an example of laziness making memoization easy in a simple functional longest common subsequence example. Take this as your cue to point out the much better LCS algorithm that already exists in the standard libraries, that I couldn't find. (-: -- Mark

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debugging space leaks in references
by Andrew Pimlott 22 Aug '06

22 Aug '06

GHC's great space profiling tools don't appear to be much help when your leaked memory is stored in references (IORefs, StablePtrs, etc). I had a real-life case where the allocation profile showed me where the leaked data came from, and I could guess that it was being held by some reference, but I couldn't tell which one. Retainer set profiling showed a big suspicious entry for "SYSTEM", but I couldn't find any way to pinpoint the problem. (It ended up being a missing freeStablePtr in hsgnutls, found by code inspection.) Here's a contrived example that just allocates a bunch of IORefs: import Control.Monad import Data.IORef main = repeatM (newIORef [1,2,3]) repeatM io = liftM2 (:) io (repeatM io) Retainer set profiling shows everything in "SYSTEM". None of the other profiling methods say anything interesting either. What I'd like to get, I think, is (1) your memory is being held in IORefs (2) allocated by this cost center and (3) being retained by this cost center. I guess I'm looking for something like a memory profiler for a traditional language. But I haven't really thought it all out (and I don't even understand everything the existing profiles tell me). Are there some tricks I've missed for this sort of debugging? And perchance would this be an interesting GHC hackathon subject? Andrew

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Haskell wiki: most popular pages
by Bulat Ziganshin 22 Aug '06

22 Aug '06

Hello haskell-cafe, The http://haskell.org/haskellwiki/Special:Popularpages page lists most popular pages on haskell wiki. I think this list is very useful because it shows us what are the questions about Haskell people most interested and gives us hints what should be improved in first place. The only catch is that most of popular pages are just ones listed on the title page. But nevertheless... On the other side, we can simplify title page by omitting links to page that are of little interest for casual readers (such as History). We can also add here links to most popular pages, such as GHC and "Hitchhikers guide". Below is beginning of this list: Haskell (252,505 views) Introduction (50,091 views) Libraries and tools (41,864 views) Books and tutorials (40,040 views) Language and library specification (32,773 views) Haskell in practice (31,698 views) Implementations (24,141 views) GHC (20,634 views) Haskell in 5 steps (16,707 views) Learning (14,088 views) Hitchhikers guide to Haskell (13,191 views) Future (12,754 views) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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