August 2006 - Haskell-Cafe

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 23 Aug '06

23 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 23 Aug '06

23 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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DiffArray is an unsaturated type?
by Neil Mitchell 22 Aug '06

22 Aug '06

Hi, I note that in the base libraries, Data.Array.Diff is defined as: type DiffArray = IOToDiffArray IOArray However, IOToDiffArray takes 3 parameters. I thought this was not allowed in Haskell 98? Its annoying from my point of view because Hoogle wants to know the arity of a type, and if it has to chase down the arity of the RHS to do it, I probably just won't bother. Is there any difference between the above definition and: type DiffArray a b = IOToDiffArray IOArray a b Is the first prefered for any reason? Thanks Neil

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Useful: putCharLn {inspire by the Int->[Char] thread
by Gene A 22 Aug '06

22 Aug '06

The thread on the use of "show" and print to display an Int value, brought up a problem I had early on... the one of cleanly displaying a Char value, on a line all by itself.. My first attempts: This was just plain hard to read: with the character t being where it was: Prelude> putChar $ head "this and that" tPrelude> ----------------------- So I tried this and of course ... type mismatch: Prelude> putStrLn $ head "this and that" <interactive>:1:16: Couldn't match `String' against `Char' Expected type: [String] Inferred type: [Char] In the first argument of `head', namely `"this and that"' In the second argument of `($)', namely `head "this and that"' ---------------------- So I did this... to manually turn it to a string and it does work, but a little cumbersome to work into other function calls: Prelude> putStrLn $ (head "this and that"):[] t ------------- so the definition of putCharLn came to life {may be in some library already and I just haven't found it yet.. but it is in my toolbox now}: Prelude> let putCharLn c = putStrLn (c:[]) Prelude> and an application of it: Prelude> putCharLn $ head "this and that" t --------------- now I also have the char to string conversion alone: c2Str c = c:[] Prelude> let c2Str c = c:[] Prelude> c2Str 'A' "A" ------------------ Now this is almost too trivial a thing to mention these gizmos... what with all the monadic constructions that greater minds toss about on this list.. and I am trying to get understanding of that still, but .... sometimes we just accept the unacceptable little irritants rather than just code a solution, no matter how simple it is. There are probably troves of simple workarounds out there that seem too trivial to mention but hey, share 'em... might hit a guy like me that says. "now why didn't I think to do that?" happy days, gene

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