April 2008 - Haskell-Cafe

catching fails on read
by Jacques Bergeron 29 Apr '08

29 Apr '08

I'm new to Haskell and have a stupid question. How can a catch a parsing error on a read? Something like let s = "e" in read s :: Int I've been searching for a trick and couldn't find one. I want to enter commands directly from the keyboard and don't want the program to abort if I do a typing error. Regards, Jacques

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Something like optimistic evaluation
by Daniil Elovkov 29 Apr '08

29 Apr '08

Hello Somewhat on the topic of optimistic evaluation, I've just thought of another way to evaluate thunks. When the program is about to block on some IO, what if we start a thread to evaluate (any) unevaluated thunks. We'll have additional system thread, but the blocked one will not actually consume any processor time. This would take place only when the program is compiled as threaded and run with -N k, k>1. The RTS knows at least about some operations that will block, those which IO operations are implemented with. for example. It could merely start a process of evaluating any (or something more clever) outstanding thunks right before going into one of those operations and stop it when it's back. Of course, it's not like optimistic evaluation because we don't avoid creating thunks. But in a sense it's similar. It could also be compared with incremental garbage collection :) Has something like that been done, discussed?

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Mutual recursive data types
by rodrigo.bonifacio 29 Apr '08

29 Apr '08

Hi all, I have the following data types: type Id = String type Action = String type State = String type Response = String data Scenario = Scenario Description [Step] data Step = Step Id Scenario Action State Response So, there is a mutual recursion between Scenario and Step. Now, consider the following function: xmlScenario2Scenario :: XmlScenario -> Scenario xmlScenario2Scenario (XmlScenario description steps) = Scenario description [xmlStep2Step x | x <-steps] How can I send "scenario" as an argument for xmlStep2Step? I've tried let and where but I get in a loop. Thanks a lot, Rodrigo. ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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pls help about subtree
by cetin tozkoparan 29 Apr '08

29 Apr '08

Assume a tree is a subtree of the other if all elements of the first tree is included in the second with the exact structure; all parent-child relations are preserved with their order. data Tree = Empty | Leaf Int | Node (Int,Tree,Tree) subtree:: Tree -> Tree -> Bool How can i start to write this code with this data structure? Example; tree1 tree2 3 / \ 5 6 / \ 7 4 13 / \ 3 12 / \ / \ 5 6 9 6 / \ / \ 7 4 11 7 / / \ 2 3 4 ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ

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force inlining in GHC
by Henning Thielemann 29 Apr '08

29 Apr '08

Whenever I try to inline a lot of nested function calls, GHC decides to specialise one of the functions and the specialised function is no longer inlined. I hoped to get the function inlined anyway by specialising it manually. Say, I want to inline genericFunc {-# INLINE genericFunc #-} genericFunc :: RealFrac a => a -> Int but GHC inserts a call to genericFunc1 specialised to a=Double where I apply genericFunc to a Double argument. Now I define {-# INLINE doubleFunc #-} doubleFunc :: Double -> Int doubleFunc = genericFunc However, in the Core output 'doubleFunc' does not get the __inline_me tag and thus will not be inlined, too. :-(

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FFI Query: How to free a CString allocated in Haskell
by Verma Anurag-VNF673 29 Apr '08

29 Apr '08

Hi, I am calling a Haskell function from my C code, wherein I export a Haskell function xyz that takes as an argument string and returns a string: Foreign export ccall xyz::CString -> IO CString xyz = do -- extract the input string and act on it-- -- at the end I return a string like this newCString str Now once I call this function from C code, I am freeing the allocated memory using free function. I want to confirm that this is the right thing to do. - Anurag

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Confused by instances
by Fraser Wilson 28 Apr '08

28 Apr '08

Hello, Haskellers, I feel like I'm missing something obvious, but here's some example code: > module Instance where > > data Value = Value Integer > > class ValueClass a where > fromValue :: Value -> a > > instance ValueClass Bool where > fromValue (Value n) = n /= 0 > > instance ValueClass String where > fromValue (Value n) = show n > > instance (Num a) => ValueClass a where > fromValue (Value n) = fromInteger n > The Bool instance compiles fine. The String instance fails, and that would be because String is a synonym for [Char], and we can't create instances for those. There's a slight hack in the Prelude which allows you to define your own list show function, and that's exactly what gets done by Show Char, right? I'm sorry about calling it a hack but, well, it is. :) What I'm really confused by is the response to instance (Num a) => ValueClass a -- what I am trying to say is "if a is an instance of Num, then can be an instance of ValueClass too, and here's how". But ghc says: Instance.hs:14:0: Illegal instance declaration for `ValueClass a' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `ValueClass a' I don't understand why what I'm doing above is different from, for example, instance Show a => Show [a], or Monad m => MonadState s (StateT s m) ... I imagine that it's related to the fact that the difference between those two and what I have is that they have either the same class on each side of the =>, or new type variables on the right. But I'm having trouble abstracting from that to the general rule. Any help would be greatly appreciated. In context, I'm attempting to wrap up different types inside a data type, and rather than extracting the value with getBoolean, getInteger, getString etc, I thought I would let type classes work for me, and just say fromValue instead. Is this silly? You can tell me, I won't be hurt. cheers, Fraser.

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announce: Glome.hs-0.3 (Haskell raytracer)
by Jim Snow 28 Apr '08

28 Apr '08

A new version of my raytracer is out. It now supports cones, cylinders, disks, boxes, and planes as base primitives (previously it only supported triangles and spheres), as well as transformations of arbitrary objects (rotate, scale, translate) and the CSG operations difference and intersection. Perlin noise and Blinn highlights have been added, as well. http://syn.cs.pdx.edu/~jsnow/glome/ Glome can parse NFF-format scene files (see http://tog.acm.org/resources/SPD/) but many features are only accessible via raw Haskell, since NFF doesn't support very many kinds of primitives. I included a TestScene.hs file that demonstrates how to create a scene with various kinds of geometry (including a crude attempt at a recursively-defined oak tree) in haskell. There isn't any documentation yet, but many of the primitives have constructors that resemble their equivalents in povray, so anyone familiar with povray's syntax should be able to figure out what's going on. -jim

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Re: [Haskell-cafe] Trouble compiling collections-0.3 (from Hackage)
by Adrian Hey 28 Apr '08

28 Apr '08

ccing Haskell Cafe in case anyone else is interested in my answer.. Bryan O'Sullivan wrote: > Adrian Hey wrote: > >> I don't think anyone is interested in working on this or maintaining >> it, so it's probably best not to use it for new stuff. > > If nobody has stepped up yet, I'll take it over. It would be a shame to > see it bitrot. I think the main problem is that the collections package contained a number of different sub-projects that should really be separated out (and indeed have been to some extent), as they were progressing at rather different rates and it was never clear when would be a good time to do a new release of collections as a whole, so it was never done. There's the abstract collections class API's which were really what Jean-Philippe Bernardy was working on. I'm not sure if he's still interested in that so you'd better ask him. I think he's more interested in Yi these days :-) Then there's the AVL tree lib which has been put in hackage as a separate package already. I kinda washed my hands of this too, but I guess I'd better change my mind about that as if I don't maintain it I don't think any one else will. Then there's the relatively trivial AVL based Data.Set/Map clones which aren't in hackage yet but I guess could be put there without too much work. Then there's all the generalised trie stuff which has stalled for the time being, at least until type families are available. You'll probably see some more action on this in the summer as it's the subject of Jamie Brandons GSoC project. Regards -- Adrian Hey

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Simplest possible Fasta shootout entry. How do I zap the ugly line? Suggest any other improvements.
by Richard Kelsall 28 Apr '08

28 Apr '08

(Extracting these questions from my previous thread for clarity.) Below is my simplest possible program to solve the Fasta shootout benchmark. http://shootout.alioth.debian.org/gp4/benchmark.php?test=fasta&lang=all http://haskell.org/haskellwiki/Shootout/Fasta I can see one remaining flaw - the line marked 'Ugly'. What's the best way to get rid of this line? Any other suggestions for simplifying or improving the program would also be interesting. This code is about three or four times slower that the current fastest GHC entry for the Fasta benchmark. I'll elaborate it for speed when I've produced the best version regardless of speed. Richard. {-# OPTIONS -O -fexcess-precision #-} -- The Computer Language Shootout : Fasta -- http://shootout.alioth.debian.org/ -- Simple solution by Richard Kelsall. -- http://www.millstream.com/ import System main = do n <- getArgs >>= readIO . head title "ONE" "Homo sapiens alu" writeLined (cycle alu) (n * 2) title "TWO" "IUB ambiguity codes" let (r1, r2) = splitAt (fromIntegral (n * 3)) (rand 42) -- Ugly !! writeLined (map (look iubs) r1) (n * 3) title "THREE" "Homo sapiens frequency" writeLined (map (look homs) r2) (n * 5) title :: String -> String -> IO () title a b = putStrLn $ ">" ++ a ++ " " ++ b look :: [(Char, Float)] -> Float -> Char look [(c, _)] _ = c look ((c, f) : cfs) r = if r < f then c else look cfs (r - f) lineWidth = 60 writeLined :: [Char] -> Integer -> IO () writeLined cs 0 = return () writeLined cs n = do let w = min n lineWidth (cs1, cs2) = splitAt (fromInteger w) cs putStrLn cs1 writeLined cs2 (n - w) rand :: Int -> [Float] rand seed = newran : (rand newseed) where im = 139968 ia = 3877 ic = 29573 newseed = (seed * ia + ic) `rem` im newran = fromIntegral newseed / fromIntegral im alu = "GGCCGGGCGCGGTGGCTCACGCCTGTAATCCCAGCACTTTGGGAGGCCGAGGCGGGCGGA\ \TCACCTGAGGTCAGGAGTTCGAGACCAGCCTGGCCAACATGGTGAAACCCCGTCTCTACT\ \AAAAATACAAAAATTAGCCGGGCGTGGTGGCGCGCGCCTGTAATCCCAGCTACTCGGGAG\ \GCTGAGGCAGGAGAATCGCTTGAACCCGGGAGGCGGAGGTTGCAGTGAGCCGAGATCGCG\ \CCACTGCACTCCAGCCTGGGCGACAGAGCGAGACTCCGTCTCAAAAA" iubs = [('a', 0.27), ('c', 0.12), ('g', 0.12), ('t', 0.27), ('B', 0.02), ('D', 0.02), ('H', 0.02), ('K', 0.02), ('M', 0.02), ('N', 0.02), ('R', 0.02), ('S', 0.02), ('V', 0.02), ('W', 0.02), ('Y', 0.02)] homs = [('a', 0.3029549426680), ('c', 0.1979883004921), ('g', 0.1975473066391), ('t', 0.3015094502008)]

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