September 2013 - Beginners

Split list by list using Continuations
by Dmitriy Matrosov 24 Sep '13

24 Sep '13

Hi. I want to write a function to split list by list. E.g. if i have input list "aXYbc" and list "XY" is separator, then result should be ["a", "bc"]. And i want to write it using Continuations. Here is my version, which implements following scheme: .. >>= f x(k+1) >>= f x(k+2) >>= f x(k+3) >>= f x(k+4) >>= .. ....(match to sep) ..........>+ | (failed) +<= (continuation backward) <=+ (add to word) | \------->..(match to sep).....+ | (succeed) +------>+.. (match to sep) .. > import qualified Data.Foldable as F > import Control.Applicative > import Control.Monad.Cont > > nullF :: F.Foldable t => t a -> Bool > nullF = null . F.toList > > addToHeadA :: Alternative f => a -> [f a] -> [f a] > addToHeadA x [] = [pure x] > addToHeadA x (y : ys) = (pure x <|> y) : ys > > type Sep a = [a] -- Word separator. > type Res5 f a = [f a] -- Result. > data SplitState5 m f a = MaybeSep5 (Sep a) (Res5 f a) > (() -> m (SplitState5 m f a)) > | Word5 (Res5 f a) > > split5M :: (Eq a, F.Foldable t, Alternative f, MonadCont m) => > Sep a -> t a -> m (Res5 f a) > split5M ks0 xs > | nullF xs = return [] > | otherwise = F.foldrM go (Word5 [empty]) xs >>= finalize > where > ksR = reverse ks0 > --go :: (Eq a, MonadCont m) => > -- a -> SplitState5 m f a -> m (SplitState5 m f a) > go _ (MaybeSep5 [] _ h) = h () > go x (MaybeSep5 [k] zs _) > | x == k = return (Word5 (empty : zs)) > go x (MaybeSep5 (k : ks) zs h) > | x == k = return (MaybeSep5 ks zs h) > | otherwise = h () > go x (Word5 zs) = callCC $ \r -> do > callCC $ \h -> go x (MaybeSep5 ksR zs h) >>= r > return (Word5 (x `addToHeadA` zs)) > finalize :: (Alternative f, MonadCont m) => > SplitState5 m f a -> m (Res5 f a) > finalize (Word5 zs) = return zs > finalize (MaybeSep5 _ _ h) = h () >> return undefined > And i have several questions about this implementation: - Is it good CPS implementation? Or there is much simpler and better one? - Can it be improved? - Can i make it more generic? - Would non-CPS implementation be better or simpler, than this one? -- Dmitriy Matrosov

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vim - quickly take suggestions from ghc in case of compile errors
by Nathan Hüsken 24 Sep '13

24 Sep '13

Hey, ghc has a nice feature, when one compiles a haskell program and has made a spelling error, it suggest similar names that could be what one meant. Example: Data.hs:24:44: Not in scope: `mont_' Perhaps you meant `month_' (line 10) Is there a vim plugin that takes advantage of this? So that I can compile my haskell project, and for errors where ghc has a suggestion insert the suggestion with a simple key press? Thanks! Nathan

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Evaluation order semantics..
by Mike Meyer 24 Sep '13

24 Sep '13

王兵兵 wbbtiger at gmail.com wrote: Since no one else answered this, I'll take a crack at it. > The Haskell language specification states that it is a non-strict language, > but nothing about the evaluation strategy (like when and how an expression > is evaluated, and to what level). It does mention the word "evaluate" > several times when talking about pattern matching. > > I have read a wonderful tutorial ( > http://en.wikibooks.org/wiki/Haskell/Laziness <http://en.wikibooks.org/wiki/Haskell/Laziness>) about lazy evaluation and > weak head normal form, but it is just an implemenation strategy of some > compiler, which I should not depend on when writing codes. > > I come from a strict language background and I just don't feel right if I > don't understand how my codes are execuated. I wonder why the language > specificition does not define the evaluation strategy. > > I hope someone can enlighten me. Thanks! I think you have already achieved enlightenment - you just don't feel right about it. The language specification doesn't define the evaluation strategy because it doesn't matter. At least, it doesn't matter in pure code. You'll get the same answer no matter what order expressions are evaluated in, so long as they are evaluated by the time they are needed. Not specifying evaluation order gives the compiler freedom to arrange it to get the best possible performance. That said, there are cases where the evaluation order matters. Come to think of it, most of them fall into two categories: the order matters for performance reasons (clearly outside the purview of a specification), or you want to insure that something is evaluated before it's actually used (like doing IO), and there are tools in the language to force evaluation in those cases.

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recursive and non recursive functions
by Dan Lior 23 Sep '13

23 Sep '13

The following snippet is from a recent stack overflow post (by someone else). 7 down vote accept I'd like to mention an important distinction: cyclic' = [0,1] ++ cyclic' cyclic'' = [0,1] ++ x where x = cyclic'' These two functions are recursive. But cyclic = let x = 0 : y y = 1 : x in x is not! It uses x internally, which is recursive, but the whole cyclic isn't. This is also why they're different when compiled into the core language. This has some important practical implications, namely that recursive functions can't be inlined, but non-recursive can. Presumably, cyclic = let x = [0,1] ++ y y = x in x is another "nonrecursive function". I don't understand the distinction. I imagine that it is related to the let…in clause, but I don't think that I understand that either. Can someone try to clarify this ? dan

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FFI - how to handle when external dll crashes
by Miro Karpis 22 Sep '13

22 Sep '13

Haskellers, I have an external dll which in some particular cases crashes. Basically first I need to send to my dll with MethodA some parameters and then call MethodB to do some calculations on those parameters. If I didn't give enough parameters then MethodB will crash the whole dll. Is there a way to handle this? Unfortunately there are no exceptions thrown from the dll. In ghci I'm getting following message: ERROR in InitNumericalSystem::initializeSystem. JuncLabel. cheers, m.

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Why is it so slow to solve "10^(10^10)"?
by yi lu 22 Sep '13

22 Sep '13

Why is it so slow to solve "10^(10^10)" in Haskell? Yi

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HXT followingSiblingAxis && transform on this and the thing before
by Rene＠gmail 22 Sep '13

22 Sep '13

Hello, I'd like to move around and transform some nodes in the following structure: <vol> <col pg="1" s="a"/> <art>Text of article 1</art> <art>Text of article 2</art> <col pg="1" s="b"/> <art>Text of article 3</art> <art>Text of article 4</art> </vol> Namely, I want to move COL in between each of the next two (or more) ART-Tags (<art><col../>...</art>. (So maybe "move" is the wrong word. I want to cut COL and paste it in multiple ART-Tags.) Plus when moving one COL in, I'd like to give the corresponding ART a new attribute with a counter (n="[1..]"), so that the articles in one column are numbered. So, to begin with, how do I get a COL-milestone together with the following ARTs (but of course without the next COL) ? (if that's the right way to do what I want to do..) I tried this: {-# LANGUAGE TupleSections, Arrows, NoMonomorphismRestriction #-} import Text.XML.HXT.Core import Control.Arrow.ArrowNavigatableTree file = "../auswahl_few16.xml" main = do runX (configSysVars [withCanonicalize no, withValidate no, withTrace 0, withParseHTML no] >>> readDocument [withErrors no, withWarnings no] file //> hasName "col" >>> addNav >>> followingSiblingAxis >>> remNav >>> putXmlTree "-") Which gives me an empty list. Then this: //> (hasName "col" <+> (addNav >>> followingSiblingAxis >>> remNav)) >>> putXmlTree "-" And there are my old results for COL, but again nothing for the siblings. In fact, it seems I have no clue, how to use followingSiblingAxis, and then again if it worked out, I'm not sure if my selection really is the one I need (maybe I get the siblings then only -- without the COL?). UPDATE: Thanks to this SO-Question (http://stackoverflow.com/questions/4767430/xpath-axis-get-all-following-nod…) using hxt-xpath I was able to evaluate (//col)[1]/following-sibling::art [1 = count (preceding-sibling::col[1] | (//col)[1])] to the first pair of ART-siblings. So that's very nice, but again not really what I want. I want to process every COL with its siblings and I hope HXT-Arrows offers a somewhat elegant solution to this. Thanks in advance, René T.

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help with FFI: passing char* to dll function + pointer freeing
by Miro Karpis 21 Sep '13

21 Sep '13

Please, I just can not figure this out: I have a method in my C dll: int somemethod(char *param, int length, double array[], int UB1, int UB2, bool isValid); I would like to use this method in Haskell, so I have defined it like: {-# OPTIONS_GHC -fglasgow-exts #-} import Foreign import Foreign.C foreign import stdcall unsafe "somemethod" c_somemethod :: Ptr CString -> Int -> Ptr (Double) -> Int -> Int -> Bool -> IO Int main = do let param = ["input"] let paramLength = length param realTable = [ 1.0, 2.0, 3.0 ] :: [Double] ub1 = 0 ub2 = 2 isValid = False realTablePtr <- newArray realTable x <- c_somemethod param paramLength realTablePtr ub1 ub2 isValid free realTablePtr putStrLn $ "c_somemethod output: " ++ show x putStrLn "Done" When I try to compile this I get error regarding the param string. I have no idea how to define it. Additionally to this, is the freeing of realTablePtr correct? compile error: Couldn't match expected type `Ptr CString' with actual type `[t0]' In the first argument of `c_somemethod', namely `param' Please, how can I ix this? cheers, m.

3 7

question about pattern guards
by TP 21 Sep '13

21 Sep '13

Hi, I have a question related to pattern guards in the syntax of Haskell 2010. Why is it not possible to combine them with a logical OR, instead of the comma that stands for a logical AND? http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#p… Thanks, TP

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Non-strictness semantics of Haskell
by 王兵兵 20 Sep '13

20 Sep '13

Hi, The Haskell language specification states that it is a non-strict language, but nothing about the evaluation strategy (like when and how an expression is evaluated, and to what level). It does mention the word "evaluate" several times when talking about pattern matching. I have read a wonderful tutorial ( http://en.wikibooks.org/wiki/Haskell/Laziness) about lazy evaluation and weak head normal form, but it is just an implemenation strategy of some compiler, which I should not depend on when writing codes. I come from a strict language background and I just don't feel right if I don't understand how my codes are execuated. I wonder why the language specificition does not define the evaluation strategy. I hope someone can enlighten me. Thanks! -- spockwang

1 0