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Sorting
by mike h 13 Dec '16

13 Dec '16

Hi, I’m trying to sort a list of tuples. A char and a count of that char (Char , Int) e.g. [ ('r',2), ('c',2),('a', 2), ('b',3), ('f',2)] e.g. ‘r’ occurs twice etc. The order should be based on the count first and then ties broken by the natural ordering of char. So [ ('r',2), ('c',2),('a', 2), ('b',3), ('f',2)] will sort as [('b',3),('a', 2), ('c',2),('f',2), ('r',2)] I initially tried variants on sortBy (compare `on` snd) and then made a type Tup = T (Char, Int) and defined Eq and then got to the point where I felt that this had become too difficult for a simple problem and concluded that I’m missing a point somewhere and need a bit of help! Many thanks M

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Problem install Gtk
by Arnim Fiebig 13 Dec '16

13 Dec '16

The same happen on Win8.1 and another Pc with win7 1. install 'HaskellPlatform-8.0.1-full-x86_64-setup-a' ok 2. copy 'gtk+-bundle_3.6.4-20130513_win64' to C:\Gtk\ ok 3. 'cabal install gtk' failed Thanks for help Guenter

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crypto random UUID generation
by Ovidiu Deac 13 Dec '16

13 Dec '16

I have to produce a crypto random UUID. I haven't found simple examples. and I used the one from hre (see type CRand) http://hackage.haskell.org/package/monadcryptorandom-0.7.0/docs/Control-Mon… My attempt is the following: cryptoRandomUUID :: IO UUID.UUID cryptoRandomUUID = do g <- newGenIO:: IO SystemRandom case runCRand impl g of Left err -> throwIO err Right (v, g') -> return v where impl = do w1 <- getCRandom w2 <- getCRandom w3 <- getCRandom w4 <- getCRandom return $ UUID.fromWords w1 w2 w3 w4 ...but the compilation fails miserably with: • Ambiguous type variable ‘e0’ arising from a use of ‘runCRand’ prevents the constraint ‘(ContainsGenError e0)’ from being solved. Relevant bindings include impl :: CRandT SystemRandom e0 Data.Functor.Identity.Identity UUID.UUID (bound at src/Party.hs:75:9) Probable fix: use a type annotation to specify what ‘e0’ should be. These potential instance exist: instance ContainsGenError GenError -- Defined in ‘Control.Monad.CryptoRandom’ • In the expression: runCRand impl g In a stmt of a 'do' block: case runCRand impl g of { Left err -> throwIO err Right (v, g') -> return v } In the expression: do { g <- newGenIO :: IO SystemRandom; case runCRand impl g of { Left err -> throwIO err Right (v, g') -> return v } } ... What's the problem here? Are there some good examples for generating crypto-randoms? Thanks!

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Parse file with existentials
by Dmitriy Matrosov 13 Dec '16

13 Dec '16

> {-# LANGUAGE GADTs #-} > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE RankNTypes #-} > {-# LANGUAGE KindSignatures #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE DeriveDataTypeable #-} > > import Prelude hiding (getLine) > import Data.Maybe > import Data.List > import Data.Typeable > import Control.Monad.Identity > import Control.Monad.Trans.Identity > import Control.Monad.Writer > import Control.Applicative > import System.FilePath Hi. I ask for an opinion about interface (implemented below) for parsing `rsync` filter files. The parser does not parse full syntax, i wrote it for determining rsync filter dependencies, when installing them using `shake`. So, i distinguish two kinds of lines: include of another filter file, which looks like . file and any other. I want to distinguish them at type level, so e.g. a record function for one constructor can't be applied to another, etc. I know, that i can prevent this at runtime by exporting only smart constructor, but i want a type check. > data RsyncFilterT = IncludeT | LineT > type IncludeT = 'IncludeT > type LineT = 'LineT > > -- Particular rsync filters distinguishable at type-level. > data RsyncFilter :: RsyncFilterT -> * where > Include :: {getInclude :: FilePath} -> RsyncFilter 'IncludeT > Line :: {getLine :: String} -> RsyncFilter 'LineT > deriving instance Show (RsyncFilter a) > deriving instance Typeable RsyncFilter For accessing records i use lenses redefined here. I redefine them with `Applicative` instead of `Functor` to make modify/set work even if value does not have a required record (by returning original (unmodified) value using `pure`). > type LensA a b = forall f. Applicative f => (b -> f b) -> a -> f a > > viewA :: LensA a b -> a -> b > viewA l = fromJust . getLast . getConst . l (Const . Last . Just) > viewAmaybe :: LensA a b -> a -> Maybe b > viewAmaybe l = getLast . getConst . l (Const . Last . Just) > > modifyA :: LensA a b -> (b -> b) -> a -> a > modifyA l f = runIdentity . l (Identity . f) > > modifyAA :: Applicative t => LensA a b -> (b -> t b) -> a -> t a > modifyAA l f = runIdentityT . l (IdentityT . f) > > setA :: LensA a b -> b -> a -> a > setA l s = modifyA l (const s) Here're lenses for `RsyncFilter` (its constructors are distinguishable at type-level, so i don't really need `Applicative` lenses here): > includeL :: LensA (RsyncFilter 'IncludeT) FilePath > includeL f z@Include {getInclude = x} = > fmap (\x' -> z{getInclude = x'}) (f x) > lineL :: LensA (RsyncFilter 'LineT) FilePath > lineL f z@Line {getLine = x} = > fmap (\x' -> z{getLine = x'}) (f x) The order of lines (may) matter, so i need to store all `RsyncFilter a` values in a list in original file order. But now the values are of different type. So.. i use existential container: > -- Generic container for any type of rsync filter. > data AnyFilter = forall (a :: RsyncFilterT). Typeable a => > AnyFilter (RsyncFilter a) > deriving instance Show AnyFilter > deriving instance Typeable AnyFilter And still i want to work on values of certain type to have some guarantees against misuse, so i need to cast `AnyFilter` back into `RsyncFilter` value: > -- Extract rsync filter from AnyFilter. > getFilter :: (forall (a :: RsyncFilterT). Typeable a => > RsyncFilter a -> b) -> AnyFilter -> b > getFilter f (AnyFilter x) = f x and here i also want to use lenses, but now the value may be of different type, that the lens expect, so i really need `Applicative` lenses here: > rsyncIncludeL' :: LensA AnyFilter (RsyncFilter 'IncludeT) > rsyncIncludeL' f z = maybe (pure z) (fmap AnyFilter . f) (getFilter cast z) > rsyncIncludeL :: LensA AnyFilter FilePath > rsyncIncludeL = rsyncIncludeL' . includeL > > rsyncLineL' :: LensA AnyFilter (RsyncFilter 'LineT) > rsyncLineL' f z = maybe (pure z) (fmap AnyFilter . f) > (getFilter cast z) > rsyncLineL :: LensA AnyFilter String > rsyncLineL = rsyncLineL' . lineL Then i define another Read/Show class just to be able to keep default Read/Show instances: > class Serialize a where > fromString :: String -> Maybe a > toString :: a -> String > > instance Serialize (RsyncFilter 'LineT) where > fromString = Just . Line > toString (Line xs) = xs > > -- RULE and PATTERN separator is space (`_` not supported). > -- Only short rule names without modifiers are supported. > instance Serialize (RsyncFilter 'IncludeT) where > fromString = go . break (== ' ') > where > go :: (String, String) -> Maybe (RsyncFilter 'IncludeT) > go (r, _ : x : xs) > | r == "." = Just (Include (x : xs)) > go _ = Nothing > toString (Include xs) = ". " ++ xs > > instance Serialize AnyFilter where > fromString x = > fmap AnyFilter (fromString x :: Maybe (RsyncFilter 'IncludeT)) > <|> fmap AnyFilter (fromString x :: Maybe (RsyncFilter 'LineT)) > toString x = fromMaybe "" $ > fmap toString (viewAmaybe rsyncIncludeL' x) > <|> fmap toString (viewAmaybe rsyncLineL' x) and a lens from String to AnyFilter, which effectively parses file and writes it back: > rsyncAnyL :: LensA String AnyFilter > rsyncAnyL f z = maybe (pure z) (fmap toString . f) (fromString z) And here is how i use this: > -- | Replace path prefix, if matched. > replacePrefix :: FilePath -> FilePath -> FilePath -> FilePath > replacePrefix old new x = maybe x (combine new . joinPath) $ > -- For ensuring that path prefix starts and ends at path > -- components (directories) boundaries, i first split them. > stripPrefix (splitDirectories old) (splitDirectories x) > > -- | Rewrite path in rsync inlcude line @line@ from source path > -- @srcdir@ to install path @prefix@ > -- > -- > usedIncludes srcdir prefix line > -- > -- and collect (rewritten) rsync include pathes in @Writer@ monad. > -- Other lines return as is. > usedIncludes :: FilePath -- ^ Source path. > -> FilePath -- ^ Install path. > -> String -- ^ Line from rsync filter file. > -> Writer [FilePath] String > usedIncludes srcdir prefix = > modifyAA (rsyncAnyL . rsyncIncludeL) $ \x -> do > let x' = replacePrefix srcdir prefix x > tell [x'] > return x' and then a shake rule: -- | Add file rule for instaling rsync filters with extension -- @ext@, rewriting source path @srcdir@ to install path @prefix@ -- in any rsync includes: -- -- > rsyncFilter ext srcdir prefix -- rsyncFilter :: String -- ^ Extension. -> FilePath -- ^ Install path. -> FilePath -- ^ Source path. -> Rules () rsyncFilter ext prefix srcdir = prefix ++ "//*" <.> ext %> \out -> do let src = replacePrefix prefix srcdir out ls <- readFileLines src let (rs, incs) = runWriter $ mapM (usedIncludes srcdir prefix) ls need incs putNormal $ "> Write " ++ out writeFileChanged out . unlines $ rs I probably won't think too much about this API, if i haven't read [Luke Palmer's post about existentials][1] . And now i doubt, did i fall into the same trap with existentials and does not see an obvious solution with functions? [1]: https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential… -- Dmitriy Matrosov

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Real World Haskell: Deprecated Libraries
by John Ky 28 Nov '16

28 Nov '16

Hello Haskellers, I have been working through the Real World Haskell chapter on Profiling and Optimisation <http://book.realworldhaskell.org/read/profiling-and-optimization.html>, and got to the section *Advanced techniques: fusion*, where I encountered some code using Data.Array.Vector from the deprecated library uvector. Does anyone know how to rewrite this to not use deprecated libraries? I have tried to use Data.Vector.Unboxed from vector, but my version consumes a lot of memory. The code I have is here: https://github.com/haskell-works/hw-tutorial-performance/blob/master/hw-tut… With that code, I get “1831 MB total memory in use”, which is terrible Build run steps I used were: stack build --executable-profiling --library-profiling --ghc-options="-fprof-auto -rtsopts -auto-all -caf-all -fforce-recomp" time /Users/jky/wrk/haskell-works/hw-tutorial-performance/.stack-work/install/x86_64-osx/lts-7.10/8.0.1/bin/hw-tutorial-performance-rwhe +RTS -sstderr -p -hy -RTS 1e7 I get very large numbers of allocations of Pair and Double. There is also a very large upfront allocation ARR_WORDS, which I feared would happen when using Data.Vector Any help appreciated. Cheers, -John

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Setting up WXHaskell on the 8.0.1 platform?
by Cleverson Casarin Uliana 27 Nov '16

27 Nov '16

Hello all, I've sent this to the WXHaskell list, but no answers. Please what is the recommended way to set up WXWidgets and WXHaskell for the 8.0.1 Haskell Platform on Windows? I tried using the Achelanne installer, but it gives errors apparently related to packages versions incompatibilities. Thanks, Cleverson

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pattern matching on a common element
by briand＠aracnet.com 25 Nov '16

25 Nov '16

Here's what I'm doing: data X = A1 String Double | A2 String Int | A3 String Double Int name c = case c of A1 name _ -> name A2 name _ -> name A3 name _ _ -> name I'm sure there's a better way...

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Equivalence (or not) of lists
by Lawrence Wickert 14 Nov '16

14 Nov '16

Hello all, I am a rank beginner to functional languages. Working through Lipovaca's book, up to Chapter 3. Ok, setup this function in editor and compiled: length' :: (Num b) => [a] -> b length' [] = 0 length' (_:xs) = 1 + length' xs skippy@skippy:~$ ghci GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> :l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) Ok, modules loaded: Main. *Main> length' [1,2,3] 3 *Main> 1:2:3:[] [1,2,3] *Main> length' 1:2:3:[] <interactive>:5:9: Could not deduce (Num [a0]) arising from the literal '1' from the context (Num a) bound by the inferred type of it :: Num a => [a] at <interactive>:5:1-16 The type variable 'a0' is ambiguous In the first argument of 'length'', namely '1' In the first argument of '(:)', namely 'length' 1' In the expression: length' 1 : 2 : 3 : [] *Main> Obviously, there is something I don't understand about the apparent non-equivalence of the lists [1,2,3] and 1:2:3:[]I am guessing that the solution is contained in that error message but I can't quite decipher it. Thanks for any help.

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How to tell haskell that 'a' is 'a' ?
by Lai Boon Hui 14 Nov '16

14 Nov '16

Hi all, skips :: [a] -> [[a]] skips xs = go 1 [] where go :: Int -> [[a]] -> [[a]] *-- Compiles if i remove this line* go n acc | n > (length xs) = acc | n == 1 = xs : (go (n+1) acc) | n < 1 = [] | otherwise = (everyN n xs) : (go (n+1) acc) everyN :: Int -> [a] -> [a] everyN n xs = case (drop (n-1) xs) of y:ys -> y : (everyN n ys) [] -> [] How can i tell haskell compiler that a is the sub program if the same as a in the main program? -- Best Regards, Boon Hui

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Request for Code Review: Dice Game Distribution
by Jan Brusch 12 Nov '16

12 Nov '16

Hi, I'm currently learning Haskell and as a first project I wrote a little helper for a dice game. It's very short. Nonetheless I would really appreciate a little Code Review or any other input. The Game: Each player writes down the numbers from 1 to 42. Each turn you roll 3 dice (6-sided). The rolling player can cross of any one number that he can construct from the three dice using the math operators +, -, *, /. First player to cross off all numbers wins. The Helper: Calculates the reachable numbers for each dice roll. It the aggregates a count for each number, from how many dice rolls it can be reached. E.g.: There are 216 possible dice rolls (6^3), the number 2 can be reached form 171 of them. The helper gives out the data in JSON format that can be read by NVD3 to be displayed in a browser. The graph helps you to make a decision on which number you should prefer to cross off with your current roll. The Code: http://lpaste.net/338358 The Graph: http://imgur.com/a/NSDT8 Looking forward to any feedback and thanks in advance Jan

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