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haskell + python numpy
by PICCA Frederic-Emmanuel 22 Mar '16

22 Mar '16

Hello, I would like to use some python code in order to do my computation with haskell. The idea is to use python and all it's numrical stack in some places, until I find the time to replace the python code by some haskell code. what I will do is read the data from hdf5 with bindings-hdf5 in haskell obtain a Storable for all my data images then I would like to create a python object which will use these data in order to generate in return another array. get back this array and put it into another hdf5 file. Indeed I want to do this with the pipes library. I would like to know if someone have some code examples for this kind of use case. interfacing haskell with python numpy object back and forth. thanks for your help Frederic

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Do IO actions *have* to be glued by the do syntax?
by Olumide 22 Mar '16

22 Mar '16

Hello List, Do IO actions *have* to be glued by the do syntax? Many, if not all, the examples that I've come across in LYH seem to suggest so. And if so, why? BTW, if possible I'd appreciate an explanation that does not resort to monads. I haven't studied them yet and I'm sure I'd struggle to understand any explanation that resorts to monads. Thanks, - Olumide

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Need help groking the statement: expression1 in term:expression2
by Olumide 22 Mar '16

22 Mar '16

Hello List, I'd appreciate help understanding the second line of following block of code (from LYH, first line added for completeness), http://learnyouahaskell.com/input-and-output#randomness randoms' :: (RandomGen g, Random a) => g -> [a] randoms' gen = let (value, newGen) = random gen in value:randoms' newGen The part I'm really struggling with is random gen in value:randoms' newGen Thanks, - Olumide

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Using Cassava with medium sized files (~50MB)
by Antoine Genton 20 Mar '16

20 Mar '16

Hello, I tried to load a ~50MB csv file in memory with cassava but found that my program was incredibly slow. After doing some profiling, I realized that it used an enormous amount of memory in the heap: 24,626,540,552 bytes allocated in the heap 6,946,460,688 bytes copied during GC 2,000,644,712 bytes copied maximum residency (14 sample(s)) 319,728,944 bytes maximum slop 3718 MB total memory in use (0MB lost due to fragmentation) ... %GC time 84.0% (94.3% elapsed) Seeing that, I have the feeling that my program lacks strictness and accumulates thunks in memory. I tried two versions, on using Data.Csv, and one using Data.Csv.Streaming. Both are giving the same result. What am I doing wrong? Here are the two sources: 1/ import Data.Csv import Data.ByteString.Lazy as BL import qualified Data.Vector as V main :: IO () main = do csv <- BL.readFile "tt.csv" let !res = case decode NoHeader csv of Right q -> q :: V.Vector(V.Vector(ByteString)) print $ res V.! 0 -------------------------------- 2/ import Data.Csv.Streaming import Data.ByteString.Lazy as BL import qualified Data.Vector as V import Data.Foldable main :: IO () main = do csv <- BL.readFile "tt.csv" let !a = decode NoHeader csv :: Records(V.Vector(ByteString)) let !xx = V.fromList $ [V.fromList([])] :: V.Vector(V.Vector(ByteString)) let !res = Data.Foldable.foldr' V.cons xx a print $ res V.! 0 The goal of the program is ultimately to have the csv loaded in memory as a Vector of Vector of ByteString for further processing later on. Thank you for your help, Antoine

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better way: contains one of a string from list in a string
by Miro Karpis 20 Mar '16

20 Mar '16

Hi, I needed a function that returns True/False if a list of strings are in a given string. I made one function below (works fine),..but I was wondering whether there is a shorter way? -- |Returns True if one of given strings are contained in given string contains :: [String] -> String -> Bool contains elements seach_elem | trueListCount == 0 = False | otherwise = True where isInStringList = [isInfixOf elem seach_elem | elem <- elements] onlyTrueList = [elem | elem <- isInStringList, elem == True] trueListCount = length onlyTrueList Cheers, -m

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Parsing
by Mike Houghton 19 Mar '16

19 Mar '16

Hi, I’m using Parsec to parse structured text and have some problems with mandatory and optional text entries and the order in which they occur. For example module{ name = some string source = … dest = ….. bundle = … bundle_dest = … zip_name = …. } In the above text the name and source are mandatory and all others are optional. I’m not quite sure how to handle the optional parts once I’ve parsed name and source. Should I enforce the rule that if dest does appear then it must be first in the list? In this situation how should I make use of the various Parsec functions such as choice, option, optionMaybe, lookAhead etc. Thanks Mike

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Multiplicaton of singletons
by Dmitriy Matrosov 16 Mar '16

16 Mar '16

> {-# LANGUAGE DataKinds, KindSignatures, TypeFamilies, UndecidableInstances #-} Hi. I've reading "Part I: Dependent Types in Haskell" by Hiromi ISHII at schoolofhaskell [1] and i can't understand why my multiplication for singletons (that was exercise) does not not work: > data Nat = Z | S Nat > > type family Plus (a :: Nat) (b :: Nat) :: Nat where > Plus 'Z b = b > Plus ('S a1) b = 'S (Plus a1 b) > > type family Mult (a :: Nat) (b :: Nat) :: Nat where > Mult 'Z b = 'Z > Mult ('S 'Z) b = b > Mult ('S a1) b = Plus (Mult a1 b) b > > data SNat :: Nat -> * where > SZ :: SNat 'Z > SN :: SNat n -> SNat ('S n) > > plusN :: SNat n -> SNat m -> SNat (Plus n m) > plusN SZ y = y > plusN (SN x) y = SN (plusN x y) > > multN :: SNat n -> SNat m -> SNat (Mult n m) > multN SZ y = SZ > multN (SN SZ) y = y > multN (SN x) y = plusN (multN x y) y The last line (multN) does not type-check with error: 1.lhs:31:25: Could not deduce (Mult ('S n1) m ~ Plus (Mult n1 m) m) from the context (n ~ 'S n1) bound by a pattern with constructor SN :: forall (n :: Nat). SNat n -> SNat ('S n), in an equation for ‘multN’ at 1.lhs:31:10-13 Expected type: SNat (Mult n m) Actual type: SNat (Plus (Mult n1 m) m) Relevant bindings include y :: SNat m (bound at 1.lhs:31:17) x :: SNat n1 (bound at 1.lhs:31:13) multN :: SNat n -> SNat m -> SNat (Mult n m) (bound at 1.lhs:29:3) In the expression: plusN (multN x y) y In an equation for ‘multN’: multN (SN x) y = plusN (multN x y) y Though, when (n ~ 'S n1) i get `SNat (Mult ('S n1) m)` and by definition of Mult this is `SNat (Plus (Mult n1 m) m)` . On the other hand, when i check the type of expression, i get `SNat (Plus (Mult n1 m) m)` by definition of plusN. I.e. exactly the same type. [1]: https://www.schoolofhaskell.com/user/konn/prove-your-haskell-for-great-safe…

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lookAhead
by Dennis Raddle 16 Mar '16

16 Mar '16

I'm working with "lookAhead" in Parsec. My goal is to simplify error messages and make them more relevant. I'm parsing strings that represent musical score text markings that convey nuances of playback expression. I worked long and hard to devise a concise syntax that has sufficient expressive power --- must be concise because there is little room in these score for text markings, especially if there are a lot of them. The result is that it looks a little cryptic. 1;%2|> <%4| <<5:4 <<5:4a etc. My first parser worked, but would give error messages like "Unexpected ';', expected digit, ';', '%', '<<', '|', .... etc., basically giving me no clue what the parser was thinking. The first challenge is that it's hard to identify the particular type of mark from the first few characters. For instance, a mark like this "1;%2|>" --- what gives it away is the ">" at the very end. That means it's a "right-facing warp" and that defines what to expect in the other characters. So it would be great if my parser first identified this as a "right-facing warp" and then when parsing the internals it can give messages that make sense in context. So I tried this, with lookAhead rightWarp = do try (lookAhead (do many1 (noneOf "<>") char '>' eof)) ... parse internals ... char '>' eof The problem is that I want to put a <?> message in here somewhere so that if this whole 'rightWarp' parser fails, it will say "expected right warp". But I have found with experiment that the parser considers the point of failure to be in different places inside the lookAhead. For instance if I give an input string that has too many characters at the end, the failure point will be the 'eof' inside the lookAhead. If I put a <?> message anywhere else, the parser will just say "expected end of input". On the other hand, if there is no '>' character, then the failure point will be "char '>'" and the message will be "expected >". What I was hoping was that there was some single place I could put a <?> message that would say "expected right warp" no matter where inside rightWarp the failure point occurs. Any ideas? Mike

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Getting a specified number of lines from stdin
by Nicolaas du Preez 11 Mar '16

11 Mar '16

Good day All, Why does liftM (take 5) $ sequence $ repeat getLine not stop at reading only 5 lines from stdin but keeps on reading more? What I’m really trying to achieve is to get input from the user until an empty line is read with the code below. But this doesn’t work as I expected, similar to the above. liftM (takeWhile (/= "")) $ sequence $ repeat getLine Regards, Nicolaas du Preez

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Showing Types
by Dániel Arató 08 Mar '16

08 Mar '16

How does GHCi show types? Is there a magic function (showType :: a -> String) that does that? Or is this feature buried somewhere deep in the compiler? Can I show, compare and reason about types in a Haskell program? This would be cool: ```haskell test = do let len = sum . map (const 1) when (isInfixOf "Integer" (showType len)) $ putStrLn "restrictive type inferred; maybe try turning off monomorphism restriction" ```

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