April 2016 - Beginners

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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Type depending on value
by Dmitriy Matrosov 13 May '16

13 May '16

> {-# LANGUAGE DataKinds, KindSignatures, GADTs, StandaloneDeriving #-} Hi. Here is natural numbers and its singleton definition, which i take from "Part I: Dependent Types in Haskell" article by Hiromi ISHII [1]: > data Nat = Z | S Nat > deriving (Show) > > data SNat :: Nat -> * where > SZ :: SNat 'Z > SN :: SNat n -> SNat ('S n) > deriving instance Show (SNat n) But i can't figure out how may i define function returning SNat value depending on Nat value: f :: Nat -> SNat n f Z = SZ f (S n) = SN (f n) This does not typecheck, because, as i understand, ghc can't infer type n. Is it possible to do this at all? [1]: https://www.schoolofhaskell.com/user/konn/prove-your-haskell-for-great-safe…

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audio generation
by Dennis Raddle 02 May '16

02 May '16

I'm writing a program that will use functions to generate audio. The Haskell code will write the audio samples to disk---no need for real time playback. I see some useful libraries for writing audio files. My question concerns efficiency when generating several million to 20 million samples (or even many times more than that if I use high-resolution sampling rates). They can be generated one at a time in sequence, so there's no need to occupy a lot of memory or postpone thunk evaluation. I'm going to need efficient disk writing. Note that I may need some pseudorandom numbers in my calculations, so I might want to calculate samples by state monadic computations to carry the generator state. What is my general strategy going to be for memory and time efficiency? I am pretty confused by Haskell "strictness" and normal head form and all that, which often doesn't seem to be very strict. Or bang patterns, etc. Is it going to be simple to understand what I need? Dennis

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Trying to prove Applicative is superclass of Functor, etc
by Silent Leaf 01 May '16

01 May '16

Hi, well aware it's trivial so I'll be terse: do we have?: > fmap = <*> . pure So we could write for example > liftA2 f fa fb = f <$> fa <*> fb this way?: > liftA2 f fa fb = pure f <*> fa <*> fb <*> Incidentally, do we have?: > liftA == liftM == fmap Thanks!

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Array update and `seq`
by Chul-Woong Yang 27 Apr '16

27 Apr '16

Hi, all When I fold a list to update Data.Array, memory usage is very high. Consider following source, which counts occurence of each element in a list (1): import Data.Array import Data.List import Control.DeepSeq go :: [Int] -> [Int] go = elems . foldl' update (array (0,99) [(i,0) | i <- [0..99]]) where update acc x = acc // [(x, acc ! x + 1)] main = putStrLn . unwords . map show . go . concat . replicate 5000 $ [0..99] Above program uses about 350MB at heap. Memory usage is same if I try to force strictness in array update with `seq` (2) : where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `seq` a' However, when I use `deepseq`, memory usage is gone (around 35Kbyte) (3): where update acc x = let v = acc ! x + 1 a' = acc // [(x,v `seq` v)] in a' `deepseq` a' What's the missing part in (2)? At (2), evaluation of updated array a' is forced and the value of array cell is also evaluated forcefully with `seq`. Any help would be appreciated deeply. -- Regards, Chul-Woong Yang

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approximate string search
by Mason Lai 26 Apr '16

26 Apr '16

Hi, I'm writing a Haskell program which is looking for whether or not a string (needle) exists within a larger string (haystack) given a Levenshtein distance. Note that it shouldn't calculate the Levenshtein distance between the needle and haystack. For example, needle: AAA haystack: TTTTTAATTTTT The needle would be "found" if the Levenshtein distance is set at 1, because dist("AAT", "AAA") == 1. The following almost works: distance(needle, haystack) - (len(haystack) - len(needle)) But it does not handle deletions correctly. I've previously written a program which does this approximate search in Java with the Wu-Manber extension of the Bitap, or shift-or, algorithm. The same strategy seems difficult to code up in Haskell because it's very "stateful" and involves a lot of bit-fiddling. (Maybe it's actually quite simple, but I'm not sure how I would go about doing this.) As a further complication, I'd prefer to keep the data packed as ByteStrings, as I'll be dealing with around 200 GiBs of data (split and parallelized over a cluster, but still a lot). I don't really know how to deal with ByteStrings without using the functions that come along in the ByteString module or unpacking the ByteString. I've been using the language-spelling package, which has a module which can calculate the Levenshtein distance between two ByteStrings. I see that it uses ListLike. I'm not really sure how it works, but I assume it makes ByteString an instance of ListLike, and then you have access to all of the ListLike methods? Does anyone have any advice on how I should proceed with this? I don't mind learning new things, but I don't really know what the best strategy is or where to look. -- Mason

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Feedback on Maybe+State+IO code
by shane pearman 26 Apr '16

26 Apr '16

I just read the M.P. Jones paper on the topic of monad transformers and have been playing a bit with some simple examples combining `Maybe` and `State` with `IO`: http://lpaste.net/6702944292505124864 I have the same "stuff" function defined 3 ways: two recursive methods, one of which takes the "maybe" function as an argument which is used to break the recursion, and a non-recursive method which is 'looped' using `iterateM_`. The functionality is that an integer is given as initial `State` and it is decremented until 0 is reached, printing each iteration and breaking by the result of the "maybe" function. I also wanted to ensure that no negative integer could enter into the `State` so the same "maybe" function is used in main to restrict entry into the "stuff" function, either using `for_` over the result or injecting the `Maybe` before the "stuff" function. The iterative calls are a little bit harder to read but the second (2) recursive call is fairly concise and readable. Basically I'm just looking for any suggestions if anything looks out of place or can be refined before I go on to do more involved error handling or logging. * There are also a couple helper functions at the bottom below "main" that I ended up not using but was wondering if they are defined somewhere.

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'No such file' - ghc Parsec Package Error
by Oliver Dean 26 Apr '16

26 Apr '16

Hello All, Silly question, I cannot seem to compile an additional .hs file using the –package parsec –o command… It has to be something to do with my directory locations no doubt. I’ve tried numerous attempts and still no luck. I have a program called simple_parser which was compiled using the ‘parser.hs' file. Now I wish to run and compile with the parsec package using the file ‘parsec.hs’… Both my .hs files are located in the same folder: 8afbc4c1:HS_Parser homefolder$ ls parsec.hs parser.hi parser.hs parser.o simple_parser Then when I try the following (and I have tried many different path routes ../../ and vice versa), I get the following: 8afbc4c1:HS_Parser homefolder$ ghc -package parsec -o simple_parser [..parsec.hs parsec.hs] <no location info>: can't find file: [..parsec.hs Sorry to be such a newbie! Any helpful direction would be greatly appreciated. Not sure if I am doing something wrong or my code is…? PS - Both .hs files have slightly differing lines of code (based on the Wiki Book Scheme -> Haskell tutorial).

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Re: [Haskell-beginners] Handling failed output
by Imants Cekusins 25 Apr '16

25 Apr '16

Hello Matt, Either Int (Int,Int) might work. Left ... by convention indicates 'other' result.

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cabal install zlib failing
by John Ky 24 Apr '16

24 Apr '16

Hello Haskellers, Does anyone know what to do in this situation? cabal install zlib Resolving dependencies... Notice: installing into a sandbox located at /home/ubuntu/hw-files/.cabal-sandbox/7.10.3 Downloading zlib-0.6.1.1... Configuring zlib-0.6.1.1... Failed to install zlib-0.6.1.1 Build log ( /home/ubuntu/hw-files/.cabal-sandbox/7.10.3/logs/zlib-0.6.1.1.log ): Configuring zlib-0.6.1.1... setup-Simple-Cabal-1.22.5.0-x86_64-linux-ghc-7.10.3: Missing dependency on a foreign library: * Missing (or bad) header file: zlib.h * Missing C library: z This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. If the header file does exist, it may contain errors that are caught by the C compiler at the preprocessing stage. In this case you can re-run configure with the verbosity flag -v3 to see the error messages. cabal: Error: some packages failed to install: zlib-0.6.1.1 failed during the configure step. The exception was: ExitFailure 1 This is what I have in my environment: ubuntu@box1179:~/hw-files$ dpkg -l | grep zlib ii zlib1g 1:1.2.3.4.dfsg-3ubuntu4 compression library - runtime ii zlib1g:i386 1:1.2.3.4.dfsg-3ubuntu4 compression library - runtime iU zlib1g-dev 1:1.2.8.dfsg-1ubuntu1 compression library - development And this: ubuntu@box1179:~/hw-files$ nix-env -q cabal-install-1.22.9.0 ghc-7.10.3 nix-1.11.2 zlib-1.2.8 Cheers, -John

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