- Beginners - Haskell.org

How to use earlier ghc version with stack
by stevix＠zoho.com 02 Apr '19

02 Apr '19

I'm going through a book on Haskell that is based on a ghc version between 7.8 and 8.0 I created a new project using the default settings and have a much more recent version of ghc installed How to I fall back to an earlier version? Using haskell stack on manjaro linux Steve

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ANN: Haskell Beginners PASHpost Community: "Haskell: The Haskell-Beginners Community"
by Benjamin L. Russell 02 Apr '19

02 Apr '19

Since the Google+ social network service is scheduled to be "sunset" (i.e., put out of service) on April 2, 2019, I have created a new Haskell beginners PASHpost community, "Haskell: The Haskell-Beginners Community" (see https://pashpost.com/group/haskell-the-haskell-beginners-community) and migrated a substantial portion of the original posts thereto (sans posts with missing links or irrelevant content). Feel free to join and contribute! -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Computer Science Document Translator/Editor "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Re: [Haskell-beginners] Integrating external dependencies with stack
by Noel Kwan 01 Apr '19

01 Apr '19

I am trying to use stack to build a web scraper with scalpel. However, it requires curl-1.8.0 as a dependency and I have no clue how to integrate this dependency. Any insight would be greatly appreciated.

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IO String and String using readFile
by Yugesh Kothari 27 Mar '19

27 Mar '19

Hi, I am trying to read data from a file. When I do- ghci> contents <- readFile "file.txt" ghci> words contents Then everything works. The same thing when done in a .hs file gives an IO String vs [String] error. Why is that so?

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A sort of fold for bits
by Mario Lang 26 Mar '19

26 Mar '19

Hi. I have written the following function for "iterating" over all set bits in a Bits instance: foldBits :: (Bits bits, Num bits) => (a -> Int -> a) -> a -> bits -> a foldBits _ a 0 = a foldBits f a n = foldBits f (f a lsb) (n `clearBit` lsb) where lsb = countTrailingZeros n The type signature mimmicks foldl. However, composing several calls to foldBits ends up pretty messy: foldBits f (foldBits f (foldBits f [] value1) value2) value3 So I was thinking, maybe I should just flip the final arguments to make composition easier: fooBits :: (Bits bits, Num bits) => (a -> Int -> a) -> bits -> a -> a fooBits _ 0 a = a foldBits f n a = foldBits f (n `clearBit` lsb) (f a lsb) where lsb = countTrailingZeros n fooBits f value3 . fooBits f value2 . fooBits f value1 $ [] This looks more tidy. However, now that I am no longer mimmicking foldl, I am sort of at a loss how to name this beast. iterBits comes to bind, but this isn't really like iterate. I know, this question might look silly, but good naming conventions seem to make it easier for me to reason about the code. Any suggestions? P.S.: Mimmicking foldl is probably wrong as well, since this isn't really a fold, is it? -- CYa, ⡍⠁⠗⠊⠕

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Learn you a Haskell for great good! group reading session
by Jaysinh shukla 17 Mar '19

17 Mar '19

Respected Member, I am trying to learn Haskell from the book Learn you a Haskell for great good! [1]. I livestream my reading sessions at this [2] Twitch channel. Group reading is helping me to learn better. It is way more productive than reading alone. If you are learning Haskell, please join us. If you already know Haskell, please join to teach. Teaching will give you a good revision. Don't worry about the commitment to attend each and every session. It is fine to attend sessions on the day you are available and skip the rest. The fun part is, all the sessions will be on weekends. You can watch videos of past sessions here [3]. I am trying to maintain an event calendar for upcoming sessions here [4]. Please share these resources with whoever learning the Haskell. Don't hesitate to ask your questions here or in a personal mail. Thanks! 1: http://learnyouahaskell.com 2: https://www.twitch.tv/jaysinhp 3: https://www.twitch.tv/jaysinhp/videos 4: https://www.twitch.tv/jaysinhp/events

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Matrix and types
by mike h 15 Mar '19

15 Mar '19

Hi, As an exercise I want to write a Matrix library. Multiplication of two matrices is only defined when the the number of columns in the first matrix equals the number of rows in the second matrix. i.e. c1 == r2 So when writing the multiplication function I can check that c1 == r2 and do something. However what I really want to do, if possible, is to have the compiler catch the error. I’d appreciate any advice on how to approach this. I don’t want a full description of exactly what to do as that way I won’t have struggled or argued with the compiler - which for me is the best way to learn Haskell :) Thanks Mike

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Error Message: Can not find C library. How can I do this?
by S. H. Aegis 01 Mar '19

01 Mar '19

Hello. I’m new to stack & FFI. I created a dll file by http://www.mingw.org/wiki/sampleDLL And I put all files into dll folder. (Those files are example_dll.cpp, example_dll.dll, example_dll.h, example_dll.o, example_exe.cpp, libexample_dll.a) I test example_exe.exe file, and confirmed that works. (Windows7 Pro, stack 1.9.3) <package.yaml> executables: test2-exe: main: Main.hs source-dirs: app ghc-options: - -threaded - -rtsopts - -with-rtsopts=-N dependencies: - test2 extra-libraries: - example_dll.a extra-lib-dirs: - dll <stack.yaml> # Extra directories used by stack for building extra-include-dirs: - dll extra-lib-dirs: - dll <Main.hs> {-# LANGUAGE ForeignFunctionInterface #-} module Main where -- import Lib import Foreign foreign import ccall "example_dll.h hello" c_hello :: IO() main :: IO () main = c_hello <example_dll.cpp> #include <stdio.h> #include "example_dll.h" __stdcall void hello(const char *s) { printf("Hello %s\n", s); } int Double(int x) { return 2 * x; } void CppFunc(void) { puts("CppFunc"); } void MyClass::func(void) { puts("MyClass.func()"); } When I try to stack build, I get error messages. *C:\Users\shaegis\Documents\Haskell\test2\dll>stack build* *Building all executables for `test2' once. After a successful build of all of them, only specified executables will be rebuilt.* *test2-0.1.0.0: configure (lib + exe)* *Configuring test2-0.1.0.0...* *Cabal-simple_Z6RU0evB_2.4.0.1_ghc-8.6.3.exe: Missing dependency on a foreign* *library:* ** Missing (or bad) C library: example_dll.a* *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* *library 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.* *-- While building package test2-0.1.0.0 using:* * C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_2.4.0.1_ghc-8.6.3.exe --builddir=.stack-work\dist\e626a42b configure --with-ghc=C:\Users\shaegis\AppData\Local\Programs\stack\x86_64-windows\ghc-8.6.3\bin\ghc.EXE --with-ghc-pkg=C:\Users\shaegis\AppData\Local\Programs\stack\x86_64-windows\ghc-8.6.3\bin\ghc-pkg.EXE --user --package-db=clear --package-db=global --package-db=C:\sr\snapshots\3233b5e2\pkgdb --package-db=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\pkgdb --libdir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\lib --bindir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\bin --datadir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\share --libexecdir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\libexec --sysconfdir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\etc --docdir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\doc\test2-0.1.0.0 --htmldir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\doc\test2-0.1.0.0 --haddockdir=C:\Users\shaegis\Documents\Haskell\test2\.stack-work\install\784ab3f0\doc\test2-0.1.0.0 --dependency=base=base-4.12.0.0 --extra-include-dirs=C:\Users\shaegis\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include --extra-include-dirs=C:\Users\shaegis\Documents\Haskell\test2\dll --extra-lib-dirs=C:\Users\shaegis\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\bin --extra-lib-dirs=C:\Users\shaegis\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\lib --extra-lib-dirs=C:\Users\shaegis\Documents\Haskell\test2\dll --enable-tests --enable-benchmarks* *Process exited with code: ExitFailure 1* How can I fix this? I did a lot of things on the webpages, but I could not. Please, help me. Sincerely, S. Chang.

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A game of life implementation
by Michel Haber 24 Feb '19

24 Feb '19

Hello everyone, I'm a new haskeller, and (like many others, I assume) I thought I'd try my hand at Conway's "Game of Life". So here is my code that seems to work (up to this point). I am looking for feedback in order to improve my Haskell code on all levels. Especially (In no particular order): 0- Find and fix bugs 1- Write more performance optimal code. 2- Good use of polymorphic types. 3- Good use of higher-order functions. 4- Good use of Haskell's common (and uncommon) abstractions. 5- Coding style (I'm finding it hard to let go of the function types :p) 6- Good code structuring allowing for reuse and updates. 7- Best options to give to the compiler. 8- Anything else that comes to your mind! So I'd really appreciate your feedback :) This is the wikipedia reference for the game of life: https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life And this is the code: START OF CODE -- Game of life Haskell Implementation import Data.List import Control.Monad.State import qualified Data.Map as M -- The cell state is isomorphic to Bool. type CellState = Bool -- The coordinates of a cell type Coord = (Int, Int) -- The board size is (length, width) type Size = (Int, Int) -- The state of the board is simply the coordinates of its live cells type Board = [Coord] -- The state carried in the State Monad, used to count tags for cells type TallyState = State (M.Map Coord (CellState, Int)) () -- The type of the game rules type Rules = (Coord, CellState, Int) -> CellState -- The type for the neighbor functions type Neighbors = Coord -> [Coord] -- Tally the live neighbors of live cells and relevant dead cells tallyBoard :: Neighbors -> Board -> TallyState tallyBoard nb = mapM_ $ tallyCoord nb -- Tally a live cell: Set its state to True (alive) and tag its neighbors -- This function takes the neighbors function as its first argument. We can use -- different neighbor functions to change the zone of influence of a cell tallyCoord :: Neighbors -> Coord -> TallyState tallyCoord nb c = do let merge (a1,b1) (a2,b2) = (a1 || a2, b1 + b2) s <- get let s' = M.insertWith merge c (True, 0) s let neighbors = nb c put $ foldl' (\acc x -> M.insertWith merge x (False, 1) acc) s' neighbors -- Extract the results from a TallyState toResults :: TallyState -> [(Coord, CellState, Int)] toResults s = map flatten . M.toList . execState s $ M.empty where flatten (x,(y,z)) = (x,y,z) -- Use A Rules and Neighbors function to advance the board one step in time advance :: Rules -> Neighbors -> Board -> Board advance rules nb = map first . filter rules . toResults . tallyBoard nb where first (x,_,_) = x -- The standard neighbors function stdNeighbors :: Neighbors stdNeighbors (x,y) = [ (a,b) | a <- [x-1, x, x+1] , b <- [y-1, y, y+1] , (a /= x) || (b /= y) ] -- Standard game rules stdRules :: Size -> Rules stdRules (a,b) ((x,y),_,_) | (x < 0) || (y < 0) || (x >= a) || (y >= b) = False stdRules _ (_,True,c) | (c == 2) || (c == 3) = True | otherwise = False stdRules _ (_,False,3) = True stdRules _ _ = False -- Main loop loop :: (Board -> Board) -> Board -> IO () loop f b = do print b unless (null b) $ loop f (f b) -- Main function main :: IO () main = do putStrLn "Choose board size (x,y)" input <- getLine putStrLn "Choose starting points" start <- getLine putStrLn "Game:" let size = read input let rules = stdRules size let initial = map read . words $ start let game = advance rules stdNeighbors loop game initial END OF CODE Thanks :)

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Use of interact
by Michele Alzetta 10 Feb '19

10 Feb '19

I solved the hacker-rank hello world n times challenge thus: hello_worlds :: Int -> IO () hello_worlds n | n < 1 = return () | otherwise = do putStrLn "Hello World" hello_worlds (n-1) main :: IO() main = do n <- readLn :: IO Int hello_worlds n I would like to solve this by using the interact function. If I leave my hello_worlds function as is and change the main function as follows: main = interact $ show . hello_worlds . read::Int I get: Couldn't match expected type ‘IO t0’ with actual type ‘Int’ • In the expression: main When checking the type of the IO action ‘main’ helloworlds.hs:14:8-44: error: • Couldn't match expected type ‘Int’ with actual type ‘IO ()’ • In the expression: interact $ show . hello_worlds . read :: Int In an equation for ‘main’: main = interact $ show . hello_worlds . read :: Int could someone please explain why this can't work? Is it possible to use interact in such a context? Thanks

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