- Beginners - Haskell.org

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

1 0

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

4 7

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.

3 4

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 :)

3 4

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

2 3

Install advice
by Lawrence Bottorff 04 Feb '19

04 Feb '19

So I've got a new machine with a fresh install of Ubuntu 18.10. How should I install latest-greatest Haskell? (Perhaps review my previous post about having so much trouble with conflicting Haskell install theories.) LB

2 1

Using a monad function inside the monad transfomer variant
by Moritz Tacke 02 Feb '19

02 Feb '19

Hi, I am running into difficulties regarding the use of monad transformers. The situation is as follows: I implemented some functions returning "RVar" results. Now, in a different part of the program, I am using STUArrays. Therefore my idea was to create a monad transformer stack which would have this type: someFunction:: a -> b -> RVarT (ST s) (STUArray s Int Double) I still want to use the functions that return RVars, so eg. rvarDouble :: RVar Double and then the definition of the transformer function would be: someFunction a b = do ... the_double <- rvarDouble .... This does not compile, complaining that; • Couldn't match type ‘Data.Functor.Identity.Identity’ with ‘ST s’ Expected type: RVarT (ST s) Double Actual type: RVar Double How can I re-user the RVar function in the RVarT monad transformer? Sincerely yours, Moritz

2 2

Parsing Terms in Brackets for Calculator
by Leonhard Applis 02 Feb '19

02 Feb '19

Hello, I'm currently doing my first steps in Haskell with a calculator and I'm stuck at the parser. I have a data Term which will build ... basically a tree of operations, and works fine. I need help for the function termify :: [Either Operator Term] -> Term It takes operators (such as +,**) and terms and output a new, bigger term and is mostly trivial. However, all attempts I've done for parsing brackets seem very ... crude and not like Haskell at all. The very first pattern match should check for the innermost brackets, and return termify for everything in between. I guess that I'm missing some really cool, haskelly solution here. Best Regards Leonhard

2 1

Install mess
by Lawrence Bottorff 29 Jan '19

29 Jan '19

I'm on Ubuntu 18.10 and I've made a real mess of my Haskell install. In the past I believe I installed from source. Then I must have tried to install through Ubuntu's repo, then I installed through the Haskell tool stack (curl -sSL https://get.haskellstack.org/ | sh). Now I can type ghci at the command line and I get 8.4.2, through stack ghci I get version 8.0.1, while starting a ghci REPL in Emacs goes with the 8.4.2. The Ubuntu repo version seems to be 8.2.2, BTW. What can I do/what should I do to straighten this out? What is the definitive install of Haskell, stack or Haskell platform, and what role should cabal play? LB

5 6

weird constraint
by Dimitri DeFigueiredo 19 Jan '19

19 Jan '19

Could someone help me make sense of this Num constraint given by GHCi? I was expecting to get: inc . const :: Num a => a -> b -> a rather than: inc . const :: Num (b -> a) => a -> b -> a Here's the transcript: GHCi, version 8.6.3: http://www.haskell.org/ghc/ :? for help Prelude> inc x = x + 1 Prelude> :t inc inc :: Num a => a -> a Prelude> :t const const :: a -> b -> a Prelude> :t inc . const inc . const :: Num (b -> a) => a -> b -> a Prelude> Thanks, Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

3 2