February 2019 - Beginners

Error Message: Can not find C library. How can I do this?
by S. H. Aegis 02 Mar '19

02 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 25 Feb '19

25 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 11 Feb '19

11 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 05 Feb '19

05 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 03 Feb '19

03 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 03 Feb '19

03 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