March 2019 - Beginners

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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