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Haskell REPL doesn't accept type declaration
by Lawrence Bottorff 11 Dec '20

11 Dec '20

If I simply put a type declaration into the GHCi (version 8.8.4), it complains: Prelude> divides :: Integer -> Integer -> Bool <interactive>:18:1-7: error: Variable not in scope: divides :: Integer -> Integer -> Bool I'm trying to use Emacs org-mode babel, and I have to wrap the code block to get it to work. For example: #+begin_src haskell :results verbatim :exports both :{ factorial :: Int -> Int factorial 0 = 1 factorial n = n * factorial (n - 1) :} #+end_src #+begin_src haskell :results verbatim :exports both factorial 0 #+end_src #+RESULTS: : 1 I understand :set +m, but this doesn't help. Why won't a running Haskell REPL with set +m take a type declaration? Lawrence Bottorff Grand Marais, MN, USA

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Usage of type
by Pietro Grandinetti 09 Dec '20

09 Dec '20

Hi, Playing around with code (very few lines) that represents long-form articles. I'd like to understand: 1- if the usage of `type' is correct, or if I should prefer `newtype', or something different altogheter. 2- what's the more idiomatic way to do the boxing and unboxing of renamed types? See last 2 functions in the code. import Data.Time (Day) import Data.List (intercalate) import Data.List.Split (splitOn) type Title = String type Author = String type Sentence = String type Paragraph = [Sentence] type Abstract = Paragraph type Content = [Paragraph] type Date = Day data Essay = Essay { title :: Title , authors :: [Author] , pubDate :: Date , startDate :: Date , abstract :: Abstract , content :: Content } deriving (Show) makeTitle :: String -> Title makeTitle x = x::Title makePar :: String -> Paragraph makePar = splitOn sep where sep = "." makeContent :: String -> Content makeContent x = map makePar $ splitOn sep x where sep = "\n\n" unboxPar :: Paragraph -> String unboxPar = intercalate ". " unboxContent :: Content -> String unboxContent x = intercalate "\n\n" $ map unboxPar x Thanks, Pete

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Desugar list comprehension
by mike h 02 Dec '20

02 Dec '20

Hi, I have sumIs2020P1' xs = do x <- xs y <- xs guard (x + y == 2020) pure (x,y) which has been desugared from a list comprehension I would like to reduce this even more using >>= So I do sumIs2020P1'' xs = (a,b) where (a,b):rest = filter (\(x,y) -> x + y == 2020) pairs pairs = xs >>= \x -> xs >>= \y -> pure (x,y) but really I would like the guard to be within the >>= sections but I could not work out how to do it! i.e. I’m looking for something like (pseudo code) pairs = xs >>= \x -> xs >>= \y -> if (x + y == 2020) then pure (x,y) else DO_NOTHING which would then allow the filter to be removed. Many Thanks Mike Dr Mike Houghton mike_k_houghton(a)yahoo.co.uk

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State in IO monad
by Dmitriy Matrosov 16 Nov '20

16 Nov '20

Hi. I want to use [telnet library][1] to run some commands on cisco switches and then parse output. Thus, first i need to login (enter username and password, etc) and then run commands and collect output. As far as i understand that library API, all of output parsing should be done in EventHandler of type TelnetPtr -> Event -> IO () But, these (mine) operations require some state (to track where am i now (entered username, entered password, etc) and to collect output). But the monad of this event handler is IO, so i can't see any simple way of adding state to it apart from ['unsafePerfromIO' trick][2], like telnetRef :: IORef TelnetRef {-# NOINLINE telnetRef #-} telnetRef = unsafePerformIO $ newIORef (TelnetRef undefined Unauth M.empty) ('undefined' is 'Telnet' pointer, which is returned by 'telnetInit' and will be initialized later..) Is there a better a way to do this? Thanks. [1]: https://hackage.haskell.org/package/libtelnet [2]: https://wiki.haskell.org/Top_level_mutable_state

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UNSUBSCRIBE
by Elan-Mentholer 30 Oct '20

30 Oct '20

UNSUBSCRIBE

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Doing a DFS from scratch ....
by Claudio Cesar de Sa 24 Oct '20

24 Oct '20

Hi everyone Initially, I hope that this list is active yet. For some days, I had been trying to do a Depth First Search on a graph from scratch following something very didactical (clear, elegant - legible, any worry about efficiency). This DFS keeps a boolean list to mark the nodes already visited and another with current or valid nodes ( the stack of DFS classic). The code is naive and well documented and is here: https://github.com/claudiosa/CCS/blob/master/haskell/dfs_graph.hs (excessively commented) but unfortunately, it is not working. My guess is that the problem starts on *new_node* function. I am not sure if in DFS, when a current node gets a next neighbour to stack, it gets all the neighbours. In this code, it is getting one node per time. The functions next_node and one_node seem very non-sense. Could someone help me? Thanks in advance claudio **************************************************************** ************ *Whatsapp: +55 47 992451825 <%2B55%2047%2092451825>* https://claudiocesar.wordpress.com/ (my links HERE) https://github.com/claudiosa https://www.udemy.com/picat-uma-linguagem-de-programacao-multiparadigma/ **************************************************************** *************

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Clarifying $ vs parentheses
by Josh Friedlander 21 Aug '20

21 Aug '20

I understand that in general $ is a) right-associative and b) lowest-priority. But if so shouldn't these two be roughly the same? λ take (succ 10) $ cycle "hello world" "hello world" But not this? λ take $ succ 10 $ cycle "hello world" <interactive>:20:8: error: • No instance for (Enum ([Char] -> Int)) arising from a use of ‘succ’ (maybe you haven't applied a function to enough arguments?) • In the expression: succ 10 In the second argument of ‘($)’, namely ‘succ 10 $ cycle "hello world"’ In the expression: take $ succ 10 $ cycle "hello world" <interactive>:20:13: error: • No instance for (Num ([Char] -> Int)) arising from the literal ‘10’ (maybe you haven't applied a function to enough arguments?) • In the first argument of ‘succ’, namely ‘10’ In the expression: succ 10 In the second argument of ‘($)’, namely ‘succ 10 $ cycle "hello world"’

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Text.Regex.Posix
by Alexander Chen 19 Aug '20

19 Aug '20

Hi, cabal install regex-posix runs without issue. added it to cabal dependencies: name: data-regex version: 0.1.0.0 -- synopsis: -- description: homepage: https://github.com/githubuser/data-regex#readme license: BSD3 license-file: LICENSE author: Alexander Chen maintainer: example(a)example.com copyright: 2020 Alexander Chen category: regex build-type: Simple cabal-version: >=1.10 extra-source-files: README.md executable data-regex hs-source-dirs: src main-is: Main.hs default-language: Haskell2010 build-depends: base >= 4.7 && < 5, regex-posix Then I do: stack install regex-posix error> WARNING: Ignoring regex-posix's bounds on base (<0 && >=4.3 && <4.15); using base-4.13.0.0. Reason: trusting snapshot over cabal file dependency information. regex-posix> configure regex-posix> Configuring regex-posix-0.96.0.0... regex-posix> build regex-posix> Preprocessing library for regex-posix-0.96.0.0.. regex-posix> C:\Users\achen\AppData\Local\Temp\stack-e2b99a8a9eb45aea\regex-posix-0.96.0.0\Wrap.hsc:96:10: fatal error: regex.h: No such file or directory regex-posix> compilation terminated. regex-posix> compiling .stack-work\dist\29cc6475\build\Text\Regex\Posix\Wrap_hsc_make.c failed (exit code 1) regex-posix> command was: C:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\../mingw/bin\gcc.exe -c .stack-work\dist\29cc6475\build\Text\Regex\Posix\Wrap_hsc_make.c -o .stack-work\dist\29cc6475\build\Text\Regex\Posix\Wrap_hsc_make.o -D__GLASGOW_HASKELL__=808 -Dmingw32_BUILD_OS=1 -Dx86_64_BUILD_ARCH=1 -Dmingw32_HOST_OS=1 -Dx86_64_HOST_ARCH=1 -Icbits -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include -I.stack-work\dist\29cc6475\build\cbits -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include -I.stack-work\dist\29cc6475\build\autogen -I.stack-work\dist\29cc6475\build\global-autogen -include .stack-work\dist\29cc6475\build\autogen\cabal_macros.h -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\msys2-20180531\mingw64\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\bytestring-0.10.10.0\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\base-4.13.0.0\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib\integer-gmp-1.0.2.0\include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3\lib/include -IC:\Users\achen\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.3/lib/include/ -- While building package regex-posix-0.96.0.0 using: C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.3.exe --builddir=.stack-work\dist\29cc6475 build --ghc-options " -fdiagnostics-color=always" Process exited with code: ExitFailure 1 Progress 1/2 ========================================================================================================================================================================================= I also get an error in my main(but seems related): module Main where import Text.Regex.Posix main :: IO () main = do let str1 = "one fish two fish red fish blue fish" let str2 = "The quick brown fox jumps over the lazy dog." print str1 =~ "one" ::Bool What am I doing wrong? best,

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Re: [Haskell-beginners] IO operations
by 鲍凯文 14 Aug '20

14 Aug '20

Hi, > * Couldn't match expected type `t0 Char' with actual type `Char' > * In the second argument of `elem', namely `x' > In the second argument of `($)', namely '-' `elem` x > In the expression: not $ '-' `elem` x Hope the other answers helped. FWIW, just wanted to point out that for me these kinds of error messages sometimes used to be confusing since the type of `elem` is `(Foldable t, Eq a) => a -> t a -> Bool`, so the typechecker tells you it wants to unify `t0 Char` (where `t0` is something `Foldable`) with `Char`. As Mr. Klink wrote, it looks like you wanted `t0` to be `[]`; I used to get confused since I was only working with lists and the extra `Foldable` genericity made it harder to see that I was still just working with lists. Best, toz >

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IO operations
by Alexander Chen 13 Aug '20

13 Aug '20

Hi, module Main where allwords :: IO Int allwords = do dict <- readFile "data/dict.txt" return $ length [x | x <- dict , not $ '-' `elem` x] main :: IO Int main = do allwords error * Couldn't match expected type `t0 Char' with actual type `Char' * In the second argument of `elem', namely `x' In the second argument of `($)', namely '-' `elem` x In the expression: not $ '-' `elem` x could someone tell me what I am doing wrong? thanks!

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