- Beginners - Haskell.org

1=2
by M Douglas McIlroy 15 Dec '20

15 Dec '20

ghc and ghci 8.6.5 accept 1=2 at top level. It seems to have no effect. What does it mean?

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Nested folds
by Pietro Grandinetti 13 Dec '20

13 Dec '20

Hello, I have a piece of code to represents Sentences, Paragraphs and the Content of an article. I added functions to count the words, code below. My questions: 1- Are these functions idiomatic? 2- Is this an efficient way to do the computation? 3- In different languages, I could (and would) give the same name `wordCount` to the three functions, because the type of the input would clarify the usage. But here GHC throws an error. What's the most idiomatic way to do this in Haskell? type Sentence = String type Paragraph = [Sentence] type Content = [Paragraph] sentWordCount :: Sentence -> Int sentWordCount = length . words parWordCount :: Paragraph -> Int parWordCount = foldr ((+) . sentWordCount) 0 contWordCount :: Content -> Int contWordCount = foldr ((+) . parWordCount) 0 I also have two more practical questions on the following two functions: makeSentence :: String -> Sentence makeSentence x = x::Sentence sentCharCount :: Sentence -> Int sentCharCount x = length $ filter (/= ' ') x 4- About `makeSentence` -- does it make sense to write a function like that just to encapsulate the String type? 5- About `sentCharCount` -- I cannot take the argument x off, the compilator complains. What's the reason? Thanks, -P

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unwanted Fractional constraint
by M Douglas McIlroy 11 Dec '20

11 Dec '20

For rational functions that take on integer values at integer arguments, for example n*(n+1)/2, is there a way to doctor the corresponding Haskell definition f n = n*(n+1)/2 so that the type signature becomes f :: Num a => a -> a rather than f :: Fractional a => a -> a Doug McIlroy

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(no subject)
by M Douglas McIlroy 10 Dec '20

10 Dec '20

Is it possible to decorate a definition like this

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Haskell REPL doesn't accept type declaration
by Lawrence Bottorff 10 Dec '20

10 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 08 Dec '20

08 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 15 Nov '20

15 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 29 Oct '20

29 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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