- Beginners - Haskell.org

How does forever works ?
by Olivier Revollat 09 Jul '18

09 Jul '18

>From what I understand, this code main = forever $ do putStrLn "OK !" is equivalent to this one : main = do putStrLn "OK !" main In the second case, it's a simple recursion, so far so good ... but when I look at the implementation of "forever" i can't wrap my head around : http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm… forever <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> a <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> = let a' <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> = a <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> *> <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.Base.html#%2A…> a' <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> in a' <http://hackage.haskell.org/package/base-4.11.1.0/docs/src/Control.Monad.htm…> How does this works ? How does this make an infinite loop ? I understand that *> discard his right argument but it doesn't help me understand how forever implement an infinite loop ...

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Passing a file containing a list as an input
by Awsaf Rahman 06 Jul '18

06 Jul '18

Hello everyone! I am really struggling with Haskell I/O. I have written a mergesort program and as an input to that program I want to use a file containing a list of 1000 integers. Let's say the contents of the file look like this [120, 400, 500 , 20, 100 ..] How can I achieve that? I am trying to write a main function that takes the file as an input and passes the list to my mergesort function.. Regards Awsaf Rahman

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Execution time of Mergesort
by Awsaf Rahman 05 Jul '18

05 Jul '18

Hello everyone! I am trying to find out the execution time of mergesort for a list size of 1 million integers. I have done the same in Erlang as well and the execution time in Erlang was around 0.93 seconds. I have implemented the program in almost the same way in Haskell as well but for some reason the Haskell implementation is taking around 12 seconds which doesn't seem right. Here is the implementation in Haskell: *{-# LANGUAGE OverloadedStrings #-}* *{-# LANGUAGE BangPatterns #-}* *import Control.Exception* *import Formatting* *import Formatting.Clock* *import System.Clock* *import Control.DeepSeq* *mergesort [] = []* *mergesort [x] = [x]* *mergesort xs = let (lhalf, rhalf) = splitAt (length xs `div` 2) xs* * in merge' (mergesort lhalf) (mergesort rhalf)* *merge' lhalf rhalf = merge lhalf rhalf []* *merge [] [] acc = reverse acc* *merge [] y acc = reverse acc ++ y* *merge x [] acc = reverse acc ++ x* *merge (l:ls) (r:rs) acc* * | l < r = merge ls (r:rs) (l:acc)* * | otherwise = merge rs (l:ls) (r:acc)* *toList :: String -> [Integer]* *toList input = read ("[" ++ input ++ "]")* *main = do* * file <- getLine* * contents <- readFile file* * let !unsortedlist = (toList contents)* * start <- getTime Monotonic* * evaluate(force (mergesort unsortedlist))* * end <- getTime Monotonic* * fprint (timeSpecs % "\n") start end* What am I doing wrong?

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Presentation
by Olivier Revollat 04 Jul '18

04 Jul '18

Hi, I'm a french (any other french here ?) web developper since 2001. I use PHP, Python essentially and I've been working with Haskell for a few month now ... I really like it even if it's quite hard to start. I like the mathematical aspect of the language. So hello everyone !

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Design help to add imports to a (Java) template
by Hilco Wijbenga 02 Jul '18

02 Jul '18

Hi all, I'm working on a Haskell (Eta) tool to generate Java ADTs. I have the following definitions: data Line = Line Int Text [JavaImport] -- The Int represents the indentation level of this line. data Template = None | One Line | Many [Line] | Template (Reader Configuration Template) I then run it like this (incomplete, I hope you get the idea): main :: IO () main = putStr $ replace (importsGoHereKey `append` "\n") importsAsText templateAsText where config = Configuration (Copyright 2018 "Me") USE_GETTERS point' = javaClassTemplate point importsAsText = runReader (mapReader javaImportsToText (javaImports point')) config templateAsText = runReader (toText tab point') config Essentially, I use the template twice: once to extract the Java imports and once to generate the result Text. Then I replace a magic string in the result Text with the import code. It works but makes me sad. ;-) I will probably pull out the JavaImport list from Line and use something like State to keep track of the necessary imports. (Now that I understand Reader, maybe State is no longer a bridge too far.) My main issue is how to avoid the magic string. I thought about adding an extra entry to Template [something like Imports ((Set JavaImport) -> Text or maybe Template)] but then I don't know how to implement "instance Monoid Template". Similarly if I use something Reader like. Any design advice or ideas would be appreciated. If it's of any help, the full code is at [1] (it's tiny). Any and all comments are welcome, by the way: I'm still a Haskell beginner. Cheers, Hilco [1] https://github.com/hilcode/java-algebraic-data-types

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How to start Haskell
by Saurabh Sihag 20 Jun '18

20 Jun '18

Hi all, I am a newbie in Haskell and I am interested to explore Haskell. Kindly help me from where I start. I have prior experience in Python and C. Thank you.

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How to define macro var with stack/cabal/etc?
by PY 20 Jun '18

20 Jun '18

Hello all, I'm using conditional compilation: #ifdef BLAH blahBlah #endif And I want to define BLAN, but no in *.hs file. For example, with `stack` option, in yaml file, in cabal file, in environment variable... How can I do it? For example, `stack -DBLAH=1 build` or something else. What are the options? === Best regards, Paul

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Difficulty understanding how to use filterM to compute powerset
by Olumide 17 Jun '18

17 Jun '18

Dear List, I'm trying to apply the following definition of filterM (from Brent Yorgey's blog https://byorgey.wordpress.com/2007/06/26/deducing-code-from-types-filterm/) import Control.Monad -- for liftM filterM' :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM' p [] = return [] filterM' p (x:xs) = let rest = filterM' p xs in do b <- p x if b then liftM (x:) rest else rest in order to understand how filterM can be used to compute the power set of a list, as follows filterM' (const [False,True]) [1,2,3] Where p in the filterM' is (const [False,True]). What confuses me is that p x in filterM'. Based on my very limited understanding p x returns b = [False, True]. How b be tested in the subsequent if-statement if it is indeed a list? What am I getting wrong? Regards, - Olumide

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life with Hutton 2ed
by Sylvester Hazel 16 Jun '18

16 Jun '18

I wonder why life/10.8 runs nonsensically on my PC even with Language.Haskell.HsColour.ANSI import? These cls and goto reappear in 11.7 that spoil the pleasure of understanding. Also putGrid/11.4 is unintuitive backsword engineering concoction that teaches nothing, while it can be very intuitively written: putGrid g = sequence_ [putStrLn j| j<-weave [' ','|',' ','|',' '] (interleave (replicate 6 '-') [(interleave '|' i)| i<-g])] interleave :: a->[a]->[a] interleave x [] = [] interleave x [y] = [y] interleave x (y:ys)= y:x:interleave x ys weave :: a->[a]->[a] weave x [] = [x] weave x [y] = x:[y,x] weave x (y:ys)= x:y:weave x ys Prelude> putGrid [[' ','O','O'],['O','X','O'],['X','X','X']] | | |O|O | | ------ | | O|X|O | | ------ | | X|X|X | |

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fmap composition
by Marco Turchetto 13 Jun '18

13 Jun '18

I'm reading the "Haskell programming" book and in chapter 16 about functors they combine two "fmap" with the "(.)" function. I really don't understand how the type of "fmap . fmap" is "(Functor f2, Functor f1) => (a -> b) -> f1 (f2 a) -> f1 (f2 b)". The thing that really freaks me out is that I thought that "(.)" arguments are ONLY two unary function. Maybe there is some curring magic underline, there is someone that can explain to me how the type of "fmap . fmap" is derived from the type of "fmap" and "(.)"?

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