July 2018 - Beginners

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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