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Re: [Haskell-beginners] oddsFrom3 function
by Doug McIlroy 18 Aug '15

18 Aug '15

> > oddsFrom3 :: [Integer] > > oddsFrom3 = 3 : map (+2) oddsFrom3 > > > > > > Thanks for your help. > > Try to expand a few steps of the recursion by hand e.g.: > > 3 : (map (+2) (3 : map (+2) (3 : map (+2) ...))) > > > As you can see, the deeper you go more 'map (+2)' are applied to '3'. Some more ways to describe the program, which may be useful: As with any recursive function, assume you know the whole series and then confirm that by verifying the inductive step. In this case oddsFrom3 = [3,5,7,9,11,...] map (+2) oddsFrom3 = [5,7,9,11,13,...] voila oddsFrom3 = 3 : map (+2) oddsFrom3 Assuming we have the whole series, we see its tail is computed from the whole by adding 2 to each element. Notice that we don't actually have to know the values in the tail in order to write the formula for the tail. Yet another way to describe the program: the "output" is taken as "input". This works because the first element of the output, namely 3, is provided in advance. Each output element can then be computed before it is needed as input. In an imperative language this would be done so: integer oddsFrom3[0:HUGE] oddsFrom3[0] := 3 for i:=1 to HUGE do oddsFrom3[i] = oddsFrom3[i-1] + 2

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oddsFrom3 function
by Debdut Karmakar 17 Aug '15

17 Aug '15

I am a haskell beginner and wondering how the following function works (in procedure) : oddsFrom3 :: [Integer] oddsFrom3 = map (+2) oddsFrom3 Thanks for your help.

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oddsFrom3 function
by Debdut Karmakar 17 Aug '15

17 Aug '15

I am a haskell beginner and wondering how the following function works (in procedure) : oddsFrom3 :: [Integer] oddsFrom3 = 3 : map (+2) oddsFrom3 Thanks for your help. -- A GNU [1] LINUX [2] Patron Links: ------ [1] http://gnu.org [2] http://www.linuxfoundation.org/

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Partial application and lazy evalutaion
by Shishir Srivastava 15 Aug '15

15 Aug '15

Hi, Is partial application feature of functions in haskell a direct outcome of lazy evaluation ? i.e Since functions are not evaluated until at the point where results are required this allows haskell functions to be partially applied because they're not evaluated and hence exist as thunk. Thanks, Shishir

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cabal sandbox add-source installs but disappears
by Dimitri DeFigueiredo 15 Aug '15

15 Aug '15

Hello all, I am having "a bit" of an issue trying to use a source dependency with cabal. I have a very simple setup with only 2 packages: ~/code/haskell/package-tests/a-package a.cabal src/MainA.hs and ~/code/haskell/package-tests/MinhaCommon MinhaCommon.cabal setup.hs src/MainCommon.hs I have setup a sandbox for the MinhaCommon package and can build it. However, the first one (a-package) depends on the second (MinhaCommon). So, I also created a sandbox for it as such: a-package>cabal sandbox init and that seems to work fine... Writing a default package environment file to /Users/dimitri/code/haskell/package-tests/a-package/cabal.sandbox.config Creating a new sandbox at /Users/dimitri/code/haskell/package-tests/a-package/.cabal-sandbox I then add the source package and install... a-package>cabal sandbox add-source '../MinhaCommon/' a-package>cabal install --only-dep Resolving dependencies... Notice: installing into a sandbox located at /Users/dimitri/code/haskell/package-tests/a-package/.cabal-sandbox Configuring MinhaCommon-0.7.0... Building MinhaCommon-0.7.0... Installed MinhaCommon-0.7.0 Updating documentation index /Users/dimitri/code/haskell/package-tests/a-package/.cabal-sandbox/share/doc/x86_64-osx-ghc-7.10.2/index.html Here's the snag! a-package>cabal configure Resolving dependencies... Configuring a-0.1.0... cabal: At least the following dependencies are missing: MinhaCommon ==0.7.0 WTF!? I just installed that! The cabal file for package 'a' just includes the MinhaCommon package thru the build-depends section (http://pastebin.com/Wpz5845k) ------------------------------------------------ name: a version: 0.1.0 build-type: Simple cabal-version: >=1.20 ---------------------------------------------- executable a main-is: MainA.hs hs-source-dirs: ./src build-depends: base >= 4.4 , unordered-containers , unix , process , stm , MinhaCommon == 0.7.0 default-language: Haskell2010 ------------------------------------------------ Any pointers on how to get this to build are much appreciated. Thanks, Dimitri

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Re: [Haskell-beginners] IntelliJ Plugin and IDEs for Haskell
by vitalij_zad＠libero.it 13 Aug '15

13 Aug '15

Hi William and welcome to the Haskell World! People complain about IntelliJ Haskell plugin to be buggy and slow. For this reason most programmers use Emacs or SpaceMacs editors. You can find an answer here: http://www.quora.com/What-is-the-best-IDE-for-programming-in-Haskell Il 09/08/2015 21:11, William Bryant ha scritto: Is the Haskell plugin for IntelliJ IDEA actually good or should I use a different IDE?I really like Intellij for Java and would like something just as good for Haskell . Any recommendations? _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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IntelliJ Plugin and IDEs for Haskell (gogobebe2 - OP)
by William Bryant 11 Aug '15

11 Aug '15

Thanks guys. I saw Atom from googling, so I'll try it. :)

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(no subject)
by William Bryant 10 Aug '15

10 Aug '15

Is the Haskell plugin for IntelliJ IDEA actually good or should I use a different IDE? I really like Intellij for Java and would like something just as good for Haskell . Any recommendations?

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Type assignment for List
by RAJATKUMAR ZALA 09 Aug '15

09 Aug '15

Hello Friends, I am totally new to Haskell. I am learning Haskell from a website on my own pace. From that site, I came to know about list in haskell and its type assignment. There is a confusable question on site. I am posting question here.... 1. Given that the expression []:xs is well typed, which of the following is not a possible type assignment for xs? Options:- 1. xs :: [Int] 2. xs :: [[Float]] 3. xs :: [[[Char]]] 4. xs :: [[[[Bool]]]] If any friend know about it then please clarify me on it. Thanks in advance Rajatkumar Zala India

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How would I increment or otherwise change a value in a record with “Simon-ness”
by Michael Litchard 06 Aug '15

06 Aug '15

The below code is from this tutorial http://dev.stephendiehl.com/hask/ it illustrates very well how to operate on values from records with "Simon-ness" (illustrated below). What I am struggling with is how to modify values inside records with "Simon-ness", say incrementing "age". I keep thinking it has to do with the way Label is defined with the constructor Get. Could I add another constructor Put? {-# LANGUAGE DataKinds #-}{-# LANGUAGE KindSignatures #-}{-# LANGUAGE MultiParamTypeClasses #-}{-# LANGUAGE FunctionalDependencies #-}{-# LANGUAGE FlexibleInstances #-}{-# LANGUAGE FlexibleContexts #-}{-# LANGUAGE StandaloneDeriving #-}{-# LANGUAGE ExistentialQuantification #-}{-# LANGUAGE ConstraintKinds #-} import GHC.TypeLits newtype Field (n :: Symbol) v = Field { unField :: v } deriving Show data Person1 = Person1 { _age :: Field "age" Int , _name :: Field "name" String } data Person2 = Person2 { _age' :: Field "age" Int , _name' :: Field "name" String , _lib' :: Field "lib" String } deriving instance Show Person1deriving instance Show Person2 data Label (l :: Symbol) = Get class Has a l b | a l -> b where from :: a -> Label l -> b instance Has Person1 "age" Int where from (Person1 a _) _ = unField a instance Has Person1 "name" String where from (Person1 _ a) _ = unField a instance Has Person2 "age" Int where from (Person2 a _ _) _ = unField a instance Has Person2 "name" String where from (Person2 _ a _) _ = unField a age :: Has a "age" b => a -> b age pnt = from pnt (Get :: Label "age") name :: Has a "name" b => a -> b name pnt = from pnt (Get :: Label "name") -- Parameterized constraint kind for "Simon-ness" of a record.type Simon a = (Has a "name" String, Has a "age" Int) spj :: Person1 spj = Person1 (Field 56) (Field "Simon Peyton Jones") smarlow :: Person2 smarlow = Person2 (Field 38) (Field "Simon Marlow") (Field "rts") catNames :: (Simon a, Simon b) => a -> b -> String catNames a b = name a ++ name b addAges :: (Simon a, Simon b) => a -> b -> Int addAges a b = age a + age b names :: String names = name smarlow ++ "," ++ name spj-- "Simon Marlow,Simon Peyton Jones" ages :: Int ages = age spj + age smarlow-- 94

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