June 2015 - Beginners

Config data
by Mike Houghton 18 Sep '15

18 Sep '15

Hi, I’ve haskell files parser.hs and email.hs email.hs is a module imported into parser and parser has the main. email has various passwords and server names needed to connect and send email. I want to have those details in a config file and read it in at start - using say config module. I’m ok with this part but the practicality of doing it eludes me... It seems to me that email module can either get the config details itself or be told them by parser. If email wants to get them how would it do this? It does not have a main that gets run so can’t load a config file.(can it ??) However parser can load the config in its main and then tell mail what the values are but how would mail save them? Of course I could chanage the signatures of the email send/receive functions in mail to take the connection details but that seems wrong. Thanks Mike

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Defining an Instance of Ord
by Matt Williams 26 Jun '15

26 Jun '15

Dear All, I am trying to produce a Map, where the (tricky) idea is that the key is a pair, (t1, t2), and the key is considered identical under ordering. Thus: (t1, t2) is the same as (t2, t1) but (t1, t3) is not the same as (t1,t2). This LOOKS like a equality definition. However, the Map key typeclass is defined as Ord, which requires me to define compare: instance Ord Edge where (Edge s1 _) `compare` (Edge s2 _) = s1 `compare` s2 I am a bit stuck on how to use compare to define this type of eqlaity - any pointers very gratefully received. BW, Matt

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Functor fmap: how to
by Imants Cekusins 26 Jun '15

26 Jun '15

There are a few Functor & fmap tutorials. Here is basic use of fmap. If it leaves you confused, please ignore it. module FunctorFmap where {- why maybe & list? Functor instances exist for Maybe & List http://hackage.haskell.org/package/base-4.8.0.0/docs/Data-Functor.html#t:Fu… expand "instances" -} -- (a -> b) add3:: Int -> Int add3 = (+ 3) -- maybe runMaybe::IO () runMaybe = do print $ maybeFmap $ Just 1 print $ maybeInfixed $ Just 1 print $ maybeFmap Nothing print $ maybeInfixed Nothing -- maybeFmap & maybeInfixed do the same thing. different syntax maybeFmap::Maybe Int -> Maybe Int maybeFmap = fmap add3 maybeInfixed::Maybe Int -> Maybe Int maybeInfixed mi = add3 <$> mi -- list runList::IO () runList = do print $ listFmap [1,2] print $ listInfixed [1,2] -- listFmap & listInfixed do the same thing. different syntax listFmap::[Int] -> [Int] listFmap = fmap add3 listInfixed::[Int] -> [Int] listInfixed l = add3 <$> l

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basic use of Applicative
by Imants Cekusins 25 Jun '15

25 Jun '15

module BasicApplicative where -- 2 arg upd2:: Int -> Int -> Int upd2 x1 x2 = x1 * x2 -- 3 arg upd3:: Int -> Int -> Int -> Int upd3 x1 x2 x3 = x1 * x2 + x3 -- pure pureMaybe::Int -> Maybe Int pureMaybe = pure pureList::Int -> [Int] pureList = pure -- maybe maybe2::Maybe Int -> Maybe Int -> Maybe Int maybe2 mi1 mi2 = upd2 <$> mi1 <*> mi2 -- list list2::[Int] -> [Int] -> [Int] list2 l1 l2 = upd2 <$> l1 <*> l2 -- same result as list2 list2p::[Int] -> [Int] -> [Int] list2p l1 l2 = pure upd2 <*> l1 <*> l2 list3::[Int] -> [Int] -> [Int] -> [Int] list3 l1 l2 l3 = upd3 <$> l1 <*> l2 <*> l3

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http-conduit: Changelogs/notes on API changes
by Sevcsik András 24 Jun '15

24 Jun '15

Hi, I'm trying to update the couchdb-conduit package to use the latest (stackage) version of it's dependencies. Is there any changelog, release notes, or even a blogpost which summarise how the API was changed between different major versions? And in general, is there a common practice for hackage packages on how are API-breaking changes are communicated? Cheers -- Minden jót, Sevcsik András

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ghc-mod and cabal "could not find module Prelude"
by Alan Buxton 24 Jun '15

24 Jun '15

Hi I've recently set up a new Haskell working environment on Ubuntu 14.04. I installed the Ubuntu packaged version of Haskell platform. My problem is that I can't now get ghc-mod check to work if there is a cabal file in the current directory. I am using: * ghc-mod 5.2.1.2 compiled by GHC 7.6.3 * cabal 1.16.0.2 See below an extract of trying to run ghc-mod check in a directory that was empty until I just ran cabal init in it: ~/tmp-ghc-mod$ ls Setup.hs tmp-ghc-mod.cabal ~/tmp-ghc-mod$ ghc-mod check Setup.hs Setup.hs:1:1:Could not find module `Prelude'It is a member of the hidden package `base'.Perhaps you need to add `base' to the build-depends in your .cabal file.It is a member of the hidden package `haskell98-2.0.0.2'.Perhaps you need to add `haskell98' to the build-depends in your .cabal file.It is a member of the hidden package `haskell2010-1.1.1.0'.Perhaps you need to add `haskell2010' to the build-depends in your .cabal file.Use -v to see a list of the files searched for. ~/tmp-ghc-mod$ mv tmp-ghc-mod.cabal tmp-ghc-mod.cabal.NOT ~/tmp-ghc-mod$ ls dist Setup.hs tmp-ghc-mod.cabal.NOT ~/tmp-ghc-mod$ ghc-mod check Setup.hs Setup.hs:2:1:Warning: Top-level binding with no type signature: main :: IO () ~/tmp-ghc-mod$ So. ghc-mod behaves as expected when there is no cabal file, but doesn't behave as expected if there is a cabal file. My google fu isn't helping me out on this: the only issues I have seen are to do with a change in format of the cabal file in newer versions of cabal. Any ideas? Thanks Alan

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Structural restrictions in type constructor
by Matt Williams 23 Jun '15

23 Jun '15

Dear All, I wonder if/ how this is possible? I have a constructor which takes 2 pairs of type t). However, I want to ensure that the pairs are matched: MyP = MyP (t, t) (t, t) But where the first pair contains the same elements as the second, but reversed in order. Any help much appreciated. BW, Matt

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Function problem
by Bruno Sotelo Klinec 22 Jun '15

22 Jun '15

Hi there! I was hoping you could help me a bit here with making a function. I am asked to write functions to make a mini robot game, which uses these data and types: data Program = Move Direction Program |Radar (Robot -> Point) Program |Shoot Point Program |Surrender data Direction = N|S|E|O type Point = (Int,Int) data Robot = R1|R2 And then I have to write functions like this: prog1::Program prog1 = Move E $ Radar R1 (2,4) $ Radar R2 (5,6) $ Shoot (5,7) prog1 My problem is the function adjust::Direction->Program->Program that takes one direction and a program and moves every shot in that program one unit in the given direction. I can't figure out how to do this, I would really appreciate you could help me, thanks!

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Haskell -> JavaScript -> nodejs
by Martin Vlk 21 Jun '15

21 Jun '15

Hi, I can successfully compile a basic Hello World into JavaScript with ghcjs and run it with nodejs. But what I ultimately need to do is write a nodejs module in Haskell and compile down to JavaScript with ghcjs. I need to be able to require other nodejs modules as well. Could somebody point me in the right direction? E.g. what approach and libraries to use... I have tried a simple thing with jsaddle/lens, but when I try to compile with ghcjs it is not finding the imported modules, even though when compiling with ghc it does find them (they are installed using cabal). Many Thanks Martin

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Help - I'm completely lost with building a really simple app with Snap and Groundhog
by Matthias Beyer 21 Jun '15

21 Jun '15

Hi, I am learning Haskell by trying to implement a really simple CRUD like web app with Snap and Groundhog - and I'm completely lost, cannot get it to work. The relevant code is here: https://github.com/matthiasbeyer/rate.hs/tree/restart_clean What the app should be able to do: - CRUD Categories, where a category can have several subcategories or Ratings - CRUD Ratings, where a rating has a title, a description and, of course, a rating (something like a number from 0-10) That's it. Backend should be postgresql, frontend should contain a list of all categories with all ratings (a simple tree) and each rating should have its own page where the description is listed. So, I guess, this is not really complex or anything, but I'm completely list with both integrating groundhog and heist, the former beeing my current step. Can someone digg into my code and tell me what I'm doing wrong? Or maybe even provide a _good_ tutorial for me? Or even be my mentor for this? Would be really nice! Good night, Haskell community! -- Mit freundlichen Grüßen, Kind regards, Matthias Beyer Proudly sent with mutt. Happily signed with gnupg.

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