- Beginners - Haskell.org

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 23 Jun '15

23 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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Reading binary data into numeric types
by Barry DeZonia 21 Jun '15

21 Jun '15

Hello, I need to write some code that will read binary data from a file into various types defined in Data.Int, Data.Word, and the floating types. I am looking to make functions with signatures like Handle->IO Word16 and Handle->IO Int32 and Handle->IO Float and others similarly defined. One complication I am running into is that the data is stored in big endian byte order. So my functions need to be flexible with byte ordering (and I need to determine the endian ordering of the current platform). Anyhow I'm pretty much a newbie and don't know how to do this. For a float in an imperitive language I might read 4 bytes, possibly reorder them based on platform endian value, and interpret the bytes as a float via a ptr. Would the process be similar in Haskell? Pointers appreciated.

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Adapting code from an imperative loop
by Matt Williams 19 Jun '15

19 Jun '15

Dear All, I have been wrestling with this for a while now. I have a list of data items, and want to be able to access them, in a Hash Map, via a short summary of their characteristics. In an imperative language this might look like: myMap = new map() for elem in myElems: key = makeKey(elem) myMap[key] = myMap[key] + elem I have been trying to do this in Haskell, but am stuck on how to hand the Map back to itself each time. I have a function :: elem -> [p,q] to make the key, but the Map.insert function has the following signature: insert :: (Hashable <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-Hashable.html…> k, Ord <https://hackage.haskell.org/packages/archive/base/4.2.0.2/doc/html/Data-Ord…> k) => k -> a -> HashMap <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#…> k a -> HashMap <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#…> k a My thought was that I needed to go through the list of the elems, and at each point add them to the Hash Map, handing the updated Map onto the next step - but this is what I cannot write. Any help much appreciated. Thanks, Matt

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'Simple' function
by Mike Houghton 19 Jun '15

19 Jun '15

Hi, I’ve been tryimg to write a function with signature asString :: IO String -> String Does someone please have the patience to explain to me what the compiler error messages really mean for these two attempts and exactly what I’m doing (!!!) If I *do not* give this function any type signature then it works i.e.. asString ioStr = do str <- ioStr return $ str and the compiler tells me its signature is asString :: forall (m :: * -> *) b. Monad m => m b -> m b which, at this stage of my Haskell progress, is just pure Voodoo. Why isn’t it’s signature asString :: IO String -> String ? Another naive attempt is asString ioStr = str where str <- ioStr and then compiler says parse error on input ‘<-’ Many Thanks Mike

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