October 2013 - Beginners

Re: [Haskell-beginners] Actions v. Functions
by MJ Williams 20 Oct '13

20 Oct '13

For example, `main = do input <- getLine ...', where `getLine' is an action as distinguished from a function such as `main'. Other examples of actions in Haskell include `print', `putStrLn' and any process that reads from or writes to a file. Of course, the list of actions doesn't end there, though the examples here should answer your question. Sincerely, Matt At 21:44 19/10/2013, you wrote: >What do you mean by "action"? > >Antoine > > >On Fri, Oct 18, 2013 at 9:26 PM, MJ Williams ><<mailto:matthewjwilliams101@gmail.com>matthewjwilliams101(a)gmail.com> wrote: >Hello >What are the formal properties of an action? >How does an action differ from a function? >Sincerely, >Matthew > >_______________________________________________ >Beginners mailing list ><mailto:Beginners@haskell.org>Beginners(a)haskell.org >http://www.haskell.org/mailman/listinfo/beginners > > >_______________________________________________ >Beginners mailing list >Beginners(a)haskell.org >http://www.haskell.org/mailman/listinfo/beginners

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Actions v. Functions
by MJ Williams 19 Oct '13

19 Oct '13

Hello What are the formal properties of an action? How does an action differ from a function? Sincerely, Matthew

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Re: [Haskell-beginners] How do I give a type for this code?
by Todd Wilson 18 Oct '13

18 Oct '13

Thanks, Daniel, for your suggestion, but I think it misses a key aspect of my situation (unless I've misunderstood). Every model is supposed to be a quadruple (m, q, f, p), with m a list, q an element of m, f a list of ordered pairs of elements of m, and p a sublist of m. By contrast, it looks like your definition makes a model either a single element, or a single pair, or a single list, etc. --Todd On Thu, Oct 17, 2013 at 4:12 AM, Daniel Trstenjak < daniel.trstenjak(a)gmail.com> wrote: > > Hi Todd, > > the problem is, that 'model_of' tries to return different types: Model a, > Model (a,a) and Model [a]. > > I think you have to use some kind of ADT also for the Model, like you > already did for ModName. > > Something like: > > data Model a = Model a > | ProductModel (a,a) > | PowerModel [a] > ... > > > Greetings, > Daniel > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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How do I give a type for this code?
by Todd Wilson 17 Oct '13

17 Oct '13

Haskell community, I'm in one of those situations where I have the code I want to write, but need to find the right types to get it to typecheck. What follows is a generic version of my situation. Suppose I want to represent the finite models of a language (in the sense of mathematical logic) with a single constant c, unary function symbol f, and predicate P. I can represent the carrier as a list m, the constant as an element of m, the function as a list of ordered pairs of elements of m, and the predicate as the list of the elements in the model that satisfy it: -- a model is (m, c, f, p) type Model a = ([a], a, [(a,a)], [a]) -- a is the "base type" I can then construct particular models and operations on models. The details aren't important for my question, just the types: unitModel :: Model () unitModel = ([()], (), [((),())], []) cycleModel :: Int -> Model Int cycleModel n = ([0..n-1], 0, [(i, (i+1)`mod`n) | i<-[0..n-1]], [0]) -- function application, assuming function is total ap :: Eq a => [(a,b)] -> a -> b ap ((x',y'):ps) x = if x == x' then y' else ap ps x -- cartesian product of models productModel :: (Eq a, Eq b) => Model a -> Model b -> Model (a,b) productModel (m1, c1, f1, p1) (m2, c2, f2, p2) = (m12, c12, f12, p12) where m12 = [(x1,x2) | x1 <- m1, x2 <- m2] c12 = (c1, c2) f12 = [(x12, (ap f1 (fst x12), ap f2 (snd x12))) | x12 <- m12] p12 = [x12 | x12 <- m12, elem (fst x12) p1 && elem (snd x12) p2] -- powerset of model powerModel :: (Eq a, Ord a) => Model a -> Model [a] powerModel (m, c, f, p) = (ms, cs, fs, ps) where ms = subsequences m cs = [c] fs = [(xs, nub (sort (map (ap f) xs))) | xs <- ms] ps = [xs | xs <- ms, elem c xs] Now, I want to give a name to all of these models: data ModName = UnitModel | CycleModel Int | Product ModName ModName | Power ModName deriving (Show, Eq) And finally, I want to associate each name with its associated model: model_of UnitModel = unitModel model_of (CycleModel n) = cycleModel n model_of (Product m1 m2) = productModel (model_of m1) (model_of m2) model_of (Power m1) = powerModel (model_of m1) I've tried a number of ways of getting this to work, in the sense of defining types so that I can use exactly this definition of model_of (some involving GADTs and type families) but haven't found a best way. How should I do it? Todd Wilson

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Signature of monadic functions
by Lorenzo Tabacchini 17 Oct '13

17 Oct '13

Hi, is there an easy way to transform a function from: Monad m => a -> m b to: Monad m => m (a -> b) ?

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capital letter name in record syntax (parsing json)
by Miro Karpis 17 Oct '13

17 Oct '13

Hi, please,...is it possible to use a capital letter in record syntax data definition? For example data Car = Car { CarName :: !Text ,CarColor :: !Text } deriving (Show,Generic) When I try this I get: parse error on input `CarName' The thing is that I'm trying to parse a json file (with Aeson package) that has records starting with capital letter. After that I would like to parse the JSON file with following code: d <- (eitherDecode <$> getJSON) :: IO (Either String [Car]) where getJSON gets the json file from url If not, what alternative do I have? thanks, m.

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Exercise of "Programming with Arrows"
by Thiago Negri 16 Oct '13

16 Oct '13

The paper "Programming with Arrows" of John Hughes gives some exercises to do [1]. I'm trying to solve it and would like to receive a feedback if I'm doing it right or not before reading the rest of the paper. I didn't find the answers in the internet (if someone could point me to it, please do so). Exercise 2 (section 2.5) is asking to create a Stream Processor that can map more than one output per input (e.g. 3 outcomes for a single consume of the stream). The paper says that implementing "first" will be tricky, and it really is. I've came up to the solution listed below, *is it right?* * * * * module SP where import Prelude hiding (id, (.)) import Control.Category import Control.Arrow data SP a b = Put b (SP a b) | Get (a -> SP a b) runSP :: SP a b -> [a] -> [b] runSP (Put b s) as = b:runSP s as runSP (Get k) (a:as) = runSP (k a) as runSP (Get k) [] = [] compose :: SP b c -> SP a b -> SP a c compose (Put a s) g = Put a (compose s g) compose (Get k) (Put a s) = compose (k a) s compose f (Get k) = Get (\a -> compose f (k a)) instance Category SP where id = arr id (.) = compose instance Arrow SP where arr f = Get (\a -> Put (f a) (arr f)) first (Put a s) = Get (\(a', c) -> Put (a, c) (delayed (a', c) s)) first (Get k) = Get (\(a, c) -> firstWithValue (k a) c) delayed :: (a, c) -> SP a b -> SP (a, c) (b, c) delayed (a, c) (Put b s) = Put (b, c) (delayed (a, c) s) delayed (a, c) (Get k) = firstWithValue (k a) c firstWithValue :: SP a b -> c -> SP (a, c) (b, c) firstWithValue (Put a s) c = Put (a, c) (firstWithValue s c) firstWithValue (Get k) _ = Get (\(a, c) -> firstWithValue (k a) c) input :: [(String, String)] input = [("a1", "a2"), ("b1", "b2"), ("c1", "c2"), ("d1", "d2")] myArrow :: SP (String, String) (String, String) myArrow = (delay "db1" >>> delay "da1") *** (delay "db2" >>> delay "da2") delay :: a -> SP a a delay b = Put b (arr id) main :: IO () main = let output = runSP myArrow input in mapM_ f output where f (a, b) = putStrLn $ "(" ++ show a ++ ", " ++ show b ++ ")" * * The output of "main" is: *SP> main ("da1", "da2") ("da1", "db2") ("da1", "a2") ("db1", "a2") ("a1", "a2") ("b1", "b2") ("c1", "c2") ("d1", "d2") [1] http://www.cse.chalmers.se/~rjmh/afp-arrows.pdf

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Problems with cabal install hakyll
by Britt Anderson 16 Oct '13

16 Oct '13

I have a fresh installation of ghc 7.6.3-1 and cabal 1.16. From this I was able to cabal update and then cabal install diagrams and pandoc, but when trying to install hakyll (either version 4 or 3) I get a lot of dependency conflicts. Can anyone give me some suggestions on how to proceed in this case? Thank you. Below follow the messages from cabal install --dry-run *** Trying version 4 *** cabal install --dry-run hakyll Resolving dependencies... cabal: Could not resolve dependencies: trying: hakyll-4.4.1.0 trying: hakyll-4.4.1.0:+checkexternal trying: http-conduit-1.9.5.1/installed-1fe... rejecting: tls-extra-0.6.6/installed-302... (conflict: tls-extra => cryptohash==0.11.1/installed-ae4..., hakyll => cryptohash>=0.7 && <0.11) rejecting: tls-extra-0.6.6, 0.6.5, 0.6.4, 0.6.3, 0.6.1, 0.6.0, 0.5.1, 0.5.0, 0.4.7.1, 0.4.7, 0.4.6.1, 0.4.6, 0.4.5, 0.4.4, 0.4.3, 0.4.2.1, 0.4.2, 0.4.1, 0.4.0, 0.3.1, 0.3.0, 0.2.3, 0.2.2, 0.2.1, 0.2.0, 0.1.9, 0.1.8, 0.1.7, 0.1.6, 0.1.5, 0.1.4, 0.1.3, 0.1.2, 0.1.1, 0.1.0 (conflict: http-conduit => tls-extra==0.6.6/installed-302...) ***Trying version 3 *** cabal install --dry-run hakyll-3.5.3.0 Resolving dependencies... In order, the following would be installed: HUnit-1.2.5.2 (new package) List-0.5.1 (new package) blaze-html-0.5.1.3 (new version) bytestring-mmap-0.2.2 (new package) entropy-0.2.2.4 (new package) crypto-api-0.12.2.2 (new package) cryptohash-0.8.4 (new version) enumerator-0.4.20 (new package) attoparsec-enumerator-0.3.1 (new package) blaze-builder-enumerator-0.2.0.5 (new package) hashable-1.1.2.5 (new version) case-insensitive-1.1 (reinstall) changes: hashable-1.2.1.0 -> 1.1.2.5 hexpat-0.20.3 (new package) highlighting-kate-0.5.5 (reinstall) changes: blaze-html-0.6.1.1 -> 0.5.1.3, filepath-1.3.0.1 added lrucache-1.1.1.3 (new package) regex-tdfa-1.1.8 (new package) syb-0.3.7 (new version) hs-bibutils-5.0 (new package) json-0.7 (new package) pandoc-types-1.9.1 (new version) citeproc-hs-0.3.9 (new package) system-filepath-0.4.8 (new package) system-fileio-0.3.11 (new package) shakespeare-1.2.0.1 (new package) hamlet-1.1.7.3 (new package) tagsoup-0.12.8 (new version) texmath-0.6.4 (reinstall) changes: pandoc-types-1.12.2.3 -> 1.9.1, syb-0.4.1 -> 0.3.7 pandoc-1.9.4.5 +blaze_html_0_5 (new version) unix-compat-0.4.1.1 (new package) unordered-containers-0.2.3.3 (reinstall) changes: hashable-1.2.1.0 -> 1.1.2.5 zlib-enum-0.2.3 (new package) snap-core-0.9.4.1 (new package) snap-server-0.9.3.4 (new package) hakyll-3.5.3.0 (new package) Warning: The following packages are likely to be broken by the reinstalls: http-types-0.8.1 pandoc-1.12.0.2 http-conduit-1.9.5.1 yaml-0.8.5.1 semigroups-0.11 void-0.6.1 zlib-conduit-1.0.0 lens-3.9.2 force-layout-0.2 diagrams-contrib-0.7 diagrams-0.7.1.1 conduit-1.0.8 blaze-builder-conduit-1.0.0 MemoTrie-0.6.1 vector-space-0.8.6 vector-space-points-0.1.2.1 diagrams-core-0.7.0.1 diagrams-svg-0.8.0.1 diagrams-lib-0.7.1.1 active-0.1.0.7 semigroupoids-3.1 semigroupoid-extras-3.0.1 profunctor-extras-3.3.3.1 monoid-extras-0.3.2.2 dual-tree-0.1.0.4 groupoids-3.0.1.1 comonad-transformers-3.1 comonads-fd-3.0.3 bifunctors-3.2.0.1 comonad-3.1 profunctors-3.3.0.1 aeson-0.6.2.1 pandoc-types-1.12.2.3 Use --force-reinstalls if you want to install anyway.

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Re: [Haskell-beginners] [Haskell-cafe] Haskell Group Dresden?
by Daniel Trstenjak 15 Oct '13

15 Oct '13

Hi Stephan, > I'm very much interested. Do you need any help organizing a first meeting? > > For what it's worth, we once had an event about Yesod at the local CCC > group[1]. well, normally I'm not the kind of guy who enjoys performing in front of a lot of people, so I'm trying to find a way to get in touch with some folks without getting a heart attack =). So looking at your yesod event, the perception to get filmed, isn't really helping here :D. I was thinking about something "more" informal or if someone else wants to talk, I have no issues to sit somewhere in the back ;). Ok, but perhaps we can work out something fitting. Greetings, Daniel

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Haskell Group Dresden?
by Daniel Trstenjak 15 Oct '13

15 Oct '13

Hi, is there any interest in having something like a Haskell Group in Dresden? Perhaps I could say something about the lens library, which would be more of a practical talk, how to define lenses and working with lenses, traversals, isos and prisms. Greetings, Daniel

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