July 2015 - Beginners

basic Functor, Applicative and Monad instances
by Imants Cekusins 18 Jul '15

18 Jul '15

Here are complete working snippets with Functor, Applicative and Monad instances. This is my first attempt to write own instances rather than use ready ones. Here we go: module Part.Monad where import Control.Applicative {-# ANN module ("HLint: ignore Use let"::String) #-} {- Val' { val = b1 } can also be written as Val' b1 -} data Val a = Val' { val::a } deriving Show data Weather = Cold | Warm deriving Show instance Functor Val where -- (a -> b) -> f a -> f b fmap ab fa = let a1 = val fa b1 = ab a1 in Val' { val = b1 } instance Applicative Val where -- a -> f a pure a = Val' { val = a } -- f (a -> b) -> f a -> f b (<*>) fab fa = let ab1 = val fab a1 = val fa b1 = ab1 a1 in Val' { val = b1 } instance Monad Val where -- a -> m a return a = Val' { val = a } -- m a -> (a -> m b) -> m b (>>=) ma amb = let a1 = val ma in amb a1 -- pure and return in this example are interchangeable main::Int -> IO() main i = do -- do: Val as monad v1 <- pure Val' { val = i } -- pure: applicative v2 <- return $ over20 <$> v1 -- <$> : functor print v2 v3 <- return $ Val' weather <*> v2 -- <*> : applicative print v3 over20::Int-> Bool over20 i | i > 20 = True | otherwise = False weather::Bool-> Weather weather False = Cold weather True = Warm

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http-client: Access response body of error responses
by Sevcsik András 13 Jul '15

13 Jul '15

Hi list, Is there any way to read the response body of an >=400 HTTP response with http-client? If the response is an error response, it throws a StatusCodeException [1], which only exposes the headers, not the whole Response value. However, CouchDB gives a response body for the error responses, describing the error, which I want to access. [1]: https://hackage.haskell.org/package/http-client-0.2.1.1/docs/Network-HTTP-C… Cheers -- Minden jót, Sevcsik András

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cabal install snap-server-0.9.5.1: At least the following dependencies are missing: attoparsec >=0.10 && <0.13 && ==0.13.0.1
by Stanislaw Findeisen 12 Jul '15

12 Jul '15

Hi Could anyone explain this to me, please? $ cabal install snap-server-0.9.5.1 Resolving dependencies... Configuring snap-server-0.9.5.1... cabal: At least the following dependencies are missing: attoparsec >=0.10 && <0.13 && ==0.13.0.1 cabal: Error: some packages failed to install: snap-server-0.9.5.1 failed during the configure step. The exception was: ExitFailure 1 Where does this unsatisfiable dependency come from? snap-server-0.9.5.1 seems to require just attoparsec (>=0.10 && <0.14): http://hackage.haskell.org/package/snap-server-0.9.5.1 . And attoparsec 0.13.0.1 is already installed: $ cabal list --installed attoparsec * attoparsec Synopsis: Fast combinator parsing for bytestrings and text Default available version: 0.13.0.1 Installed versions: 0.13.0.1 Homepage: https://github.com/bos/attoparsec License: BSD3 * attoparsec-enumerator Synopsis: Pass input from an enumerator to an Attoparsec parser. Default available version: 0.3.4 Installed versions: 0.3.4 Homepage: https://john-millikin.com/software/attoparsec-enumerator/ License: MIT I am trying to install hakyll 4.6.*, and snap-server seems to be one of the requirements. Hakyll 4.6.* is an old version but AFAIR I need that due to conflicting versions of containers. Thanks! -- http://people.eisenbits.com/~stf/ http://www.eisenbits.com/ OpenPGP: 9EC2 5620 2355 B1DC 4A8F 8C79 0EC7 C214 E5AE 3B4E

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Monad problem
by René Klačan 12 Jul '15

12 Jul '15

Hi all, I've been trying to create working example with "ig" https://hackage.haskell.org/package/ig-0.2.2 - library over instagram API and I am facing little monad problem. Can someone advise me please how to make this small piece of code work? {-# LANGUAGE OverloadedStrings #-} import Network.HTTP.Client import Instagram code = "xxx_some_code" redirectUrl = "http://localhost:9988/instagram/oauth2/callback" credentials = Credentials "xxx_some_api_id" "xxx_some_api_secret" main :: IO () main = do manager <- newManager defaultManagerSettings token <- runInstagramT credentials manager $ getUserAccessTokenURL2 redirectUrl code print token I am getting following error: src/Main.hs:14:9: No instance for (Control.Monad.Trans.Resource.Internal.MonadResource IO) arising from a use of ‘getUserAccessTokenURL2’ In the second argument of ‘($)’, namely ‘getUserAccessTokenURL2 redirectUrl code’ In a stmt of a 'do' block: token <- runInstagramT credentials manager $ getUserAccessTokenURL2 redirectUrl code In the expression: do { manager <- newManager defaultManagerSettings; token <- runInstagramT credentials manager $ getUserAccessTokenURL2 redirectUrl code; print token } Thanks Rene

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Fwd: cabal install snap-server-0.9.5.1: At least the following dependencies are missing: attoparsec >=0.10 && <0.13 && ==0.13.0.1
by Imants Cekusins 10 Jul '15

10 Jul '15

> There is also a stack tool, Thank you. It is very interesting. Why were new tools introduced instead of adding new functionality to cabal? A mix of packaging tools may lead to confusion.

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-fglasgow-exts in .cabal
by Imants Cekusins 10 Jul '15

10 Jul '15

How to specify defaultExtensions in cabal (1.22) file? e.g. -fglasgow-exts or TypeSynonymInstances ?

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Loch package builds?
by Imants Cekusins 08 Jul '15

08 Jul '15

Did anyone try to install loch package recently: https://wiki.haskell.org/Debugging#Source-located_errors This error comes up on install: Debug/Trace/Location.hs:70:8: Expecting one more argument to ‘C.Exception’ Expected a type, but ‘C.Exception’ has kind ‘* -> Constraint’ In the type signature for ‘ppr’: ppr :: C.Exception -> String Failed to install loch-0.2 cabal: Error: some packages failed to install: loch-0.2 failed during the building phase. The exception was: ExitFailure 1 ?

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case statement
by Imants Cekusins 07 Jul '15

07 Jul '15

interestingly, the below test fails. May I ask: If "case" a1 is not the same as "where" a1, what is its value? Why does x=1 match against a1? note: result1 indent is aligned with a1 on the previous line ------ test module TestCase where import Test.Hspec main::IO() main = hspec $ do describe "TestCase" $ do it "case 1 - pass" $ do result1 1 `shouldBe` "one" it "case 2 - fail" $ do result1 2 `shouldBe` "two" where a1 = 2 result1 x = case x of a1 -> "one" 2 -> "two"

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