- Haskell-Cafe - Haskell.org

a simpler way to declare typeclass instances
by Alexey Muranov 29 Oct '14

29 Oct '14

Hello, i am trying to understand how typeclasses work. I know that they can be used as follows (from the manual): data Foo = Foo {x :: Integer, str :: String} instance Eq Foo where (Foo x1 str1) == (Foo x2 str2) = (x1 == x2) && (str1 == str2) I am wondering, why is the following seemingly unambiguous syntax not allowed too? data Foo = Foo { x :: Integer, str :: String } instance Eq Foo (Foo x1 str1) == (Foo x2 str2) = (x1 == x2) && (str1 == str2) If it was allowed, it seems that it could also be applied to records: class HasName r where name :: r -> String data Bird = Bird { name :: String, wingNumber :: Integer } data Person = Person { name :: String, likesBirds :: Bool } instance HasName Bird instance HasName Person Alexey.

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Polymorphic case for this lens transformer?
by Hans Höglund 28 Oct '14

28 Oct '14

Hi all, I have been using this lens transformer http://lpaste.net/113159. However, I can't seem to find a way to implement the polymorphic case (i.e. Lens instead of Lens'). What am I missing? Hans - Hans Höglund Composer, conductor and developer hans [at] hanshoglund.se hanshoglund.com https://twitter.com/hanshogl https://soundcloud.com/hanshoglund http://github.com/hanshoglund

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Tracking down where an instance comes from
by Mateusz Kowalczyk 28 Oct '14

28 Oct '14

Hi, I'm in a situation where we have a 140+ module program (Yi) with some 30+ packages of dependencies and somewhere along the lines, this little guy gets in: instance [safe] Show (a -> b) -- Defined in ‘Text.Show.Functions’ Of course this automatically infects everything to import Yi. This is bad. What would be the best way to track down through which import this actually comes in? There's nothing directly importing Text.Show.Functions and I wonder if there's a better way than ‘remove imports one by one going deeper each time you hit it’ which might take a while, especially as there seems to be no way to unload instances except restarting GHCi. -- Mateusz K.

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pontarius-xmpp with tls
by Никита Тимофеев 27 Oct '14

27 Oct '14

Who can show a working example code creates a client Session with the server with enabled TLS? -- Timofeev N.D.

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inlinable vs inline
by Charlie Durham 27 Oct '14

27 Oct '14

Hi, I wanted to get some clarifications on how inlinable and inline behave. One particular case is where you have a module exporting some functions with helper functions that are not exported. You go ahead and add inlinable to the exported functions so that the call site can specialize on them. Is there any penalty to adding inline vs inlinable to the helper functions? For example, are there cases where a helper function will not inline and thus can not be specailized at the call site and it would be better to declare the helper function inlinable? I would be interested to hear any nuances related to the differences and practical consequences of these pragmas. Thanks! Charlie Durham

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Wiki Account
by Alper Tekinalp 27 Oct '14

27 Oct '14

Hi. In wiki page it says: " If you would like an account please email "nominolo" (at the email service from Google) or on the haskell-cafe mailing list." I want to create a wiki account. My preferred user name: alpert Can anyone help? Best regards. -- Alper Tekinalp atekinalp.blogspot.com

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Haskell Cheat Sheet
by Rudy Matela 26 Oct '14

26 Oct '14

Hello, All, Some time ago, I was looking for a Haskell Cheat Sheet, to help me remember Haskell's syntax and common functions. I've found one, but it was quite long (14 pages), not what I was looking for. So, I've started building a Haskell Cheat Sheet with the most common language features condensed in two pages. It still needs a lot of improvement (and some content). I'm using LaTeX and I've built a "cls" (so it can be used to create Sheets for other languages as well), it is kind of a hack for now. If someone wants to use it as a reference, the first version can be found on [1] and the TeX source can be found on GitHub [2]. I would appreciate help on it: feel free to fork and make pull requests with new additions (or mail me asking for push permissions). Regards, Rudy [1]: https://matela.com.br/pub/cheat-sheets/haskell-ucs-0.1.pdf [2]: https://github.com/rudymatela/ultimate-cheat-sheets

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Call-for-help: Add XDGBDS support to `directory` package
by Herbert Valerio Riedel 25 Oct '14

25 Oct '14

Hello *, I'd like to invite everyone with knowledge of the XDG Base Directory Specification[1] to contribute to https://github.com/haskell/directory/issues/6 >From what I gathered, it appears to me that adhering to the XDGBD Spec (on Linux at least) is desirable and "strongly encouraged" by Linux distributions[2], so I think this ought to be implemented sooner rather than later (maybe even in time to be part of GHC 7.10.1) This issue was originally filed against GHC, since it affects the location of the `~/.ghc` folder (and fwiw this would also affect cabal's `~/.cabal` folder location). Thanks, hvr [1]: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html [2]: https://wiki.debian.org/XDGBaseDirectorySpecification

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Parse CSV / TSV file in Haskell - Unicode Characters
by Volker Strobel 24 Oct '14

24 Oct '14

Hi, I'm trying to parse a tab-delimited file using cassava/Data.Csv in Haskell. However, I get problems if there are "strange" (Unicode) characters in my CSV file. I'll get a parse error (endOfInput) then. According to the command-line tool "file", my file has a "UTF-8 Unicode text" decoding. My Haskell code looks like this: {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE OverloadedStrings #-} import qualified Data.ByteString as C import qualified System.IO.UTF8 as U import qualified Data.ByteString.UTF8 as UB import qualified Data.ByteString.Lazy.Char8 as DL import qualified Codec.Binary.UTF8.String as US import qualified Data.Text.Lazy.Encoding as EL import qualified Data.ByteString.Lazy as L import Data.Text.Encoding as E -- Handle CSV / TSV files with ... import Data.Csv import qualified Data.Vector as V import Data.Char -- ord csvFile :: FilePath csvFile = "myFile.txt" -- Set delimiter to \t (tabulator) myOptions = defaultDecodeOptions { decDelimiter = fromIntegral (ord '\t') } main :: IO () main = do csvData <- L.readFile csvFile case EL.decodeUtf8' csvData of Left err -> print err Right dat -> case decodeWith myOptions NoHeader $ EL.encodeUtf8 dat of Left err -> putStrLn err Right v -> V.forM_ v $ \ (category :: String , user :: String , date :: String, time :: String, message :: String) -> do print message I tried using decodingUtf8', preprocessing (filtering) the input with predicates from Data.Char, and much more. However the endOfFile error persists. My CSV-file looks like this: a - - - RT USE " Kenny" • Hahahahahahahahaha. #Emmen #Brandstapel a - - - Uhm .. wat dan ook ????!!!! 👋 Or more literally: a\t-\t-\t-\tRT USE " Kenny" • Hahahahahahahahaha. #Emmen #Brandstapel a\t-\t-\t-\tUhm .. wat dan ook ????!!!! 👋 The problem chars are the 👋 and • (and in my complete file, there are many more of similar characters). What can I do, so that cassava / Data.Csv can read my file properly? I've also posted this question at StackOverflow a few days ago: http://stackoverflow.com/questions/26499831/parse-csv-tsv-file-in-haskell-u… Best, Volker

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Timing execution of individual functions in ghci?
by Magnus Therning 24 Oct '14

24 Oct '14

When playing around with various implementations of a single function it would be nice to time the execution. Is there some way to do that easily from within GHCi? Or am I forced to look to criterion for that? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Heuristic is an algorithm in a clown suit. It’s less predictable, it’s more fun, and it comes without a 30-day, money-back guarantee. -- Steve McConnell, Code Complete

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