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Polymorphic functions over string libraries
by gonzaw 29 Oct '14

29 Oct '14

Hi. I was wondering what would be the best way to create a polymorphic function over any possible string library (Text, String, Bytestring, etc). For instance, imagine I have to read a file with text, transform this text in some way and output it to another file, or to the console. If I wanted to use String, I'd just do this: / transform :: String -> String main = readFile "input.txt" >>= writeFile "output.txt" . transform / But if I wanted to use Text instead, I'd have to use this: / import qualified Data.Text.IO as T transform :: Text -> Text main = T.readFile "input.txt" >>= T.writeFile "output.txt" . transform / Idem for ByteString. I was wondering if there was a way to create these computations in a generic way, for any kind of string library, something like this: / class StringLibrary s where: sReadFile :: FilePath -> IO s sWriteFile :: FilePath -> s -> IO () ... / So then I'd just have this: / transform :: StringLibrary s => s -> s main = sReadFile "input.txt" >>= sWriteFile "output.txt" . transform / Now I can perform the computation I want without being tied down to a specific library. At times when I create some quick scripts, I find myself using one library (for example using String to get it finished more quickly, since I have less experience with the other ones), but find that it's too slow or has some problem that is solved by using one of the other libraries. Yet swapping from one to the other is more cumbersome than expected at times. In the example above, I could easily swap between them, just by forcing the compiler to typecheck to a specific one (for instance by changing the type of "transform"). Or if I wanted to, I could leave it as it is and export it as a library of my own. Is there a way to do something like this in Haskell, with existing libraries? In terms of using the string datatype as some sort of container of characters, I think there are libraries like Lens and mono-traversable that allow you to do stuff like this. But I'm not too familiar with them (at least using them in this way). -- View this message in context: http://haskell.1045720.n5.nabble.com/Polymorphic-functions-over-string-libr… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Munich Haskell Meeting
by Heinrich Hördegen 29 Oct '14

29 Oct '14

Dear all, tomorrow, the 30th of October, our monthly Munich Haskell Meeting will take place again at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please click the button! Everybody is welcome! Heinrich

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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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