February 2014 - Haskell-Cafe

Q: haskell type inference algorithm paper
by Di Xu 25 Feb '14

25 Feb '14

Hi, all I'm new to haskell, I'm learning haskell for its type system, and I found haskell type inference is excellent, I'm interested in its type inference algorithm, could you give me some paper or material reference that describe the haskell type inference algorithm? I want to port it to typed clojure.

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Haskell minimal (portable) development environment
by dabd 25 Feb '14

25 Feb '14

Hi, I am currently reading through Packer documentation http://www.packer.io/ and vagrant http://www.vagrantup.com/. I would like to set up a minimal reproducible Haskell development environment on Manjaro Linux (64-bit) running on VirtualBox. Does anyone have a Packer template for this distro or a Vagrant box for Haskell development on any other distro? Thanks.

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Fwd: manage effects in a DSL
by Corentin Dupont 25 Feb '14

25 Feb '14

Oleg: Very interresting, thanks. I have some questions: - What do you mean by "The type Cont Int a describes an impure computation, which may abort with an Int value, for example". Aborting with an Int value is akin to exceptions? - for me it's not clear when to choose an "applicative" or a "monadic" DSL? Betsides, what's the rational behind the name "let_" (never seen it before)? Linsey, Jacques: Thanks for the pointer! I learned about data kinds. I tried to apply your suggestions to add a phantom type parameter to Exp. I came up to (I dropped the Free monad idea, which seems unrelated to the problem): > data Eff = Effect | NoEffect > -- first type parameter is used to track effects > data Exp :: Eff -> * -> * where > ReadAccount :: Exp r Int --ReadAccount can be used in whatever monad (with or without effect) > WriteAccount :: Exp NoEffect Int -> Exp Effect () --WriteAccount takes an effect-less expression, and returns an effectfull expression > SetVictory :: Exp NoEffect Bool -> Exp Effect () -- same for SetVictory > OnTimer :: Exp Effect () -> Exp Effect () --OnTime can program whatever expression to be triggered every minutes, in particular effectful ones > Return :: a -> Exp r a > Bind :: Exp r a -> (a -> Exp r b) -> Exp r b This is the (simplified) game state: > data Game = Game { bankAccount :: Int, > victory :: Exp NoEffect Bool, > timerEvent :: Exp Effect ()} > -- victory when account > 100 > victoryRule' :: Exp Effect () > victoryRule' = SetVictory $ do > m <- readAccount > --WriteAccount (return $ m + 1) --won't compile (good) > return (m > 100) > --increase my bank account by 1 every minute > myTimer :: Exp Effect () > myTimer = OnTimer $ do > m <- readAccount > writeAccount (return $ m + 1) Do you have a better name suggestion for Eff? Other pointers where this idiom is realised?? Thanks!! Corentin

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segfault on fromIntegral (floor $ sqrt (-123.123)) == -123.123
by Erik Rantapaa 24 Feb '14

24 Feb '14

I'm getting a segfault on this expression entered into ghci: GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> fromIntegral (floor $ sqrt (-123.123)) == -123.123 Segmentation fault: 11 I'm running the 32-bit version of the Haskell Platform on OSX. Is this a bug? a problem with my setup? "Just don't take square roots of negative numbers - you should know better!"???

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Munich Haskell Meeting
by Heinrich Hördegen 24 Feb '14

24 Feb '14

Dear all, Wednesday, the 26th of February, our Monthly Haskell Meeting is taking place once again in Munich, Cafe Puck. We start at 19h30. If you plan to join, please go to http://www.haskell-munich.de/dates and hit the button. This time, we will have a guest from India, Raman, who will bring along some boards and shows us how to run PicoLisp [1] on bare metal. There is also work in progress for running Hugs [2] on bare metal. If you are interested in embedded systems, this will be surely an enlightening meeting. Raman is looking also for people from the Haskell community who are willing to help him embed Hugs or some other Haskell programming system on bare metal. If you are interested, I can make the contact. With hope to see many of you on Wednesday, Heinrich [1] http://picolisp.com/wiki/?home [2] http://www.haskell.org/hugs/

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#haddock lives
by Mateusz Kowalczyk 24 Feb '14

24 Feb '14

Hi all, This is just a small heads up that the IRC channel, #haddock (on Freenode) is now revived. Previously any Haddock chatter would have taken place across #ghc, #hackage, #hspec and #haskell which made it difficult to track. Please use this channel if you have questions. Thanks -- Mateusz K.

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Re: [Haskell-cafe] Is the wiki really this out of date?
by Dominic Steinitz 24 Feb '14

24 Feb '14

Daniil Frumin <difrumin <at> gmail.com> writes: > > There are also some interesting charts on ircbrowse: > http://ircbrowse.net/haskell > Very cool! Unless someone objects, I will update the 2009 data on the wiki with these more up-to- date presentations of the data.

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Want to create your first Haskell project?
by Roman Cheplyaka 23 Feb '14

23 Feb '14

Hi, I'd like to see an html test reporter for the tasty testing framework (http://documentup.com/feuerbach/tasty) I could do that myself, but I thought it would be a nice little project for someone who's about to write their first useful piece of Haskell. To participate you need to know Haskell on LYAH level and be good at html/css/js. If you're interested, get in touch. I'll provide help, review your code etc. Roman

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Data.Foldable UArray
by Marcus D. Gabriel 23 Feb '14

23 Feb '14

Hello, I wanted to make a simple Data.Foldable UArray, and I naively modelled it on > instance Ix i => Foldable (Array i) where > foldr f z = Prelude.foldr f z . elems with, of course, > instance Ix i => Foldable (UArray i) where > foldr f z = Prelude.foldr f z . elems which did not work yielding the following type message Could not deduce (IArray UArray a) arising from a use of `elems' from the context (Ix i) bound by the instance declaration at ... Possible fix: add an instance declaration for (IArray UArray a) In the second argument of `(.)', namely `elems' In the expression: Data.List.foldr f z . elems In an equation for `foldr': foldr f z = Data.List.foldr f z . elems I clearly do not understand something because I cannot make this work, and I am not sure why. With the Haskell type system or even with ghc extensions, can one even make a Data.Foldable UArray? If so, how? Thanks in advance, - Marcus

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[ANN] cabal-bounds: A command line program for managing the bounds/versions of the dependencies in a cabal file.
by Daniel Trstenjak 23 Feb '14

23 Feb '14

Hi all, If you have several cabalized projects, then it can be quite time consuming to keep the bounds of your dependencies up to date. Especially if you're following the package versioning policy, then you want to raise your upper bounds from time to time, to allow the building with newer versions of the dependencies. `cabal-bounds` tries to automate this update process to some degree. So a typical update process might look like: $> # update the version infos of all libraries $> cabal update $> # drops the upper bound of all dependencies in 'myproject.cabal', most likely you want to ignore 'base' $> cabal-bounds drop --upper --ignore=base myproject.cabal $> # create a cabal sandbox for building of 'myproject' $> cabal sandbox init $> # build 'myproject' $> cabal install $> # update the upper bound of all dependencies in 'myproject.cabal' by the cabal build information $> cabal-bounds update --upper --ignore=base myproject.cabal dist/dist-sandbox-*/setup-config Perhaps the currently most annoying thing is, that you have to live with the reformating of your `cabal` file done by the pretty printer of the `Cabal` library. https://github.com/dan-t/cabal-bounds Greetings, Daniel

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