- Haskell-Cafe - Haskell.org

How do I generate random numbers using random-fu, with platform-agnostic code?
by Andrew Pennebaker 13 Sep '12

13 Sep '12

I can't figure out how to use Data.Random.Source.IO to generate random numbers in a multiplatform way. I can generate random numbers in Unix using Data.Random.Source.DevRandom, and there is an example in the GitHub documentation for Windows using Data.Random.Source.MWC, but there is no example code for using Data.Random.Source.IO. Cheers, Andrew Pennebaker www.yellosoft.us

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Fwd: How Type inference work in presence of Functional Dependencies
by Dan Doel 13 Sep '12

13 Sep '12

Copying the mailing list, because I forgot. On Thu, Sep 13, 2012 at 5:18 AM, satvik chauhan <mystic.satvik(a)gmail.com> wrote: > Consider the code below : > > {-# LANGUAGE > MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts > #-} > class Foo a c | a -> c > instance Foo Int Float > f :: (Foo Int a) => Int -> a > f = undefined > > Now when I see the inferred type of f in ghci > >> :t f > >> f :: Int -> Float > > Now If I add the following code > > g :: Int -> Float > g = undefined > > h :: (Foo Int a) => Int -> a > h = g > > > I get the error > > Could not deduce (a ~ Float) > > > I am not able to understand what has happened here ? The restriction "Foo > Int a" should have restricted the type of h to "Int -> Float" as shown in > the inferred type of f. The answer, I believe, is that the difference between the fundep implementation and type families is local constraint information. Fundeps do no local propagation. So in your first definition, you've locally provided 'Int -> a', which is acceptable to GHC. Then it figures out externally to the function that '(Foo Int a) => Int -> a' is actually Int -> Float. In the second definition, you're trying to give 'Int -> Float', but GHC only knows locally that you need to provide 'Int -> a' with a constraint 'Foo Int a' which it _won't_ use to determine that a ~ Float. This is not inherent to fundeps. One could make a version of fundeps that has the local constraint rules (easily so by translating to the new type families stuff). But, the difference is also the reason that overlapping instances are supported for fundeps and not type families. But I won't get into that right now. -- Dan

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How Type inference work in presence of Functional Dependencies
by satvik chauhan 13 Sep '12

13 Sep '12

Consider the code below : {-# LANGUAGE MultiParamTypeClasses,FlexibleInstances,FunctionalDependencies,UndecidableInstances,FlexibleContexts #-} class Foo a c | a -> c instance Foo Int Float f :: (Foo Int a) => Int -> a f = undefined Now when I see the inferred type of f in ghci > :t f > f :: Int -> Float Now If I add the following code g :: Int -> Float g = undefined h :: (Foo Int a) => Int -> a h = g I get the error Could not deduce (a ~ Float) I am not able to understand what has happened here ? The restriction "Foo Int a" should have restricted the type of h to "Int -> Float" as shown in the inferred type of f. - Satvik

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Re: [Haskell-cafe] [ANN] Cumino 0.2 - Now supports pretty indentation through stylish-haskell
by Roman Cheplyaka 12 Sep '12

12 Sep '12

Ah, okay. I was just confused by the fact that it uses tmux, and thought that I was misusing it. Yes, I also usually keep ghci in a separate window (and I am an xmonad user, too). I just thought that this offers a different experience and wanted to try it out. Anyway, thanks for clarifying. On Wed, Sep 12, 2012 at 5:24 PM, Alfredo Di Napoli < alfredo.dinapoli(a)gmail.com> wrote: > Hi Roman, > > Cumino was thought to operate in another terminal. Personally, but this is > only a personal taste, having to switch between terminal tabs or tmux panes > is not the fastest workflow. Being an Xmonad user, i can easily swap > between terminals (read ghci and vim) simply with mod + j or mod + k. > So the answer to your answer is: You can't, Cumino will always start in > another terminal window. Is the same behaviour of Slime, though. > > Sent from my iPad > > On 12/set/2012, at 17:06, Roman Cheplyaka <roma(a)ro-che.info> wrote: > > So, suppose that I'm in a terminal vim session, and I want to start ghci > (in the current terminal). What do I do? > > <localleader>cc starts a new terminal, which is not what I want. > > On Wed, Sep 12, 2012 at 3:03 PM, Alfredo Di Napoli < > alfredo.dinapoli(a)gmail.com> wrote: > >> Hi everyone, >> >> in case you have missed it, I've released a Vim plugin called Cumino: >> >> http://adinapoli.github.com/cumino/ >> >> It does one simple thing: It allows communication between Vim and tmux, >> in particular to a ghci session. With Cumino you can fire-up Vim, >> load a ghci session and interact with it with only few keystrokes. The >> plugin also supports visual selection: you can select for example a >> function (even with all its signature!) >> and you can send it to ghci. The visual selection supports imports, >> custom types and typeclasses. >> >> It's a simple idea but so damn useful, imho. >> >> This release also adds the possibility to prettify the code using the >> excellent stylish-haskell: select a snippet, simply indent in the usual way >> ( = ) and voilà, now >> your code is indented! >> >> Feedback are highly appreciated, as well as contributions. >> There are still some issues with some terminals (for example urxvt does >> not work right now) but the plugin has been tested against gnome-terminal, >> xterm and mlterm. >> >> I'll post in reddit too for completeness! >> >> Bye! >> Alfredo >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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[Hackage] Bug report and proposal
by Stayvoid 12 Sep '12

12 Sep '12

Hello, I found a bug: "source" points to the wrong page. [1] I know that bugs should be reported via Github. But I don't have an account and don't want to create one. There might be others who don't want to use Github. Why rely on external bug tracker? I assume that it has been chosen because of its features, but it doesn't have the most important one (i.e. anonymous posting). What's better: to have more features or to have less bugs? Is it possible to move away from Github? What about a separate mailing list for bugs? I'm sorry if this sounds to harsh; I'm not good at writing. I don't want to attack or blame anyone. I'm also sorry if this message is not appropriate, but I haven't found a better place. [1] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… Cheers

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Re: [Haskell-cafe] [ANN] Cumino 0.2 - Now supports pretty indentation through stylish-haskell
by Alfredo Di Napoli 12 Sep '12

12 Sep '12

Could you please tell me what your desired behaviour would be? In praticular, do you want a ghci session in another tab or in a tmux pane perhaps? Otherwise I can't see any viable way to let vim and ghci cooperate inside the SAME window. Bye, Alfredo Sent from my iPad On 12/set/2012, at 17:05, Roman Cheplyaka <roma(a)ro-che.info> wrote: > So, suppose that I'm in a terminal vim session, and I want to start ghci (in the current terminal). What do I do? > > <localleader>cc starts a new terminal, which is not what I want. > > On Wed, Sep 12, 2012 at 3:03 PM, Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> wrote: > Hi everyone, > > in case you have missed it, I've released a Vim plugin called Cumino: > > http://adinapoli.github.com/cumino/ > > It does one simple thing: It allows communication between Vim and tmux, in particular to a ghci session. With Cumino you can fire-up Vim, > load a ghci session and interact with it with only few keystrokes. The plugin also supports visual selection: you can select for example a function (even with all its signature!) > and you can send it to ghci. The visual selection supports imports, custom types and typeclasses. > > It's a simple idea but so damn useful, imho. > > This release also adds the possibility to prettify the code using the excellent stylish-haskell: select a snippet, simply indent in the usual way ( = ) and voilà, now > your code is indented! > > Feedback are highly appreciated, as well as contributions. > There are still some issues with some terminals (for example urxvt does not work right now) but the plugin has been tested against gnome-terminal, xterm and mlterm. > > I'll post in reddit too for completeness! > > Bye! > Alfredo > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] type variable in class instance
by oleg＠okmij.org 12 Sep '12

12 Sep '12

Corentin Dupon wrote about essentially the read-show problem: > class (Typeable e) => Event e > > data Player = Player Int deriving (Typeable) > data Message m = Message String deriving (Typeable) > > instance Event Player > > instance (Typeable m) => Event (Message m) > > viewEvent :: (Event e) => e -> IO () > viewEvent event = do > case cast event of > Just (Player a) -> putStrLn $ show a > Nothing -> return () > case cast event of > Just (Message s) -> putStrLn $ show s > Nothing -> return () Indeed the overloaded function cast needs to know the target type -- the type to cast to. In case of Player, the pattern (Player a) uniquely determines the type of the desired value: Player. This is not so for Message: the pattern (Message s) may correspond to the type Message (), Message Int, etc. To avoid the problem, just specify the desired type explicitly > case cast event of > Just (Message s::Message ()) -> putStrLn $ show s > Nothing -> return () (ScopedTypeVariables extension is needed). The exact type of the message doesn't matter, so I chose Message (). BTW, if the set of events is closed, GADTs seem a better fit > data Player > data Message s > > data Event e where > Player :: Int -> Event Player > Message :: String -> Event (Message s) > > viewEvent :: Event e -> IO () > viewEvent (Player a) = putStrLn $ show a > viewEvent (Message s) = putStrLn $ show s

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IO vs MonadIO
by Sergey Mironov 12 Sep '12

12 Sep '12

Hi. Just a brief question. System.IO functions are defined in IO monad and have signatures like Foo -> IO Bar. Would it be better to have all of them defined as (MonadIO m) => Foo -> m Bar? What are the problems that would arise? Sergey

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Pattern matching: multi constructors to same branch
by Thiago Negri 11 Sep '12

11 Sep '12

Is it possible to capture more than one constructor in a single pattern matching? I mean, is it possible to generalize the following pattern matching of A and B to a single branch? g f C = [f C] g f v@(A _ n) = f v : g n g f v@(B _ n) = f v : g n For example: g f C = [f C] g f v@(A|B _ n) = f v : g n Or: g f v = case v of C -> [f C] A|B -> f v : g (n v) Thanks.

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Either Monad and Laziness
by Eric Velten de Melo 11 Sep '12

11 Sep '12

Hello, I am currently trying to rewrite the Graphics.Pgm library from hackage to parse the PGM to a lazy array. The current implementation parses it straight to UArray, which is strict. The behaviour I want to achieve is like this: I want the program when compiled to read from a file, parsing the PGM and at the same time apply transformations to the entries as they are read and write them back to another PGM file. The problem is that even using a lazy array structure, because the parser returns an Either structure it is only possible to know if the parser was successful or not after the whole file is read, therefore requiring it to read the entire file before applying the transformations, ruining the property of laziness. Is there some way to keep this from happening? Should I even want to make it like this? Not really a real life situation, but imagine I want to read a reaaaaally large PGM file which does not fit into RAM memory and I don't want to be forced to have the whole array in the memory at once. One alternative I thought was parsing only the PGM header and then read the rest of the input without using Parsec and the Either Monad. In the event the data is corrupted, though, I would not know how to recover from it. Any thoughts? Hopefully I'm not saying anything really stupid. Eric

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