January 2015 - Haskell-Cafe

StateT and ContT
by Ivan Lazar Miljenovic 21 Jan '15

21 Jan '15

In the transformers library, there are two implementations for callCC for use with StateT (both lazy and strict) [1,2], the second of which is documented to not satisfy the laws of a monad transformer. [1]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-… [2]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-… However, in mtl the MonadCont class [3] uses the second definition for the StateT instance. Is there a good reason for this, or just to match the behaviour of pre-transformers mtl [4] ? [3]: http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Cont-Class.… [4]: http://hackage.haskell.org/package/mtl-1.1.1.1/docs/src/Control-Monad-State… (I did raise an issue for this on mtl's Github issue tracker, but it hasn't had any responses for two months.) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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MonoidHashMap?
by Konstantine Rybnikov 21 Jan '15

21 Jan '15

Hi! I'm working with a lot of HashMap's and it's very frustrating how many times I've "lost" my data because of usage of either a Monoid instance of a HashMap (which is defined as H.union, which, upon collision, takes value from first hm and discards from second), or just using fromList in the wrong place. Whereas the data I'm working is is mostly defined as (Monoid v => HashMap k v), so what I need "by default" is actually something like `H.unionWith (<>)`. What I was wondering is this: is something like MonoidHashMap is desired to be in unordered-containers, or is this use-case only popular in my programs? I'm asking because I have a feeling that this thing might be useful quite a lot for others also. If not -- sorry for bothering :) Cheers!

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garbage collection for a data structure
by Elise Huard 20 Jan '15

20 Jan '15

Hi, I was wondering if there was a way to check whether a particular data structure gets garbage collected in a program. A friendly person pointed me to System.Mem.Weak on the Haskell-Beginner list - however I've been unable to verify how it works, so I'm bumping it to this list. See the following toy program: I was trying to see whether the output would contain "garbage collected". I wondered if performGC is a nudge rather than an immediate "garbage collect now" instruction, and performGC is not actually performed? Or I've misunderstood finalizers in this context and they would not actually be executed when z gets garbage collected? import System.Mem.Weak import System.Mem (performGC) import Control.Concurrent (threadDelay) main :: IO () main = do let x = 5 y = "done" z = 3 a <- mkWeak z x (Just (putStrLn "garbage collected")) performGC threadDelay 20000000 print y Thank you, Elise

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New Functional Programming Job Opportunities
by Functional Jobs 20 Jan '15

20 Jan '15

Here are some functional programming job opportunities that were posted recently: Functional Software Developer at Moixa Technology http://functionaljobs.com/jobs/8778-functional-software-developer-at-moixa-… Cheers, Sean Murphy FunctionalJobs.com

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http-client: proxy environment variable support
by Michael Snoyman 19 Jan '15

19 Jan '15

Neil Mitchell opened an issue[1] for http_proxy and https_proxy environment variable support in http-client. I've written that support, and it's ready to go, but there's an open question: what should the default behavior be? In particular, should environment variables, by default, be checked to determine the proxy, or not? Arguments each way: In favor of using environment variables: * Matches behavior of many other tools and libraries * Allows application users control without requiring a code change from application writers Against using environment variables: * It's a change in behavior vs what http-client does today (though that could certainly be seen as just a missing feature) * Environment variables will implicitly change the behavior of code, which generally speaking can be problematic I'm leaning towards having the default behavior be: * If the user explicitly chooses a proxy setting on the manager, use that * If the user explicitly sets a proxy value on the Request, use that * If the environment variable is set, use that * Otherwise, no proxy In addition to whether we should make this change in behavior, I'm also interested in whether people think this should require a major version bump. [1] https://github.com/snoyberg/http-client/issues/94

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enable-executable-profiling failing with cabal 1.22
by Michael Jones 19 Jan '15

19 Jan '15

Using cabal 1.22 for some reason does not like —enable-executable-profiling. I am working on a rebuilt machine that had 1.18 from the distribution, with 1.22 installed afterwards. I found a little info on people having this problem, but I failed to fix the problem. Note than the library profiling option is accepted. Only the executable fails. Is there any know way to start with platform 2014 with GHC 7.8.3 and install cabal 1.22 and can use this option?

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This code produces an infinite output... why? it's a bug in ghc?
by mariopal 19 Jan '15

19 Jan '15

This code produces an infinite output... why? it's a bug in ghc? > -- Glasgow Haskell Compiler, Version 7.6.3 on Debian Sid > import System.Locale (defaultTimeLocale) > import Data.Time > > > mkDated n = formatTime defaultTimeLocale "%FT%TZ" $ addUTCTime (3600*n) $ UTCTime (fromGregorian 2015 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0) > > main = mapM_ putStrLn $ map mkDated [0..2]

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Code-completion problem with emacs, ghc-mod and auto-complete
by Илья Бирюков 19 Jan '15

19 Jan '15

Hi, Cafe! I'm struggling to configure my emacs for haskell development with ghc-mod and auto-complete. It works well for the most part, but I've run into the issue when completing qualified names. In the following example I don't get any completion after BS: module Playground where import qualified Data.ByteString.Lazy as BL doWork :: BL.By -- no completion after dot Completion for unqualified names and module name in imports works just fine. Maybe someone has run into this issue before? My .emacs: (ac-config-default) (auto-complete-mode t) (add-hook 'haskell-mode-hook 'turn-on-haskell-indentation) (autoload 'ghc-init "ghc" nil t) (add-hook 'haskell-mode-hook (lambda () (ghc-init))) (defun my-ac-haskell-mode () (setq ac-sources (append ac-sources '(ac-source-ghc-mod)))) (add-hook 'haskell-mode-hook 'my-ac-haskell-mode) (add-hook 'haskell-mode-hook 'turn-on-haskell-doc-mode) (add-hook 'haskell-mode-hook 'turn-on-haskell-indent) (ac-define-source ghc-mod '((depends ghc) (candidates . ghc-select-completion-symbol) (symbol . "s") (cache))) -- Regards, Ilya Biryukov.

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haskell.org slow
by Hector Urroz 17 Jan '15

17 Jan '15

Anyone know when haskell.org <http://haskell.org/> will be usable again? Page loads take 20+ seconds. Info at https://status.haskell.org <https://status.haskell.org/> indicates there's a DB problem, but there's no ETA and performance has been bad for over 10 days.

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Number of widgets extant and displayed varying over time? (FRP, reactive-banana)
by Jeffrey Brown 17 Jan '15

17 Jan '15

Dear list, I want to write an application in which the set of widgets in existence, and the subset of them that is displayed, depends on user input. I am using reactive-banana. So far the simplest spec I have imagined for it is this: Initially there is just a text entry box and an empty (hence invisible) collection of labels. The user can do two things: enter the word "add", or enter an integer. Entering the word "add" causes a label to be added to the collection, but not displayed. The labels require no text content. Entering an integer N causes the first N labels to be displayed onscreen. The text entry box remains visible. I am totally baffled. In particular, the Behavior paradigm, though it is elegant and beautiful whenever I study it, I have no idea how to apply. Thank you, Jeff

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