December 2014 - Haskell-Cafe

Ordered JSON objects?
by Tobias Dammers 18 Dec '14

18 Dec '14

I'm building a "thing" that is supposed to use JSON 'documents' as its building blocks, providing various (pluggable) storage backends and presentation frontends. Non-techie/semi-techie (frontend dev, web dev, etc.) compatibility is needed, which is why I want to keep the data model "simple" and dynamically typed. Anyway, one thing I'm running up against is that I am going to need ordered key/value collections, which I believe is something JSON does not support out-of-the-box: "objects" ({"foo":15, "bar":23}) are conceptually unordered key/value collections (and Aeson treats them as such, using hash maps as the intermediate storage format), so I lose ordering; OTOH, flat lists ([15, 23]) do not have keys, and lists of "pairs" (two-element lists) produce uglier syntax ([["foo",15], ["bar",23]]), but they are the "correct" solution. Ideally, someone is going to step up and tell me that both I and Aeson have misunderstood JSON, and objects are really supposed to be ordered after all, in which case I'd need an alternative to Aeson (or someone needs to patch it?). Or I could bite the bullet and go with [["foo",15], ["bar",23]]. Or I could ditch JSON as the canonical format and go with something custom, or, shudder, XML. Thoughts? Hints? Ideas? - Tobias (tdammers on freenode)

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how to handle FILE* with c2hs
by Johannes Waldmann 17 Dec '14

17 Dec '14

Hi. I could use an example of how to handle files (FILE *) with c2hs, but could not find any. I want to call some "void dump (..., FILE * f)" function. I want to use this for a handle that I obtained in Haskell land. Sure I can rewrite/add the C code so that it takes a filepath (string) and opens the file by itself, but should I? - J.W.

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PVP question
by Michael Snoyman 17 Dec '14

17 Dec '14

I'm a little bit uncertain of the PVP guidelines in a certain case[1], so I'd like to get some guidance/clarity. Suppose I have a library which provides the function: myFunction :: IO () myFunction = forever $ putStrLn "Still here" >> threadDelay 10^6 Later, I realize (or someone points out to me) that I've over-specified the type signature, and really myFunction should be: myFunction :: IO a In this case, does the PVP specify that we should have a minor or a major version bump? I'm not certain if this counts as a breaking change or not. [1] https://github.com/fpco/streaming-commons/pull/13

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Haskell Polishing [was: Missing `$` like operators in Haskell]
by Giacomo Tesio 16 Dec '14

16 Dec '14

2014-12-12 14:22 GMT+01:00 Johan Tibell <johan.tibell(a)gmail.com>: > > Everything doesn't have to have an operator! We use operators a bit too > much. It hurts readability of code when lines start looking like APL. > > Probably this has been discussed already (and I'd like to read previous material, btw, so please give me some reference), but as a long time programmer with experience in many different paradigms and languages I too feel Haskell a bit cumbersome from this point of view. I mean, the language in itself is the best I know, but libraries have different levels of quality and in general they miss organization: they often overlap in intent and interface, for example. Anyone tried to address this issue? And if so, how? I can see how backward compatibility prevents Haskell to be polished, but it's a pity. It's like an handbrake for it's evolution and adoption! Giacomo

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Request For Feedback: Emulating OO-style dispatch for binding ...
by aditya siram 16 Dec '14

16 Dec '14

I am writing a GUI binding but unfortunately could not find a OO-style dispatching scheme that I was happy with so I wrote my own ( https://gist.github.com/deech/6528a75e480378182052 ). The design goal is to: - provide a single function that will dispatch to a custom implementation based on the type of argument it is given - allow the user to extend and subclass the hierarchy and override functions in a way that doesn't involve recompiling the binding - use as few new extensions as possible - provide as familiar an API as possible, thereby allowing users to leverage existing documentation maintained by the C++ project. The look-and-feel of the API is on lines 128-140. It should just be a stand-alone file that can be loaded into GHCI. Thanks for your feedback! -deech

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Year-end deadline for Haskell.org donations
by Adam Foltzer 16 Dec '14

16 Dec '14

Dear Haskellers, HaskellWiki, Hackage, Hoogle, and all the other Haskell.org sites we depend on are hosted with funds donated by you, the Haskell community. As many of us are in tax jurisdictions where December 31st is a deadline for charitable giving, I’d like to remind you that donations to support these activities are tax-deductible. You can donate online right now at https://co.clickandpledge.com/advanced/default.aspx?wid=69561. Last year was the first time we were able to seek your help while offering tax relief, and you came through: we raised nearly $900 in the span of a few days! This year, with almost three weeks left before the end of the year, I'd love to see us double that. It has been quite a year for Haskell.org: we migrated of most of our infrastructure, including Hackage, to Rackspace in order to improve reliability. We implemented new proxying to improve speed and performance, and improved the documentation and standardization of our server administration practices. Many of these accomplishments and much else are detailed by Austin Seipp at the new Haskell.org infrastructure blog at https://blog.haskell.org/post/the_new_haskell_org/. Let's keep the momentum going in 2015! Thank you and best wishes for a happy new year, Adam Foltzer & The Haskell.org Committee

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No handles from createProcess
by Henk-Jan van Tuyl 15 Dec '14

15 Dec '14

L.S., I am experimenting with the createProcess function from the package "process"; this function does not return any handle, on both Ubuntu and Windows. I am doing something wrong, or is this a bug? (I am using GHC 7.8.3 on both platforms.) The program I am running: ---✂---------------------------------------------- import System.IO import System.Process f = do ( maybeStdinHandel , maybeStdoutHandel , maybeStderrHandel , processHandle ) <- createProcess (proc "cat" ["a.txt"]) case maybeStdinHandel of Just stdinHandel -> do putStrLn "stdin" _ -> putStrLn "No stdin handle" case maybeStdoutHandel of Just stdoutHandel -> do putStrLn "stdout" putStrLn =<< hGetContents stdoutHandel _ -> putStrLn "No stdout handle" case maybeStderrHandel of Just stderrHandel -> do putStrLn "stderr" putStrLn =<< hGetContents stderrHandel _ -> putStrLn "No stderr handle" main = f ---✂---------------------------------------------- The output: ---✂---------------------------------------------- $ ./ProcessTry No stdin handle No stdout handle No stderr handle asdf ---✂---------------------------------------------- (asdf is the contents of file a.txt) Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Converting non-primitive(?) recursive function to use scanl (or foldl maybe?)
by Cody Goodman 15 Dec '14

15 Dec '14

I've had some trouble wrapping my brain around how to use a scan to build this list: code below, but here's an lpaste link: http://lpaste.net/116494 {-# LANGUAGE OverloadedStrings #-} import Control.Monad import Data.Maybe import Text.Regex type RegexRes = Maybe (String, String, String, [String]) re = mkRegex "\$(\\w+)\$:([[:digit:]]+)" getParenNum :: String -> [String] getParenNum s = case matchRegexAll re s of Nothing -> [] Just (_,_,after,[word,num]) -> (word ++ num):getParenNum after getParenNumOnce' :: String -> String getParenNumOnce' s = case matchRegexAll re s of Nothing -> [] Just (_,_,after,[word,num]) -> (word ++ num) -- trying to accumulate the results of regexMatching repeatedly over a string f1 str = (\ (_, _, xs, acc) -> Just (xs, acc)) <=< (matchRegexAll re) $ str f2 str = fromMaybe "" $ (\ (_, _, after, [word,num]) -> Just after) <=< (matchRegexAll re) $ str -- f3 :: RegexRes -> RegexRes -- f3 :: RegexRes -> String f3 match = fromMaybe [] $ (\(_,_,after,[word,num]) -> Just (word ++ num)) =<< match f4 str = scanl (const f3) "" [(matchRegexAll re str)] f5 acc str = fromMaybe [] $ (\ (_, _, after, [word,num]) -> Just (acc ++ [word ++ num])) <=< (matchRegexAll re) $ str -- λ> scanl f5 [] ["(fish):1 sausage (cow):3 tree (boat):4"] -- [[],["fish1"]] str = "(fish):1 sausage (cow):3 tree (boat):4" -- λ> foldl (const . f3 . (matchRegexAll re)) "(fish):1 sausage (cow):3 tree (boat):4" ["(fish):1 sausage (cow):3 tree (boat):4"] -- "fish1" -- λ> -- want ["fish1","cow3","boat4"] -- λ> -- I feel like I should be using something in place of const that smartly chooses whether to pass String or... idk main = undefined

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Typesafe API calls
by Fabian Bergmark 13 Dec '14

13 Dec '14

I just released a library on hackage (https://hackage.haskell.org/package/apis-0.0.0) The goal of the library is to provide typesafe API calls to a large amount of existing APIs. By typesafe I mean that, for each API the library provides two types, Input and Output, and a function :: Input -> IO Output. Internally the library knows how to serialize Input to make an HTTP request and how to deserialize the response into Output. Whats cool about this library is that it uses Template Haskell to generate the API interfaces. A combination of Open Data Tables, an internal YQL engine and JSON Schemas, all written in Haskell makes this possible. Right now only two APIs are included (Swedish weather data and Swedish public announcement), but more APIs are soon to follow. I have also implemented FFI exports of all types and functions so that, in combination with SWIG, this library can be used in almost any language. So far I have tried Java and Python with success. If anyone would like to try it and respond with feedback it would be greatly appreciated! If you want any specific API supported, I can try to import that right away Fabian Bergmark

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Standard output of exec
by Ernesto Rodriguez 13 Dec '14

13 Dec '14

Dear All, I have a program which invokes an external program via exec. I wish to capture the standard output of the program without using external (temporary) files. Is it possible to create an in memory handle which I can tell exec to use as standard output of the invoked program. It would be great if it can be done without any extra dependencies. Best, -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

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