- Haskell-Cafe - Haskell.org

Long-running request/response protocol server using enumerator/iterator/iterIO/pipes/conduits/...
by Nicolas Trangez 28 Jun '12

28 Jun '12

Hello Cafe, Some time ago I tried to implement a network service using iteratee (or enumerator, can't remember), but gave up in the end. More recently I wanted to create something similar (a similar protocol), but failed again. So I'm looking for some example code or something similar (Google only helped slightly). First of all, I don't care which API/library to use, I guess for my purpose all of enumerator, iteratee, iterIO, pipes, conduits,... are OK, so all feedback is welcome. Here's the catch. Most examples out there implement some server which accepts a single client request, interprets it, creates a response, returns this, and closes the connection (or something alike, think HTTP). The protocol I'd like to implement is different: it's long-running using repeated requests & responses on a single client connection. Basically, a client connects and sends some data to the server (where the length of this data is encoded in the header). Now the server reads & parses this (binary) data, sets up some initial state for this client connection (e.g. opening a file handle), and returns a reply. Now the client can send another request, server parses/interprets it using the connection state, sends a reply, and so on. Might sound easy (and actually it's pretty easy in most other languages I know, including an OCaml implementation), yet I fail to figure out how to get this done using some enumerator-style library. Thanks for any help, I'll most likely write up something if I get things working for future reference. Nicolas

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Re: [Haskell-cafe] Data.Bitmap, readBitmap exception
by Alexander Foremny 28 Jun '12

28 Jun '12

Hello Serge, thank you very much for you response. Unfortunately this does not enlighten me. But maybe this is helpful to someone else who could given an answer. Regards, Alexander Foremny 2012/6/28 Serge Le Huitouze <serge.lehuitouze(a)gmail.com>: > Hi Alexander, > >> >> I am using bitmap-0.2 [1] to load a Bitmap file [2]. However, the >> following sample GHCi session fails for me. >> >> (...) >>> Prelude Data.Bitmap Data.Bitmap.Pure.File> readBitmap "b.bmp" :: IO (Bitmap Word8) >>> *** Exception: decodeCType: unexpected integer (7077888) >> >> I don't have any idea where 7077888 comes from. Am I doing something >> wrong? Is my bitmap corrupted in some way? The number 7077888 has no >> apparent significance except that it is a perfect cube [3]. So I guess >> this is not a problem with endianness. > > Sorry, no personal experience with Data.Bitmap... > > In case it's a hint, decimal 7077888 is 0x006C0000, and 6C is ascii > code for "l". > > HTH (?). > > --Serge

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Template-haskell loading packages
by Christopher Done 28 Jun '12

28 Jun '12

'Ello, I'm using TH in a big project and whenever TH starts-up in the GHC (6.4.2) compilation process it loads a number of packages: [119 o 119] Compiling Main ( src/Main.hs, dist/build/eudl/eudl-tmp/Main.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package array-0.4.0.0 ... linking ... done. Loading package strict-0.3.2 ... linking ... done. Loading package split-0.1.4.3 ... linking ... done. Loading package safe-0.3.3 ... linking ... done. Loading package bytestring-0.9.2.1 ... linking ... done. Loading package deepseq-1.3.0.0 ... linking ... done. Loading package containers-0.4.2.1 ... linking ... done. Loading package binary-0.5.1.0 ... linking ... done. Loading package cereal-0.3.5.2 ... linking ... done. Loading package entropy-0.2.1 ... linking ... done. Loading package largeword-1.0.1 ... linking ... done. Loading package tagged-0.4.2.1 ... linking ... done. Loading package crypto-api-0.10.2 ... linking ... done. Loading package pureMD5-2.1.0.3 ... linking ... done. Loading package dlist-0.5 ... linking ... done. Loading package old-locale-1.0.0.4 ... linking ... done. Loading package data-default-0.4.0 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package utf8-string-0.3.7 ... linking ... done. Loading package printf-mauke-3000.0.5.0.1 ... linking ... done. Loading package haskell-lexer-1.0 ... linking ... done. Loading package pretty-show-1.2 ... linking ... done. Loading package parallel-3.2.0.3 ... linking ... done. Loading package transformers-0.2.2.0 ... linking ... done. Loading package mtl-2.0.1.0 ... linking ... done. Loading package text-0.11.2.1 ... linking ... done. Loading package parsec-3.1.3 ... linking ... done. Loading package unix-2.5.1.1 ... linking ... done. Loading package network-2.3.0.14 ... linking ... done. Loading package old-time-1.1.0.0 ... linking ... done. Loading package HTTP-4000.2.3 ... linking ... done. Loading package base64-bytestring-0.1.1.1 ... linking ... done. Loading package blaze-builder-0.3.1.0 ... linking ... done. Loading package blaze-html-0.4.3.4 ... linking ... done. Loading package filepath-1.3.0.0 ... linking ... done. Loading package directory-1.1.0.2 ... linking ... done. Loading package syb-0.3.6.1 ... linking ... done. Loading package hs-bibutils-4.12 ... linking ... done. Loading package json-0.5 ... linking ... done. Loading package pandoc-types-1.9.1 ... linking ... done. Loading package time-1.4 ... linking ... done. Loading package xml-1.3.12 ... linking ... done. Loading package citeproc-hs-0.3.4 ... linking ... done. Loading package extensible-exceptions-0.1.1.4 ... linking ... done. Loading package regex-base-0.93.2 ... linking ... done. Loading package regex-pcre-builtin-0.94.2.1.7.7 ... linking ... done. Loading package highlighting-kate-0.5.1 ... linking ... done. Loading package process-1.1.0.1 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package tagsoup-0.12.6 ... linking ... done. Loading package temporary-1.1.2.3 ... linking ... done. Loading package texmath-0.6.0.6 ... linking ... done. Loading package digest-0.0.1.1 ... linking ... done. Loading package zlib-0.5.3.3 ... linking ... done. Loading package zip-archive-0.1.1.8 ... linking ... done. Loading package pandoc-1.9.4.1 ... linking ... done. Loading package convertible-1.0.11.1 ... linking ... done. Loading package HDBC-2.3.1.1 ... linking ... done. Loading package HDBC-postgresql-2.3.2.1 ... linking ... done. Loading package haskelldb-2.1.3 ... linking ... done. Loading package haskelldb-hdbc-2.1.0 ... linking ... done. Loading package haskelldb-hdbc-postgresql-2.1.0 ... linking ... done. Loading package QuickCheck-2.4.2 ... linking ... done. Loading package has-0.5.0.1 ... linking ... done. Loading package feed-0.3.8 ... linking ... done. Loading package MonadCatchIO-mtl-0.3.0.4 ... linking ... done. Loading package xhtml-3000.2.1 ... linking ... done. Loading package cgi-3001.1.8.2 ... linking ... done. Loading package fastcgi-3001.0.2.3 ... linking ... done. Loading package ranges-0.2.4 ... linking ... done. Loading package email-validate-0.2.8 ... linking ... done. Loading package digestive-functors-0.2.0.0 ... linking ... done. Loading package digestive-functors-blaze-0.2.1.0 ... linking ... done. Loading package curl-1.3.7 ... linking ... done. Loading package csv-0.1.2 ... linking ... done. Loading package MonadRandom-0.1.6 ... linking ... done. Loading package HUnit-1.2.4.3 ... linking ... done. Loading package hslogger-1.1.5 ... linking ... done. Loading package regex-posix-0.95.2 ... linking ... done. Loading package regex-compat-0.95.1 ... linking ... done. Loading package MissingH-1.1.1.0 ... linking ... done. Loading package ConfigFile-1.1.1 ... linking ... done. The TH is declared in a separate module, e.g. {-# LANGUAGE TemplateHaskell, QuasiQuotes #-} module Test where import Language.Haskell.TH alias :: String -> Name -> Q [Dec] alias name typ = do d <- newtypeD ctx conName [] con instances return [d] where instances = [''Show,''Num,''Eq,''Ord,''Enum,''Real,''Integral] ctx = return [] conName = mkName name con = recC conName [field] field = return (mkName ("un"++name), NotStrict, ConT typ) aliases :: Name -> [String] -> Q [Dec] aliases typ names = fmap concat $ mapM (flip alias typ) names if I put import Test aliases ''Int ["UserId","EventId"] in the Main module of my project and build the project, I get the above things being loaded. I'm not sure why the modules need to be dynamically linked considering they're not actually being used in the TH. And the module is clearly staged separate to the context in which it's used. I get that I can reify names to things, like “alias "Foo" ''SomePackage'sType” but those types exist at compile time, surely it can't require the actual packages? Can anyone explain? Ciao!

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Long-running request/response protocol server ...
by oleg＠okmij.org 28 Jun '12

28 Jun '12

Nicolas Trangez wrote > The protocol I'd like to implement is different: it's long-running using > repeated requests & responses on a single client connection. Basically, > a client connects and sends some data to the server (where the length of > this data is encoded in the header). Now the server reads & parses this > (binary) data, sets up some initial state for this client connection > (e.g. opening a file handle), and returns a reply. Now the client can > send another request, server parses/interprets it using the connection > state, sends a reply, and so on.'' That is very simple to implement in any Iteratee library; I will use IterateeM for concreteness. The desired functionality is already implemented, in decoding of chunk-decoded inputs. Your protocol is almost the same: read a chunk of data (tagged with its size), and do something about it. After the chunk is handled, read another chunk. The iteratee library takes care of errors. In particular, if the request handler finished (normally or with errors) without reading all of the chunk, the rest of the chunk is read nevertheless and disregarded. Otherwise, we deadlock. The complete code with a simple test is included. The test reads three requests, the middle of which causes the request handler to report an error without reading the rest of the request. module SeveralRequests where import IterateeM import Prelude hiding (head, drop, dropWhile, take, break, catch) import Data.Char (isHexDigit, digitToInt, isSpace) import Control.Exception import Control.Monad.Trans -- Tell the iteratee the stream is finished and write the result -- as the reply to the client -- If the iteratee harbors the error, write that too. reply :: MonadIO m => Iteratee el m String -> Iteratee el m () reply r = en_handle show (runI r) >>= check where check (Right x) = liftIO . putStrLn $ "REPLY: " ++ x check (Left x) = liftIO . putStrLn $ "ERROR: " ++ x -- Read several requests and get iter to handle them -- Each request is formatted as a single chunk -- The code is almost identical to IterateeM.enum_chunk_decoded -- The only difference is in the internal function -- read_chunk below. -- After a chunk is handled, the inner iteratee is terminated -- and we process the new chunk with a `fresh' iter. -- If iter can throw async errors, we have to wrap it -- accordingly to convert async errors into Iteratee errors. -- That is trivial. reply_chunk_decoded :: MonadIO m => Enumeratee Char Char m String reply_chunk_decoded iter = read_size where read_size = break (== '\r') >>= checkCRLF iter . check_size checkCRLF iter m = do n <- heads "\r\n" if n == 2 then m else frame_err (exc "Bad Chunk: no CRLF") iter check_size "0" = checkCRLF iter (return iter) check_size str@(_:_) = maybe (frame_err (exc ("Bad chunk size: " ++ str)) iter) read_chunk $ read_hex 0 str check_size _ = frame_err (exc "Error reading chunk size") iter read_chunk size = take size iter >>= \r -> checkCRLF r $ reply r >> reply_chunk_decoded iter read_hex acc "" = Just acc read_hex acc (d:rest) | isHexDigit d = read_hex (16*acc + digitToInt d) rest read_hex acc _ = Nothing exc msg = toException (ErrorCall $ "Chunk decoding exc: " ++ msg) -- If the processing is restarted, we report the frame error to the inner -- Iteratee, and exit frame_err e iter = throwRecoverableErr (exc "Frame error") (\s -> enum_err e iter >>= \i -> return (return i,s)) -- Test -- A simple request_handler iter for handling requests -- If the input starts with 'abc' it reads and returns the rest -- Otherwise, it throws an error, without reading the rest of the input. request_handler :: Monad m => Iteratee Char m String request_handler = do n <- heads "abc" if n == 3 then stream2list else throwErrStr "expected abc" test_request_handler :: IO () test_request_handler = run =<< enum_pure_1chunk input (reply_chunk_decoded request_handler >> return ()) where input = -- first request "6"++crlf++ "abcdef" ++ crlf++ -- second request "8"++crlf++ "xxxdefgh" ++ crlf++ -- third request "5"++crlf++ "abcde" ++ crlf++ "0"++crlf++ crlf crlf = "\r\n" {- *SeveralRequests> test_request_handler REPLY: def ERROR: expected abc REPLY: de -}

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Scribd's AI challenge has Haskell support
by aditya bhargava 28 Jun '12

28 Jun '12

Hey all, I work at scribd.com. A few weeks ago we had released an AI game. It allowed you to use Javascript to program a bot. I've been wanting to add Haskell as a supported language for some time, and it's finally done! I'm pretty excited since I think we are the first big company to do something like this. Supporting Haskell was trivial thanks to GHC 7.4's Safe Haskell extension. I hope more companies support Haskell in the future! Here's the game: *http://www.scribd.com/jobs/botrace* Here's the tutorial I wrote for Haskell: * http://egonschiele.github.com/robot-fruit-hunt/* Here's the Haskell API: * https://github.com/egonSchiele/robot-fruit-hunt/blob/master/haskell/Haskell… * Here's a sandbox that allows you to test bots locally: * https://github.com/egonSchiele/robot-fruit-hunt* Comments and feedback welcome! Adit -- adit.io

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Haskell Weekly News: Issue 233
by Daniel Santa Cruz 27 Jun '12

27 Jun '12

Welcome to issue 233 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 17 to 23, 2012. Quotes of the Week * ciaranm: if it's ugly, it's imperative. if it's incomprehensible, it's functional. * xplat: when life gives you lemons, make lemonads Top Reddit Stories * What are everyone's favorite "aha" moments in learning Haskell? Domain: self.haskell, Score: 93, Comments: 156 On Reddit: [1] http://goo.gl/8YSyY Original: [2] http://goo.gl/8YSyY * Unit testing isn't enough. You need static typing too. Domain: evanfarrer.blogspot.ca, Score: 82, Comments: 22 On Reddit: [3] http://goo.gl/b73gf Original: [4] http://goo.gl/JF4ND * Snap Framework 0.9 Released Domain: snapframework.com, Score: 60, Comments: 7 On Reddit: [5] http://goo.gl/7fP9D Original: [6] http://goo.gl/7SoU6 * Haskell’s record system is a cruel joke Domain: labs.scrive.com, Score: 52, Comments: 100 On Reddit: [7] http://goo.gl/XKIch Original: [8] http://goo.gl/wOvyc * Polymorphic Update with van Laarhoven Lenses Domain: r6.ca, Score: 51, Comments: 9 On Reddit: [9] http://goo.gl/qvpPg Original: [10] http://goo.gl/34TbY * Building a blog with Snap and Heist Domain: bonus500.github.com, Score: 45, Comments: 30 On Reddit: [11] http://goo.gl/UDHCs Original: [12] http://goo.gl/eo9a3 * Totality vs. Turing completeness (McBride)[PDF, 35 slides, ±] Domain: personal.cis.strath.ac.uk, Score: 42, Comments: 22 On Reddit: [13] http://goo.gl/qua0n Original: [14] http://goo.gl/8auaP * GHC's imprecise (non-deterministic) "error" Domain: stackoverflow.com, Score: 41, Comments: On Reddit: [15] http://goo.gl/6WQWx Original: [16] http://goo.gl/MOCeT * Hackage comments — a simple user script Domain: chrisdone.com, Score: 39, Comments: 17 On Reddit: [17] http://goo.gl/t9oBH Original: [18] http://goo.gl/CmXuB * A Notation for Comonads (Orchard, Mycroft) [pdf] Domain: cl.cam.ac.uk, Score: 38, Comments: 48 On Reddit: [19] http://goo.gl/S4lz9 Original: [20] http://goo.gl/uw8ZY * Taming list syntax in Haskell Domain: labs.scrive.com, Score: 36, Comments: 29 On Reddit: [21] http://goo.gl/YLTZL Original: [22] http://goo.gl/UJpcB * Shell scripting with type-safety using Haskell « Linux-Magazin Online Domain: linux-magazin.de, Score: 36, Comments: 32 On Reddit: [23] http://goo.gl/yEMQ0 Original: [24] http://goo.gl/U3zO5 * Roy, A Statically Typed, Functional Language for Javascript. The Functional Web [PDF] Domain: brianmckenna.org, Score: 32, Comments: 9 On Reddit: [25] http://goo.gl/SBhec Original: [26] http://goo.gl/v0iH2 * [ANN] GenCheck - a generalized property-based testing framework Domain: permalink.gmane.org, Score: 30, Comments: On Reddit: [27] http://goo.gl/GCUGJ Original: [28] http://goo.gl/c2Y0s Top StackOverflow Questions * How do exceptions in Haskell work? votes: 53, answers: 1 Read on SO: [29] http://goo.gl/MOCeT * Has anyone ever compiled a list of the imports needed to avoid the “not polymorphic enough” definitions in Haskell's standard libraries? votes: 19, answers: 1 Read on SO: [30] http://goo.gl/kyXde * Why does adding a polymorphic type signature degrade performance? votes: 13, answers: 2 Read on SO: [31] http://goo.gl/Lf18k * To write or not to write `module Main where` in Haskell votes: 10, answers: 1 Read on SO: [32] http://goo.gl/IDSOx * Why aren't FingerTrees used enough to have a stable implementation? votes: 10, answers: 3 Read on SO: [33] http://goo.gl/wzw4e * What is the difference between PArray and [::] in Data Parallel Haskell? votes: 9, answers: 0 Read on SO: [34] http://goo.gl/FpUJX * Why discarded values are () instead of ⊥ in Haskell? votes: 8, answers: 5 Read on SO: [35] http://goo.gl/jIK26 * Is it possible to embed Haskell in a C library opaquely? votes: 8, answers: 1 Read on SO: [36] http://goo.gl/3H5tu * NoMonomorphismRestriction helps preserve sharing? votes: 8, answers: 2 Read on SO: [37] http://goo.gl/FDdeA * Why doesn't Haskell/GHC support record name overloading votes: 7, answers: 2 Read on SO: [38] http://goo.gl/NsHKM Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/vi425/what_are_everyones_favorite_… 2. http://www.reddit.com/r/haskell/comments/vi425/what_are_everyones_favorite_… 3. http://evanfarrer.blogspot.ca/2012/06/unit-testing-isnt-enough-you-need.html 4. http://www.reddit.com/r/haskell/comments/vbwrv/unit_testing_isnt_enough_you… 5. http://snapframework.com/blog/2012/06/17/snap-0.9-released 6. http://www.reddit.com/r/haskell/comments/v779m/snap_framework_09_released/ 7. http://labs.scrive.com/2012/06/haskell-lacks-a-serious-record-system/ 8. http://www.reddit.com/r/haskell/comments/vdg55/haskells_record_system_is_a_… 9. http://r6.ca/blog/20120623T104901Z.html 10. http://www.reddit.com/r/haskell/comments/vhbf4/polymorphic_update_with_van_… 11. http://bonus500.github.com/sc2blog/ 12. http://www.reddit.com/r/haskell/comments/vg3q2/building_a_blog_with_snap_an… 13. https://personal.cis.strath.ac.uk/conor.mcbride/pub/Totality.pdf 14. http://www.reddit.com/r/haskell/comments/vazfp/totality_vs_turing_completen… 15. http://stackoverflow.com/questions/11070690/how-do-exceptions-in-haskell-wo… 16. http://www.reddit.com/r/haskell/comments/v6sec/ghcs_imprecise_nondeterminis… 17. http://chrisdone.com/hackage-comments/ 18. http://www.reddit.com/r/haskell/comments/v8gvc/hackage_comments_a_simple_us… 19. http://www.cl.cam.ac.uk/~dao29/drafts/codo-notation-orchard12.pdf 20. http://www.reddit.com/r/haskell/comments/v6ik6/a_notation_for_comonads_orch… 21. http://labs.scrive.com/2012/06/taming-list-syntax-in-haskell/ 22. http://www.reddit.com/r/haskell/comments/v9ilw/taming_list_syntax_in_haskel… 23. http://www.linux-magazin.de/Online-Artikel/Shell-scripting-with-type-safety… 24. http://www.reddit.com/r/haskell/comments/vdrt0/shell_scripting_with_typesaf… 25. http://brianmckenna.org/files/IC-Roy.pdf 26. http://www.reddit.com/r/haskell/comments/vhkgf/roy_a_statically_typed_funct… 27. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19371 28. http://www.reddit.com/r/haskell/comments/va5ao/ann_gencheck_a_generalized_p… 29. http://stackoverflow.com/questions/11070690/how-do-exceptions-in-haskell-wo… 30. http://stackoverflow.com/questions/11076062/has-anyone-ever-compiled-a-list… 31. http://stackoverflow.com/questions/11145503/why-does-adding-a-polymorphic-t… 32. http://stackoverflow.com/questions/11112371/to-write-or-not-to-write-module… 33. http://stackoverflow.com/questions/11151732/why-arent-fingertrees-used-enou… 34. http://stackoverflow.com/questions/11103707/what-is-the-difference-between-… 35. http://stackoverflow.com/questions/11076938/why-discarded-values-are-instea… 36. http://stackoverflow.com/questions/11137443/is-it-possible-to-embed-haskell… 37. http://stackoverflow.com/questions/11145904/nomonomorphismrestriction-helps… 38. http://stackoverflow.com/questions/11100322/why-doesnt-haskell-ghc-support-…

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ANNOUNCE : Leksah 0.12.1.2 (fixes some metadata issues)
by Hamish Mackenzie 27 Jun '12

27 Jun '12

This release has an important bug fix for the metadata download. When metadata was downloaded using libcurl it was not treated as binary data. If you used one of our binary installers or if you built Leksah with the -flibcurl flag then it is likely you have bad metadata files. To fix this please install Leksah 0.12.1.2, then delete your ~/.leksah-0.12/metadata folder to force Leksah to download the metadata files again. The first time you run leksah after doing this it will take a while for leksah to update the metadata (you can track its progress by looking in the metadata directory). If you choose to generate your metadata locally or you used wget to download (the default if you cabal installed leksah) then your metadata should be ok. There are some other bug fixes in this release including one for metadata generation for users with lots of .cabal files (some lazy IO was leaving file handles open). Source is in Hackage and https://github.com/leksah Binary Installers Use "ghc --version" to work out which one you need. OS X http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.3.dmg http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.4.dmg http://leksah.org/packages/leksah-0.12.1.2-ghc-7.4.1.dmg Windows http://leksah.org/packages/leksah-0.12.1.2-ghc-6.12.3.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.4.1.exe

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cabal doens't forget old dependencies
by Sjoerd Visscher 27 Jun '12

27 Jun '12

Hi all, I tried to install reactive-banana. This failed due to a dependency conflict, and then I noticed there was a newer version of reactive-banana. So I did cabal update, and tried to install again. But whatever I do, cabal keeps trying to install fclabels, but fclabels is no longer a dependency of reactive-banana-0.6.0.0! How can I let cabal forget this dependency? Thanks, -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog

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Unambiguous choice implementation
by Bartosz Milewski 27 Jun '12

27 Jun '12

I'm reading Conal Elliot's paper, Push-Pull FRP. At some point he needs an unambiguous choice operator, essentially to implement select: a future that waits for one of its future arguments to fire. His implementation of unamb creates two threads racing on a shared MVar. By his own admission, this is very inefficient. My question is, is there a better implementation? -- [:Bartosz:]

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
by John Lato 27 Jun '12

27 Jun '12

> Message: 12 > Date: Wed, 27 Jun 2012 00:19:30 +0200 > From: Tillmann Rendel <rendel(a)informatik.uni-marburg.de> > Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with > Monads" > Cc: haskell-cafe(a)haskell.org > Message-ID: <4FEA3572.5060200(a)informatik.uni-marburg.de> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Hi, > > MightyByte wrote: >> Of course every line of your program that uses a Foo will change if you switch >> to IO Foo instead. > > But we often have to also change lines that don't use Foo at all. For > example, here is the type of binary trees of integers: > > data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer) > > A function to add up all integers in a tree: > > amount:: Tree -> Integer > amount (Leaf x) = x > amount (Branch t1 t2) = amountt1 + amountt2 > > All fine so far. Now, consider the following additional requirement: "If > the command-line flag --multiply is set, the function amount computes > the product instead of the sum." > > In a language with implicit side effects, it is easy to implement this. > We just change the third line of the amount function to check whether to > call (+) or (*). In particular, we would not touch the other two lines. > > How would you implement this requirement in Haskell without changing the > line "amount (Leaf x) = x"? Why would you do that even in an imperative language? The program logic turns into a spaghetti mess, and it's much harder to test. I would write Tree like this: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) and instead of an "amount" function I would use a fold. If you like, you can use "ala" from Control.Newtype *Main Data.Foldable Data.Monoid> let t1 = Branch (Leaf 1) (Branch (Leaf 4) (Leaf 5)) :: Tree Int *Main Data.Foldable Data.Monoid> ala Sum foldMap t1 10 *Main Data.Foldable Data.Monoid> ala Product foldMap t1 20 Now the amount calculation can be something like amount :: Num a => Tree a -> IO a amount tree = multFlag >>= \b -> if b then ala Product foldMap tree else ala Sum foldMap tree although I probably wouldn't actually write it unless it was called in more than one place. There are other ways to write it too; the important part is that checking the configuration is completely separate from the tree traversal. Plus, if it changes again (now there's another flag that says to ignore values == n or something, you can use the same fold, just change the function that's passed to it (or the monoid instance if you're using that). Plus, this type of code is much simpler to debug, test, and maintain than imperative-style magic functions. Cheers, John

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