January 2011 - Haskell-Cafe

combined parsing & pretty-printing
by Ozgur Akgun 05 Feb '11

05 Feb '11

Dear Café, I working on a DSL represented by a algebraic data type with many constructors. I can write (separately) a parser and a pretty-printer for it, and I am doing so at the moment. However, this way it feels like repeating the same information twice. Is there any work to combine the two? Best, Ozgur

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Yesod and concurrency
by Ertugrul Soeylemez 03 Feb '11

03 Feb '11

Hello everybody, how well do WAI, Yesod and the 'persistent' package play with concurrency? For example, I'd like to write a program, which concurrently provides two related sites as well as a few background workers, which do something with the database. My idea would look something like this: main :: IO () main = withMyAppPool $ \pool -> do forkIO $ worker1 ... -- background worker forkIO $ worker2 ... -- background worker forkIO $ worker3 ... -- background worker forkIO $ toWaiApp ... -- site 1 forkIO $ toWaiApp ... -- site 2 Will I run into problems with this? Thanks. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Loading bitmap with xlib
by Francesco Mazzoli 02 Feb '11

02 Feb '11

I am trying to load a bitmap and display it with the Xlib bindings, but I wasn't able to find a way. The C Xlib functions would be XReadBitmapFile() of XReadBitmapFileData() to read the data, but it seems that those functions haven't been included in the haskell bindings. From http://hackage.haskell.org/packages/archive/X11/latest/doc/html/src/Graphic…: -- XCreateBitmapFromData omitted (awkward looking type) -- XReadBitmapFileData omitted (awkward looking type) Does anybody know how to do that?

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Gedit Haskell mode plugin indentation
by yukkuri yu!yu!yu! 02 Feb '11

02 Feb '11

Here is my first try to write a Haskell mode layout indentation plugin for gedit - GNOME Text Editor: ftp://neverb.net/soft/mine/gedit_haskell_mode/haskell_indentation-latest.ta… Curent version: v0.4 To install the plugin 1) Create directory ~/.gnome2/gedit/plugins 2) Copy tarball/plugin/* contents to ~/.gnome2/gedit/plugins/ 3) In gedit go to Edit -> Preferences -> Plugins 4) Check the box left from "Haskell Indentation" plugin 5) (Possibly needed) Restart gedit.

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plugins and internal error: stg_ap_v_ret
by Michael Snoyman 01 Feb '11

01 Feb '11

Hi all, I'm trying to convert wai-handler-devel to use plugins instead of hint, but cannot even get some basic usages to work properly. I've put together a minimal example that loads a WAI application from a separate file and runs it, but this immediately causes the program to crash saying: loader: internal error: stg_ap_v_ret (GHC version 6.12.3 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Is this an actual bug in GHC, or am I misusing the plugins package? The two source files: MyModule.hs {-# LANGUAGE OverloadedStrings #-} module MyModule where import Network.Wai import Data.ByteString.Lazy.Char8 () myapp _ = responseLBS status200 [("Content-Type", "text/plain")] "myapp" loader.hs import System.Plugins.Make import System.Plugins.Load import Network.Wai.Handler.Warp (run) main :: IO () main = do MakeSuccess _ obj <- makeAll "MyModule.hs" [] LoadSuccess _ app <- load_ obj [] "myapp" run 3000 app Thanks, Michael

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Automatically move cursor focus in split window in haskell-mode in emacs on load (C-c C-l) command?
by JETkoten 01 Feb '11

01 Feb '11

With the community server down it looks like I can't join the haskell-mode mailing list, and also I thought someone might have already done this, so I've posted here. I'm new to Haskell and emacs... and don't know any elisp at all! I'm trying to set up my haskell-mode so that when I've finished inputting code in a buffer that's in haskell-mode, and I then do C-c C-l to split the screen and load the code into ghci in the lower buffer, the cursor focus will automatically jump down to the bottom half of the split screen and I can instantly begin to interact with ghci. The way it defaults to now is that the cursor stays in the topmost editing half of the split screen, and I inevitably begin typing and mess up my code and have to do some backspacing and then mouse over to the bottom half. :) I checked on #haskell and #emacs last night, and someone suggested either "doing it with elisp" or keyboard macros, but I'm not sure how to do the first, and I think the second is not what I'm looking for. I'd like for haskell-mode to otherwise remain the same, and to still use the same C-c C-l key sequence, but I'd just like it to always be set up to add the step in the load sequence of jumping the cursor focus down to the lower half of the split for me. Any ideas? (Thanks in advance.)

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Packages all screwed up
by Arnaud Bailly 01 Feb '11

01 Feb '11

Hello, I recently tried to upgrade some package (eg. yesod) and it seems that, in the process, I screwed up my Haskell packages setup. When I am trying to do a simple: > ghc --make Crete1941 It fails with message: Loader\Communication.hs:14:7: Could not find module `System.Process': Use -v to see a list of the files searched for. which is quite annoying ! Is there a way to reconstruct a sane baseline ? Thanks in advance Arnaud PS: In case it might be helpful, here is the output of ghc-pkg check -v There are problems in package pandoc-1.6: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package zip-archive-0.1.1.6: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package url-2.1.2: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package filestore-0.3.4.3: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package happstack-server-0.5.0.2: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package happstack-data-0.5.0.2: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist There are problems in package feed-0.3.7: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package SHA-1.4.1.3: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist There are problems in package hashed-storage-0.5.3: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist There are problems in package yesod-0.6.0.2: dependency "cereal-0.3.0.0-835aef1b61feb29c879c2cdae5c16608" doesn't exist dependency "clientsession-0.4.0.3-83420d6960f048ed87e049bcfa4d3d35" doesn't exist dependency "neither-0.1.0-50e3e802c117062181b91d4cbd45270b" doesn't exist dependency "wai-0.2.2-11adeb24d79e447264a1c6aa73a257e8" doesn't exist There are problems in package xss-sanitize-0.2.2: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package web-routes-0.23.0: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package wai-extra-0.2.4: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist dependency "wai-0.2.2-11adeb24d79e447264a1c6aa73a257e8" doesn't exist dependency "zlib-bindings-0.0.0-c2a6f489bd57d072e884afc1d6466b99" doesn't exist There are problems in package snap-server-0.2.11: dependency "cereal-0.3.0.0-835aef1b61feb29c879c2cdae5c16608" doesn't exist dependency "monads-fd-0.1.0.2-8e71857db7c17ae96ca5bff72f20d9fa" doesn't exist There are problems in package snap-core-0.2.11: dependency "cereal-0.3.0.0-835aef1b61feb29c879c2cdae5c16608" doesn't exist dependency "monads-fd-0.1.0.2-8e71857db7c17ae96ca5bff72f20d9fa" doesn't exist There are problems in package pureMD5-2.1.0.2: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist dependency "cereal-0.3.0.0-835aef1b61feb29c879c2cdae5c16608" doesn't exist There are problems in package persistent-0.3.0.1: dependency "neither-0.1.0-50e3e802c117062181b91d4cbd45270b" doesn't exist There are problems in package hsx-0.7.0: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package HStringTemplate-0.6.5: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package HSlim-0.1: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package hint-0.3.3.0: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package haskeline-0.6.3.1: dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package happstack-state-0.5.0.2: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist dependency "happstack-data-0.5.0.2-80c87686add000961445ac7ad8f41385" doesn't exist dependency "happstack-util-0.5.0.2-781b128492ee5ce69eb3c01f6e6a02bb" doesn't exist There are problems in package happstack-ixset-0.5.0.2: dependency "happstack-data-0.5.0.2-80c87686add000961445ac7ad8f41385" doesn't exist dependency "happstack-util-0.5.0.2-781b128492ee5ce69eb3c01f6e6a02bb" doesn't exist There are problems in package happstack-0.5.0.2: dependency "happstack-data-0.5.0.2-80c87686add000961445ac7ad8f41385" doesn't exist dependency "happstack-server-0.5.0.2-9d48ec66664b8dd67d495020314c5d2b" doesn't exist dependency "happstack-util-0.5.0.2-781b128492ee5ce69eb3c01f6e6a02bb" doesn't exist dependency "utf8-string-0.3.6-02c7018b15768d3d72b99557d0f847ff" doesn't exist There are problems in package crypto-api-0.1.2.4: dependency "cereal-0.3.0.0-835aef1b61feb29c879c2cdae5c16608" doesn't exist There are problems in package bytestring-show-0.3.3: dependency "binary-0.5.0.2-0da250ded3b79a6e76d15de8e7ffc885" doesn't exist There are problems in package attoparsec-iteratee-0.1.1: dependency "monads-fd-0.1.0.2-8e71857db7c17ae96ca5bff72f20d9fa" doesn't exist There are problems in package regex-compat-0.93.1: dependency "regex-posix-0.94.2-0a3c1e41d8c2d26547372e229b3f776c" doesn't exist There are problems in package haskell-platform-2010.2.0.0: dependency "regex-posix-0.94.2-0a3c1e41d8c2d26547372e229b3f776c" doesn't exist The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. pandoc-1.6 zip-archive-0.1.1.6 url-2.1.2 filestore-0.3.4.3 happstack-server-0.5.0.2 happstack-data-0.5.0.2 feed-0.3.7 SHA-1.4.1.3 hashed-storage-0.5.3 yesod-0.6.0.2 xss-sanitize-0.2.2 web-routes-0.23.0 wai-extra-0.2.4 snap-server-0.2.11 snap-core-0.2.11 pureMD5-2.1.0.2 persistent-0.3.0.1 hsx-0.7.0 HStringTemplate-0.6.5 HSlim-0.1 hint-0.3.3.0 haskeline-0.6.3.1 happstack-state-0.5.0.2 happstack-ixset-0.5.0.2 happstack-0.5.0.2 crypto-api-0.1.2.4 bytestring-show-0.3.3 attoparsec-iteratee-0.1.1 regex-compat-0.93.1 haskell-platform-2010.2.0.0 ConfigFile-1.0.6 MissingH-1.1.0.3 hsp-0.5.2 HJScript-0.5.0

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Splittable random numbers
by Simon Peyton-Jones 01 Feb '11

01 Feb '11

Hi Cafe A while back there was a thread<http://www.mail-archive.com/haskell-cafe@haskell.org/msg79633.html> about a good implementation of a (pseudo) random number generator with a good "split" operation. There's lots of material on generators that generate a linear sequence of random numbers, but much less on how to generate a tree of random numbers, which is what Haskell's System.Random API requires. I happened to meet Burton Smith recently, who wrote some early papers about this stuff (eg "Pseudo random trees in Monte-Carlo<http://portal.acm.org/citation.cfm?id=1746034>"), so I asked him. His reply is below, along with some follow-up comments from his colleagues Tolga Acar and Gideon Yuval. The generator uses crypto functions, so it's probably more computationally expensive than common linear-sequence generators, but in exchange you get robust splitting. Does anyone feel like taking the idea and turning it into a Haskell library? (Or even a Haskell Wiki page?) I'm taking the liberty of cross-posting to the libraries list. Simon From: Burton Smith Sent: Tuesday, November 02, 2010 3:58 PM To: Simon Peyton-Jones Cc: Gideon Yuval (Gideon Yuval); Tolga Acar Subject: Random number generation With some help from Gideon and Tolga, I think the solution to the "arbitrary tree of random numbers problem" is as follows: The generator G is a pair comprising a crypto key G.k and an integer counter (the "message") G.c. The (next G) operation returns a pair: 1. a random integer r obtained by encrypting G.c with G.k, and 2. a new generator G' such that G'.k = G.k and G'.c = G.c + 1. The (split G) operation is similar, returning the same G', except that instead of returning a random integer r it returns a third generator G'' such that G''.k = r and G''.c = 0. A suitable block cipher system might be 128-bit AES (Rijndael). Unencumbered implementations exist in a variety of languages, and performance is pretty good and will improve dramatically as hardware support improves. I'd pick both crypto key size and the size of the result r to be 128 bits, and employ a 64 bit counter c. Other crypto options exist. From: Simon Peyton-Jones Sent: Wednesday, November 03, 2010 3:11 AM To: Burton Smith; Gideon Yuval (Gideon Yuval) Cc: Tolga Acar; Simon Peyton-Jones Subject: RE: Random number generation Burton, Gideon, Tolga Aha, that's interesting. I'd never seen a random number generator based on crypto, but it seems like an attractive idea. As I understand it, successive calls to 'next' will give you encrypt(0), encrypt(1), encrypt(2), encrypt(3),.... Is this standard? Does it have provably good randomness properties, (cycle length, what else?) like other RNGs? Or does it simply seem very plausible? Can I send it round to the Haskell mailing list, in the hope that someone will turn the idea into a library? (Ideally I'd like to make claims about the randomness properties in doing so, hence my qns above.) From: Gideon Yuval (Gideon Yuval) Sent: Wednesday, November 03, 2010 7:15 AM To: Simon Peyton-Jones; Burton Smith Cc: Tolga Acar Subject: RE: Random number generation As long as the key, and the non-counting part of the counter, are kept" secret", anyone who can distinguish these pseudorandoms from real random, in less than 2^128 steps, has a nice paper for crypto-2011 (this is known as "provable security") concerning a weakness in AES128. One exception: real randoms have a birthday paradox; the pseudorandoms suggested do not. If you care, you can: (1) Limit the counter to 2^32 steps (paradox has 2^-64 probability) or even 2^16 (2^-96), then rekey; or (2) XOR 2 such encrypted counters, with different keys; or (3) XOR 3 successive values for the same counter (just possibly cheaper; top-of-head idea). More hard-core: swap the position of key & message: encrypting a constant "secret" with 1,2,3,4.... Gives pseudorandoms with no birthday paradox. From: Tolga Acar Sent: 03 November 2010 15:50 To: Gideon Yuval (Gideon Yuval); Simon Peyton-Jones; Burton Smith Subject: RE: Random number generation Simon, The general idea is not really that new in the crypto area with constraints Gideon describes, of course. That is typically called a PRNG - Pseudo Random Number Generator, or in another parlance, Deterministic Random Bit Generators (DRBG). The DRBG constructions based on hash functions and block ciphers are even standardized in NIST publication SP800-90 (even though I may not recommend every one of them). As for the construction below, that is based on the AES block cipher, that essentially takes advantage of the PRP (Pseudo Random Permutation) property of the AES block cipher, as each block cipher ought to be. So, as Gideon outlines below, if you fix the key, the PRP gives you a random-looking (or, in other terms, indistinguishable from random) output that no one without the secret key and the "state" can generate or easily predict. Assuming an ideal cipher (and AES is a close approximation to it), the probability is believed to be the birthday paradox - no counterexample or a proof exists without assumptions; so we stick to the birthday bound. There ought to be papers on this somewhere. If not, that condensed information is spread across many papers and is part of the crypto folklore, I'd say. From: Burton Smith Sent: 03 November 2010 19:03 To: Simon Peyton-Jones Cc: Gideon Yuval (Gideon Yuval); Tolga Acar Subject: RE: Random number generation Just two points of further clarification: 1. All PRNGs used in the technical computing space fail the birthday paradox criterion (i.e. have full period), so we really need not worry about this. Also, there are other mitigating factors, e.g. suppose we are using the PRNG to generate pseudorandom residues mod n << 2^128; the paradox is happily present there. 2. The big innovation in this scheme is that the rekeying operation done by split creates a new generator with independence guaranteed by "provable security" in the sense Gideon mentioned - if someone can find something nonrandom in the correlation between G' and G'', say, then it amounts to a weakness in AES128 and is publishable. So it's yet another example of reducibility, common in our field: "if you can easily transform a known/famous hard problem P into this other problem Q, Q must be hard".

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Re: [Haskell-cafe] "Cabal ignores ld-options?" - 2011 remix
by Duncan Coutts 01 Feb '11

01 Feb '11

On Sun, 2011-01-30 at 05:20 -0500, Ryan Newton wrote: > > I am trying to link a ".a" file built by a separate makefile into my > library. GHC has no problem with it, but I need to convince cabal to > do it for the package to be hackage-friendly. There's a thread about > this back in 2007: > > http://www.haskell.org/pipermail/cabal-devel/2007-April/000507.html > > Is there a method for this now, or is the state of affairs the same as > before? Having a Haskell package link against an external C library is straightforward, you just list it in the extra-libraries field. The external library must already be installed on the system however. Bundling a copy of the external library with a cabal package is not quite so easy. You will have to write code in the Setup.hs to call the separate makefile during the build (and perhaps extra steps at configure time too). Your best bet is probably to use the "simple" build system's hooks API. You can list the library in the extra-libraries field in the .cabal file but you will also need to modify the check for external libraries that are done in the postConf hook (since the separate .a library will not be available at configure time): modify the package description that gets passed to the postConf hook to exclude the library. Duncan

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Inheritance and Wrappers
by MattMan 31 Jan '11

31 Jan '11

tldr: Can I make arbitrary instances of one class instantiate another without using wrappers? I'm new to Haskell, and am trying to implement some simple typeclasses for doing algebra. For example I have type class (simplified for the sake of argument) class AbGroup a where add :: a -> a -> a I would like any type instantiating Num to also be an abelian group: instance (Num a) => AbGroup a where add i j = i+j, but this doesn't compile because a is a variable, not a type constructor(or something). The problem is fixed, in a sense, If I introduce a wrapper class data Wrapper a = Wrap a instance (Num a) => AbGroup (Wrapper a) where add (Wrap i) (Wrap j) = Wrap(i+j) However, this is clumsy. Is there something else I can do? Thanks! -- View this message in context: http://haskell.1045720.n5.nabble.com/Inheritance-and-Wrappers-tp3365126p336… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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