- Haskell-Cafe - Haskell.org

Automatically move cursor focus in split window in haskell-mode in emacs on load (C-c C-l) command?
by JETkoten 02 Feb '11

02 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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Automatically move cursor focus in split window in haskell-mode in emacs on load (C-c C-l) command?
by Steven Collins 02 Feb '11

02 Feb '11

I'm using haskell-mode-2.8.0 and emacs 22.2.1 on Ubuntu 9.10 The following elisp code works for me to change the focus to the GHCi buffer (window) after the C-c C-l command in Haskell Mode. Try this in your .emacs file. (defadvice inferior-haskell-load-file (after change-focus-after-load) "Change focus to GHCi window after C-c C-l command" (other-window 1)) (ad-activate 'inferior-haskell-load-file) I have this after the haskell mode configuration section of my .emacs file. Remeber to go to the Emacs-lisp menu and 'Byte-compile and Load' after adding the above code to your .emacs file. To understand what the elisp code is doing you can go to the help menu in emacs, then drill down via 'Search Documentation ' , then 'Find any object by name'. Which will put you in the mini-buffer. Type in either defadvice or ad-activate to see the documentation for these functions. If you are a little rusty on elisp this documentation may be a little hard to understand. Basically this is what's happening. The function that is bound to the C-c C-l command is inferior-haskell-load-file. Which is an elisp function name and part of the elisp haskell mode code. The defadvice is kind of overloading this function. We are telling emacs that 'after' executing the 'inferior-haskell-load-file' function, execute our new function who's name is 'change-focus-after-load'. The body of our function is just one expression (other-window 1). This is the kind of stuff that makes it easy to modify (hack) the functionality of emacs to do what you want. I hope this helps and enjoy hacking in Haskell Steven

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Timeout exceptions sometimes don't work, even in pure (non FFI) code
by Job Vranish 01 Feb '11

01 Feb '11

I'm trying to test some properties with quickcheck. If these tests fail, they will almost certainly fail by nontermination. I've been using the 'within' function to catch these nontermination cases. However, I was surprised to find that this doesn't always work. 'within' uses the 'timeout' function under the hood. Here is an example that demonstrates the problem: import System.Timeout import Control.Exception works :: Int -> Int works x = sum $ cycle [x] doesntWork :: Int -> Int doesntWork x = last $ cycle [x] test1 = timeout 1 $ evaluate $ works 5 == 5 -- terminates test2 = timeout 1 $ evaluate $ doesntWork 5 == 5 -- never terminates test1 returns Nothing as expected, but test2 never terminates. Why? I thought timeout exceptions are supposed to always work with pure (non FFI) Haskell code. Is there any way I can work around this? I'm using ghc 6.12.2 Thanks, - Job

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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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Re: [Haskell-cafe] Haskell function help
by Daniel Fischer 01 Feb '11

01 Feb '11

Just a few further remarks, - partition requires the import of Data.List - with an import of Data.Maybe, assign can be much shorter stated as assign (atom,value) formula = catMaybes (map (assignClause (atom,value)) formula)

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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 01 Feb '11

01 Feb '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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companies using Haskell or other FP languages near Rochester?
by Edward Amsden 01 Feb '11

01 Feb '11

I'm an undergraduate CS student (4th year status) looking for an co-op or internship in the Rochester area. I'd really like to do something with functional programming or Haskell specifically, but I'm not sure if anyone in this area does that. Staying near Rochester is important because I'd also like to take one or two classes at my school (Rochester Institute of Technology) over the summer. Any ideas? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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Haskell WalkSat algorithm implementation
by Manolache Andrei-Ionut 31 Jan '11

31 Jan '11

I need some help if possible with the following problem.....The WalkSat algorithm takes a formula, a probability 0 =< p =< 1, and a boundary of maximum flips maxflips and returns a model that satisfies the formula or failure. The algorithm begins by assigning truth values to the atoms randomly, ie. generating a random model. Then it proceeds to flip the value of an atom from one of the unsatisfied clauses until it is able to find a model that satisfies the formula or reaches the maximum number of flips. In order to select which atom from the currently selected clause to flip, it follows two strategies: 1. Either flip any atom randomly. 2. Or flip the atom that maximizes the number of satisfied clauses in the formula. The choice to flip randomly is followed with probability p. 1.atomsClause :: Clause ! [Atom] This function must take a Clause and return the set of Atoms of that Clause. Note that the set should not contain any duplicates. 2. atoms :: Formula![Atom] This function must take a Formula return the set of Atoms of a Formula. Note that the set should not contain any duplicates. 3. isLiteral :: Literal ! Clause ! Bool This function returns True if the given Literal can be found within the Clause. 4. flipSymbol :: Model ! Atom ! Model This function must take a Model and an Atom and flip the truth value of the atom in the model. Now I've done the first 3 I need some help with the last one, Here is the code: module Algorithm where import System.Random import Data.Maybe import Data.List type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) atomsClause :: Clause -> [Atom] atomsClause = nub . map snd atoms :: Formula -> [Atom] atoms =atomsClause . concat isLiteral :: Literal -> Clause -> Bool isLiteral (b,a) cs = or[x==b && y==a|(x,y)<-cs] flipSymbol :: Model -> Atom -> Model flipSymbol = undefined ---the last one Thank you.

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Typing problem
by michael rice 31 Jan '11

31 Jan '11

I'm mapping a function over a list of data, where the mapping function is determined from the data. g f l = map (g l) l So g serialize "prolog" -> [4,5,3,2,3,1] But I'm having typing problems trying to do a similar thing with a function that statistically normalizes data. See: http://people.revoledu.com/kardi/tutorial/Similarity/Normalization.html#Sta… So g normalize [2,5,3,2] -> [-0.7071067811865475,1.414213562373095,0.0,-0.7071067811865475] Is my typing for normalize too loose. Should I be using Floating rather than Num? Michael =======Code============== {- See Problem 42, pg. 63, Prolog by Example, Coelho & Cotta Generate a list of serial numbers for the items of a given list, the members of which are to be numbered in alphabetical order. *Main> serialize "prolog" [4,5,3,2,3,1] *Main> serialize "int.artificial" [5,7,9,1,2,8,9,5,4,5,3,5,2,6] *Main> ["prolog"] >>= serialize [4,5,3,2,3,1] *Main> ["int.artificial"] >>= serialize [5,7,9,1,2,8,9,5,4,5,3,5,2,6] -} import Data.Map hiding (map) import Data.List {- serialize :: [Char] -> [Int] serialize l = map (f l) l where f = ((!) . fromList . ((flip zip) [1..]) . (sort . nub)) -} serialize :: (Ord a, Integral b) => [a] -> a -> b serialize = ((!) . fromList . ((flip zip) [1..]) . (sort . nub)) g f l = map (f l) l normalize :: (Num a, Num b) => [a] -> a -> b normalize l = let (total,len) = sumlen l avg = total/len stdev = sqrt $ ((/) (len-1)) $ sum $ map ((** 2.0) . (subtract avg)) l in ((/) stdev) . (subtract avg) sumlen :: (Num a, Integral b) => [a] -> (a,b) sumlen l = sumlen' l 0 0 where sumlen' [] sum len = (sum,len) sumlen' (h:t) sum len = sumlen' t (sum+h) (len+1) ========================= Prelude> :r [1 of 1] Compiling Main ( serialize2.hs, interpreted ) serialize2.hs:34:32: Could not deduce (Integral a) from the context (Num a, Num b) arising from a use of `sumlen' at serialize2.hs:34:32-39 Possible fix: add (Integral a) to the context of the type signature for `normalize' In the expression: sumlen l In a pattern binding: (total, len) = sumlen l In the expression: let (total, len) = sumlen l avg = total / len stdev = sqrt $ ((/) (len - 1)) $ sum $ map ((** 2.0) . (subtract avg)) l in (/ stdev) . (subtract avg) serialize2.hs:36:61: Could not deduce (Floating a) from the context (Num a, Num b) arising from a use of `**' at serialize2.hs:36:61-66 Possible fix: add (Floating a) to the context of the type signature for `normalize' In the first argument of `(.)', namely `(** 2.0)' In the first argument of `map', namely `((** 2.0) . (subtract avg))' In the second argument of `($)', namely `map ((** 2.0) . (subtract avg)) l' serialize2.hs:37:18: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the type signature for `normalize' at serialize2.hs:33:25 `a' is a rigid type variable bound by the type signature for `normalize' at serialize2.hs:33:18 In the expression: (/ stdev) . (subtract avg) In the expression: let (total, len) = sumlen l avg = total / len stdev = sqrt $ ((/) (len - 1)) $ sum $ map ((** 2.0) . (subtract avg)) l in (/ stdev) . (subtract avg) In the definition of `normalize': normalize l = let (total, len) = sumlen l avg = total / len .... in (/ stdev) . (subtract avg) Failed, modules loaded: none.

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A few days to go before the old server goes down
by Henk-Jan van Tuyl 31 Jan '11

31 Jan '11

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The implementation of Control.Exception.bracket
by Leon Smith 31 Jan '11

31 Jan '11

There is a common idiom used in Control.Concurrent libraries, as embodied in the implementation of bracket: http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/src/… bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r Is there any particular reason why bracket is not implemented as: bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `finally` after a return r Is there some subtle semantic difference? Is there a performance difference? It seems like a trivial thing, but I am genuinely curious. Best, Leon

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Re: [Haskell-cafe] ANNOUNCE: genprog-0.1
by Jan Snajder 31 Jan '11

31 Jan '11

(Moved from haskell(a)haskell.org) Hi Kenneth, I'm sorry for the very late reply. On Wed, 2010-12-08 at 10:01 +0100, Kenneth Hoste wrote: > Hi Jan, > > On 11/29/2010 12:55 PM, Jan Snajder wrote: > > Dear Haskellers, > > > > I am pleased to announce the release of genprog-0.1, a genetic > > programming library. > > > > (snip) > > Very interesting, and nice job on the code (elegant, well-structured, > well-documented, ...)! Thank you. This was my first shot at building a Haskell library and I'm flattered to receive such positive comments :-) > Genetic programming recently got my attention because one of the bots in > the Google AI contest was built using this technique (see > http://ai-contest.com/forums/viewtopic.php?f=17&t=1136) and it > performed really well (way better than my hand-crafted bot). Interesting, thanks for the link. > I have a bit of experience with genetic algorithms, on a practical and > pragmatic level. The field of genetic programming is something I hope to > look at and play with in the upcoming months (just for fun). > > I was kind of planning to implement my own genetic programming library > in Haskell as I became familiar with the field, but after diving into > your code it quickly became clear to me you've done a way better job > than I would have. > > I really like the example you provided for evolving an expression that > computes a specified integer value. I kept this example deliberatly simple so to make it clear how you can use the library. The example is actually a degenerated case of symbolic regression, in which you are given only a single instance to train at, rather than a set of instances. You can find much more interesting examples in Koza's book. > I plan to start playing with that > and improve the example (faster convergence to a perfect solution, and > also tweaking the current GP config to obtain smaller solutions). > > > A couple of questions (some fairly unrelated to Haskell): > > How hard would it be to extend the current version to support the > evolution of actual Haskell programs? As far as I can tell, the current > version has support for (simple?) self-defined expressions, but this > seems like a long way from supporting Haskell programs with > multi-argument functions that operate on lists, etc., even if you just > limit it to non-monadic Haskell98 code. Actually, I have no idea. :-) I don't know if there is a data structure that can represent an AST of a Haskell program. I guess there is, because I guess at some point a compiler like GHC has to maintain such a structure internally, but I never looked into this. At present, I am more than satisfied to be able to evolve custom-made data structures. Another issue with evolving arbitrary Haskell (or whatever) programs is the "closure property". When doing crossover, you have to ensure that what you get by exchanging two subtrees still is a valid program. In other words, if F is a set of (arbitrary arity) functional nodes and T is a set of terminal nodes, then, given a functional node F(x1,...,xi,...xn), you should be able to substitute for argument xi each element of F `union` T. Obviouslly, this will not work for programms in general and you would have to either introduce specialized crossover operations or treat the nodes differently depending on their types. In genprog, I have kept things simple and closure propery is ensured by allowing crossover to take place only at nodes that are of the same type as the root node. This is certainly a limitation, but still I think one can do interesting stuff with it. > > Have you considered playing with dynamic values for the various > parameters that steer the evolution, i.e. population size, > crossover/mutation probabilities, etc.? > One thing I've always wanted to try (but never really got to it) is e.g. > increasing the mutation probability as the evolution seems to be getting > stuck in a (local?) optimum. Also, shrinking the population size if > enough progress is being made could be interesting, to speed up the > evolutionary process. Are you aware of studies that do such a thing? Yes, definitely! Variable mutation probability is often used in genetic algorithms to improve the results. The idea is to have higher mutation probability in the beginning, so to encourage exploration, and then decrease the mutation probability as the evolution progresses, so to encourage exploitation. This increases the chances of not getting stuck in a local optima. But I didn't include this into genprog because I don't need variable mutation right now. I will probably do it in the next release. The most straightforward to do it, I guess, would be make the mutation prob. (and pop. size) a function of the evolution state. There are also other things that I plan to include in the next release. I hope I will find time to do it. Btw., mutation is actually rarely used in GP (unlike in GA). Koza, for example, used mutation in one example only. The thing is that, if you think of mutation as a replacement of a subexpression with a randomly generated subexpression, then you already can do this with crossover. Now, this only true for this default type of mutation. If you have a custom-defined mutation operator, one that changes a node depending on its value and/or type, than this cannot be accomplished by crossover anymore. Whether or not you need custom-defined mutations depends on the task at hand. E.g., if the expressions you're evolving feature values from a very large domain set, than a crossover operation alone will not get you enough genetic diversity. For this reason I've defined in genprog the mutation operation as a function of an expression that is being mutated. > Are you planning to actively maintain and extend this library? Are you > using it for research purposes, or did you hack this together just for > fun (if so, nice job!). What do you plan to use it for? Yes, I will maintain the library actively. I did this for fun, of course :-), and also for research, which I also do for fun :-) I am using the library for solving specific problems related to natural language processing. > Anyway, great job, I hope I'll find the time to play around with this. So do I. Please let me know if you have any suggestions on how to improve the library. If you do interesting stuff with it, please let us know what it is. Cheers, Jan

2 1

Instantiation problem
by Patrick Browne 31 Jan '11

31 Jan '11

Below is some code that is produces information about the *types* used for measuring (e.g. metres). The following evaluation returns 1.00 which the convert factor for metres. convertFactorToBaseUnit (unit (LengthInMetres 7)) . The next evaluation returns the type, Metre, of data being measured unit (LengthInMetres 7) Using the particular definitions below is it possible to make an instance of MetricDescription for centimetres? I have an error on the defintion of the unit function in the definition of the MetricDescription instance. As far as possible I would like to retain the data types and class structures. Thanks, Pat class (Unit unit) => MetricDescription description unit | description -> unit where unit :: description -> unit valueInUnit :: description -> Double valueInBaseUnit :: description -> Double valueInBaseUnit d = (convertFactorToBaseUnit(unit d)) * (valueInUnit d) data Metre = Metre deriving Show data Centimetre = Centimetre deriving Show -- Each member of the Unit class has one operator convertFactorToBaseUnit -- that takes a measurement unit (say metre) and returns a conversion factor for that unit of measurement class Unit unit where convertFactorToBaseUnit :: unit -> Double -- An instance for metres, where the convert factor is 1.0 instance Unit Metre where convertFactorToBaseUnit Metre = 1.0 -- An instance for metres, where the convert factor is 0.1 instance Unit Centimetre where convertFactorToBaseUnit Centimetre = 0.1 data LengthInMetres = LengthInMetres Double deriving Show data LengthInCentimetres = LengthInCentimetres Double deriving Show -- This seems fine instance MetricDescription LengthInMetres Metre where valueInUnit (LengthInMetres d) = d unit l = Metre -- This is the instance that I cannot get to work -- The unit 2 function seems to be the problem. -- instance MetricDescription LengthInCentimetres Centimetre where -- valueInUnit (LengthInCentimetres d) = d -- unit 2 = Centimetre This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Re: [Haskell-cafe] Code from Haskell School of Expression hanging.
by michael rice 31 Jan '11

31 Jan '11

The sample program (the "active" version) here runs fine, so the OpenGL GLFW must have been installed properly. Must be the SOE stuff causing the problem. http://www.haskell.org/haskellwiki/GLFW Michael --- On Sat, 1/29/11, Daniel Fischer <daniel.is.fischer(a)googlemail.com> wrote: From: Daniel Fischer <daniel.is.fischer(a)googlemail.com> Subject: Re: [Haskell-cafe] Code from Haskell School of Expression hanging. To: "michael rice" <nowgate(a)yahoo.com> Date: Saturday, January 29, 2011, 9:37 PM On Sunday 30 January 2011 03:27:55, michael rice wrote: > I'm using the OpenGL stuff (GLFW). Same set of problems? No idea, I haven't used a graphics library for a long time.

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: jhc 0.7.7 is out.
by Roman Cheplyaka 31 Jan '11

31 Jan '11

Greetings! A few questions about the inclusion of parsec: 1. It is parsec-2, not parsec-3, right? 2. Does this change consist of merely inclusion parsec as a standard library, or are there any compiler improvements in this release that made it possible to build parsec? * John Meacham <john(a)repetae.net> [2011-01-29 20:47:20-0800] > Announcing jhc 0.7.7! This release fixes a large number of bugs that cropped up > when compiling haskell out in the wild as well as adds some more features. A > major one being the garbage collector is enabled by default. > > http://repetae.net/computer/jhc/ > > Changes: (including some changes from the unannounced 0.7.6 release) > > * The Garbage Collector is now enabled by default. > * new standard libraries > * transformers > * parsec > * QuickCheck > * report proper errors with line numbers for various issues with compiled code. > * New option '-C' that compiles to C code and stops, useful for targeting other > platforms or building shared libraries. > * Nintendo Wii added as target (thanks to Korcan Hussein) > * Fix major performance bug that kept WRAPPERs from being inlined in > certain places. > * Typechecking speed greatly increased. > * monomorphism-restriction flag is now respected > * empty class contexts now work > * unicode in haskell source supported now > * Type Defaulting now works properly > * RULES parse like ghc now for compatibility > * 'do' 'where' on same indent now parses > * Build system fixes and cleanups > * irrefutable lambda pattern bindings desugar properly now. > * GHC parsing regression tests have been ported to jhc, helped find > and fix many bugs. > * Certain optimizations would discard RULES, these have been fixed. > * Removed quadratic behavior in optimizer, speeds things up noticibly. > * Garbage collector improvements, caches are pre-initialized. > * Fix shiftL/R implementations for built in types. > * All Num, Real, and Integral magic removed from compiler. This is a very > good thing. > * improved help messages > > John > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell -- Roman I. Cheplyaka :: http://ro-che.info/ Don't worry what people think, they don't do it very often.

2 1

Code from Haskell School of Expression hanging.
by jean legrand 30 Jan '11

30 Jan '11

Maybe you could try The HGL package http://hackage.haskell.org/package/HGL and then replace import SOE by import Graphics.SOE.

1 0

Clear screen in ghci in haskell-mode in emacs?
by JETkoten 30 Jan '11

30 Jan '11

I'm looking for a way to clear the screen in ghci within emacs. I saw a feature request from a couple of years back for :clear, but it was deleted saying something along the lines of "Ctrl-L works fine for me". This is true in my system terminal, however, in emacs, Ctrl-L is already bound to something else. Is there a way to clear the ghci screen that doesn't involve Ctrl-L?

3 4

Haddock : Synopsis box too big for monitor.
by aditya siram 30 Jan '11

30 Jan '11

Hi all, The new Synopsis box in the API docs expands beyond the monitor for functions that have long signatures. An example is the TreeView documentation [1] in the gtk package. I have only tested Firefox. -deech [1] http://hackage.haskell.org/packages/archive/gtk/0.12.0/doc/html/Graphics-UI…

2 1

Code from Haskell School of Expression hanging.
by michael rice 30 Jan '11

30 Jan '11

I got Hudak's Haskell School of Expression today and just tried a couple of the graphics programs from the software I downloaded at the book site. Both seem to hang on opening a window. Is there a fix for this problem? I'm on a Fedora 14 Linux (32-bit) box running GHC version 6.12.3. Michael ============ *SimpleGraphics> :l Snowflake.lhs [1 of 2] Compiling SOE ( SOE.hs, interpreted ) [2 of 2] Compiling Snowflake ( Snowflake.lhs, interpreted ) Ok, modules loaded: Snowflake, SOE. *Snowflake> main ============ This code was automatically extracted from a .lhs file that uses the following convention: -- lines beginning with ">" are executable -- lines beginning with "<" are in the text, but not necessarily executable -- lines beginning with "|" are also in the text, but are often just expressions or code fragments. > module Snowflake where > import SOE > m = 81 :: Int -- multiple of 3 for triangle size > x = 250 :: Int -- x and y coordinates of > y = 250 :: Int -- center of snowflake > colors = [ Magenta, Blue, Green, Red, Yellow ] > snowflake :: Window -> IO () > snowflake w = do > drawTri w x y m 0 False -- draw first triangle w/flat top > flake w x y m 0 True -- begin recursion to complete job > flake :: Window -> Int -> Int -> Int -> Int -> Bool -> IO () > flake w x y m c o = do > drawTri w x y m c o -- draw second triangle > let c1 = (c+1)`mod`5 -- get next color > if (m<=3) then return () -- if too small, we're done > else do > flake w (x-2*m) (y-m) (m`div`3) c1 True -- NW > flake w (x+2*m) (y-m) (m`div`3) c1 True -- NE > flake w x (y+2*m) (m`div`3) c1 True -- S > flake w (x-2*m) (y+m) (m`div`3) c1 False -- SW > flake w (x+2*m) (y+m) (m`div`3) c1 False -- SE > flake w x (y-2*m) (m`div`3) c1 False -- N > drawTri :: Window -> Int -> Int -> Int -> Int -> Bool -> IO () > drawTri w x y m c o = > let d = (3*m) `div` 2 > ps = if o > then [(x,y-3*m), (x-3*m,y+d), (x+3*m,y+d)] -- side at bottom > else [ (x,y+3*m), (x-3*m,y-d), (x+3*m,y-d)] -- side at top > in drawInWindow w > (withColor (colors !! c) > (polygon ps)) > main > = runGraphics ( > do w <- openWindow "Snowflake Fractal" (500,500) > drawInWindow w (withColor White > (polygon [(0,0),(499,0),(499,499),(0,499)])) > snowflake w > spaceClose w > ) > spaceClose :: Window -> IO () > spaceClose w > = do k <- getKey w > if k==' ' || k == '\x0' > then closeWindow w > else spaceClose w

4 6

ANN: nanoparsec 0.1.1
by Maciej Piechotka 30 Jan '11

30 Jan '11

Nanoparsec is currently simply a port of attoparsec on the ListLike (the abstraction of lists used by iteratee). It allows to achive in parsing a near-attoparsec levels of speed (benchmarks from attoparsec library shown a 0.450 ± 0.028 for attoparsec, 0.479 ± 0.043 for nanoparsec and 1.532 ± 0.084 for parsec 3) combining the flexibility of stream of parsec 3 with the iterative approach and speed of attoparsec. Changes since version 0.1: - Lowered conditions on base to allow using GHC 6.12.x (haven't been tested) - Added IsString instance and (.*>) (<*.) helper functions Regards

2 2

"Cabal ignores ld-options?" - 2011 remix
by Ryan Newton 30 Jan '11

30 Jan '11

Hi cafe, 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? Thanks, -Ryan

1 0

Re: [Haskell-cafe] Fwd: web-routes and forms
by Jasper Van der Jeugt 30 Jan '11

30 Jan '11

Hello, As Jeremy said, the HTML returned by formHtml is meant to be placed inside the a <form> tag: it does not include a <form> tag. You should use it like this: > H.form ! A.enctype (H.stringValue $ show enctype) > ! A.method "POST" ! A.action "/" $ do > html -- The HTML rendered by formHtml > H.input ! A.type_ "submit" ! A.value "Submit" (with or without the submit button). Cheers, Jasper On Jan 27, 2011 10:54 AM, "Corentin Dupont" <corentin.dupont(a)gmail.com> wrote: > Hello Jasper, > Do you have an idea? > > Thanks, > Corentin > > ---------- Forwarded message ---------- > From: Jeremy Shaw <jeremy(a)n-heptane.com> > Date: Thu, Jan 27, 2011 at 2:22 AM > Subject: Re: web-routes and forms > To: Corentin Dupont <corentin.dupont(a)gmail.com> > Cc: haskell <haskell-cafe(a)haskell.org> > > > On Wed, Jan 26, 2011 at 4:33 PM, Corentin Dupont > <corentin.dupont(a)gmail.com> wrote: > >> Now turning to digestive functors, I don't see where do goes the "A.action >> actionURL" part that was in traditionnal forms? >> It seems I need it for routing the result of the form. > > I think you will find formHtml is returning you the stuff that goes > inside the <form> tag, but does not actually include the form tag > itself ? > > I am not sure how to modify the attrs using blaze-html. I think that > is a missing feature of the digestive-functors-blaze package. In > digestive-functors-hsp there is a function: > > > setAttrs :: (EmbedAsAttr x attr, XMLGenerator x, Monad m, Functor m) => > Form m i e [HSX.GenXML x] a > -> attr > -> Form m i e [HSX.GenXML x] a > setAttrs form attrs = mapView (map (`set` attrs)) form > > > You probably need something similar for blaze. > > - jeremy

2 4

tricky recursive type instance method
by Frank Kuehnel 29 Jan '11

29 Jan '11

Hi folks, how do I make this work: I want a division algebra over a field k, and I want to define the conjugation of complex numbers, i.e. conj (C 1 2) but also the conjugation of tensors of complex numbers conj (C (C 1 2) (C 1 4)) ghci load that stuff butt barfs on a simple > conj (C 1 2) with instance Real a => DAlgebra a a -- Defined at Clifford.hs:20:10-31 instance (Real r, Num a, DAlgebra a r) => DAlgebra (Complex a) r here's the code: -- for a normed division algebra we need a norm and conjugation! class DAlgebra a k | a -> k where -- need functional dependence because conj doesn't refer to k conj :: a -> a abs2 :: a -> k -- real numbers are a division algebra instance Real a => DAlgebra a a where conj = id abs2 x = x*x -- Complex numbers form a normed commutative division algebra data Complex a = C a a deriving (Eq,Show) instance Num a => Num (Complex a) where fromInteger a = C (fromInteger a) 0 (C a b)+(C a' b') = C (a+a') (b+b') (C a b)-(C a' b') = C (a-a') (b-b') (C a b)*(C a' b') = C (a*a'-b*b') (a*b'+b*a') instance (Real r, Num a, DAlgebra a r) => DAlgebra (Complex a) r where conj (C a b) = C a (conj (-b)) abs2 (C a b) = (abs2 a) + (abs2 b) Thanks for you help!

4 3

Playing sounds in Haskell
by Daniel Díaz 29 Jan '11

29 Jan '11

Hi, cafe, I'm interested in write a program that plays sounds in real time. There is a library with functions able to play sounds, from a file? I'm on Windows, and I don't need special features, only play sounds in a comfortable way. Thanks in advance. -- Daniel Díaz

4 3

Using IsString with attoparsec
by Yitzchak Gale 29 Jan '11

29 Jan '11

Using the IsString instance for Parser in attoparsec is really nice, but unfortunately, you can't use it out of the box for the most common case. One would like to be able to write parsers in this style: "(" *> stuff <* ")" But the types of *> and <* are too general - there is no way for the type checker to figure out which instance of IsString to use for the discarded parameter. I suggest adding the following type-specialized variants to Data.Attoparsec.Char8: (<*.) :: Applicative f => f a -> f ByteString -> f a (<*.) = (<*) (.*>) :: Applicative f => f ByteString -> f a -> f a (.*>) = (*>) Once attoparsec-text gets its IsString instance, I would add the same thing to Data.Attoparsec.Text, with ByteString replaced by Text. I have been using those for both attoparsec and attoparsec-text, and I find them extremely useful. Thanks, Yitz

3 3

ANNOUNCE: Parsec 3.1.1
by Antoine Latter 29 Jan '11

29 Jan '11

Parsec is a monadic combinator library that is well-documented, simple to use, and produces good error messages. Parsec is not inherently lazy/incremental and is not well-suited to handling large quantities of simply formatted data. Parsec 3 added to Parsec the ability to use Parsec as a monad transformer and generalizes the input Parsec accepts. This is a minor point release over Parsec 3.1.0. The changes in this release are: - A bug introduced in 3.1.0 which reduced the quality of error messages when failing to parse has been fixed - Typos in some of the Haddock documentation have been fixed - The package 'syb` is no longer a dependency Please note that there are a few changes which have not yet made it to the up-stream source repository, as code.haskell.org is experiencing a temporary outage. Link on Hackage: http://hackage.haskell.org/package/parsec-3.1.1 I'd like to thank Derek Elkins for prior his maintainer-ship of the package and for getting the Summer-Of-Code work released as parsec-3.0.

1 0

3GB allocation, 75% CPU usage from a callback function
by Edward Amsden 29 Jan '11

29 Jan '11

I recently got the jack package from hackage working again. For those unfamiliar, jack is a callback-based audio server. Writing a client entails importing the C library or its bindings (the Sound.JACK module in my case), creating a client and some ports (using provided library functions), and then registering callbacks for audio processing. I've written a simple program that outputs a sine wave through JACK. The server's sample rate is 44100, which means that this function must be called 44100 times/second (it is buffered, so generally that would be in chunks of 64, 128, or 256). It is an IO function, which gives the only opportunity to keep track of time: (Note that the function produced by newSinWave is the one actually registered as a callback: newSinWave :: Int -> Float -> IO (CFloat -> IO CFloat) newSinWave sampleRate freq = do ioref <- newIORef (0::Integer) let multiplier = 2 * pi * freq / (fromIntegral sampleRate) return (\_ -> {-# SCC "sinWave" #-} do t <- readIORef ioref modifyIORef ioref (+1) return $ fromRational $ toRational $ sin (fromIntegral t * multiplier)) I profiled this since when my program registered with the jack server and started taking callbacks, it was using about 75% cpu (in contrast, the echo program included with the jack package uses less than 2%). The following two lines are of concern to me: "total alloc = 3,040,397,164 bytes (excludes profiling overheads)" My program uses 3GB of virtual memory over a 15 second run? " sinWave Main 341 1173295 100.0 100.0 0.0 0.0" and ~100% of that 75% cpu time is being spent in my callback. Is there something I'm doing wrong? At the very least, it shouldn't be using 3GB of memory. The only thing that needs to be saved between callbacks is the IORef, which is storing an Int. I assume that evaluating that whole construct in haskell may be too much timewise to put in a sound callback (or perhaps not), but 3GB of memory is ridiculous. Thoughts/hints/"you're doing it wrong" anyone? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

4 4

GPL License of H-Matrix and prelude numeric
by gutti 29 Jan '11

29 Jan '11

Hi all, I realised that haskell in the base libraries tries to consistently stick to the permissive BSD license to also give the language traction in the commercial world. The H-Matrix and the prelude numeric packages however, which in my mind cover really important ground work, are issued under the resprictive GPL. Is that only because they link into other external GNU - libaries with GPL license ? -- If yes would be repackaging the haskell-native bits in an extra BSD package an option ? Cheers Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/GPL-License-of-H-Matrix-and-prelude-nu… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

13 42

Merry monad mixup?
by michael rice 28 Jan '11

28 Jan '11

The first and third work, but not the second. Why? Michael ============== f :: String -> IO () f s = do putStrLn s {- g :: [String] -> IO () g l = do s <- l putStrLn s -} {- h :: [Int] -> [Int] h l = do i <- l return (i+1) -} ============== serialize2.hs:29:9: Couldn't match expected type `[String]' against inferred type `IO String' In a stmt of a 'do' expression: s <- l In the expression: do { s <- l; putStrLn s } In the definition of `g': g l = do { s <- l; putStrLn s }

4 5

source line annotations
by Evan Laforge 28 Jan '11

28 Jan '11

One of the first things I did when starting a larger haskell project was to write a preprocessor that would replace certain tokens with (token_srcpos (filename, func_name, lineno)) and then write x and x_srcpos versions of the various 'throw' and logging functions. This is extremely handy to have, and since logs are part of my app's UI, it's part of the app itself. I found out later that jhc has a pragma that basically does the same thing, only built in. My preprocessor works but has some problems. Namely, replacing non-qualified tokens requires parsing the source because you want to replace only function calls, not declarations, import / export lists, etc. I get around this because I always import qualified, but still I need to do some hacking to get qualified tokens ('lex' mysteriously doesn't understand them... aren't they in haskell 2010 now?) and not screw up on odd corners like backslash string continuations. Then I start wanting to e.g. replace 'throw' in the same module it's defined in, which really lets the dragons out because now I'm trying to replace unqualified names. Another problem is that the names to replace are hardcoded in the preprocessor. Not a problem for me, but would be for a general tool. My preprocessor works well, but occasionally I do have to go in and fix yet another odd corner that came up. Initially I thought I would only simplistically replace tokens and avoid using a full syntax parser, but to really do it correctly a full parser is needed. And of course this is specific to my style (qualified imports) and project. To do this in full generality really requires the compiler's help. But as I use it now, it's been extremely useful, especially for exceptions which may not carry a unique or easily greppable msg. Or any msg at all. Are there any more sophisticated tools out there that do this? Interest in extending ghc to support SRCLOC_ANNOTATE? Reasons why I don't actually want this after all?

5 9

IsString instances for attoparsec-text
by Yitzchak Gale 28 Jan '11

28 Jan '11

Hi Felipe, Thanks so much for implementing the attoparsec-text package. Could you please add an IsString instance for Parser Text, parallel to the one in attoparsec? You may have missed this because of it being an orphan instance in attoparsec. It should be something like: instance IsString (Parser T.Text) where fromString = string . T.pack Thanks, Yitz

2 1

Re: [Haskell-cafe] Haskell for children? Any experience?
by John Peterson 28 Jan '11

28 Jan '11

Hi all, I've had good luck with Haskore as a way to bring kids into functional programming. It's nice in that you can get a lot done in just 2 or 3 hours. Lately I've switched over to a 3D game engine using FRP to make the programming simple. I've been running a summer camp (https://www.western.edu/academics/computerscience/computer-camp.html) using this software with a lot of success. Unfortunately I had to abandon Haskell to get to our game engine, so I've rebuilt FRP on top of Python. The programming style is functional but it would be nice to have a real type system. We're in the process of getting a public release together this term. I really need to get all my web content moved off Yale - I'll be happy to share the software / papers with anyone that's interested. My slides and papers should re John Peterson jpeterson(a)western.edu

1 0

Cabal proxy configuration
by Azeem -ul-Hasan 28 Jan '11

28 Jan '11

My university requires the students to use internet through an authenticated http proxy. How do I configure cabal for that? I am using Ubuntu. Please be detailed about it. Thanks Azeem

2 3

Re: [Haskell-cafe] tricky recursive type instance method
by Antoine Latter 28 Jan '11

28 Jan '11

On Thu, Jan 27, 2011 at 10:09 PM, Frank Kuehnel <kuehnelf(a)gmail.com> wrote: > Hi Antoine, > > I've turned on the OverlappingInstances option > > this is what I get, when I execute > >> conj ((C 1 2) :: (Complex Int)) > > Overlapping instances for DAlgebra (Complex Float) (Complex Float) > arising from a use of `conj' > Matching instances: > instance [overlap ok] Real a => DAlgebra a a > -- Defined at Clifford.hs:21:10-31 > instance [overlap ok] (Real r, Num a, DAlgebra a r) => > DAlgebra (Complex a) r > I'm guessing GHC can't pick which instance to use, because neither one follows the rules for being more specific than the other. I tried to read the manual on it, but I don't have a clear answer for you: http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/type-class-extension… If I change the `DAlgebra a a` instance to `DAlgebra Integer Integer` I can get examples to work fine. Antoine > > On Jan 27, 2011, at 8:00 PM, Antoine Latter wrote: > >> On Thu, Jan 27, 2011 at 9:35 PM, Frank Kuehnel <vincef13(a)mac.com> wrote: >>> Hi folks, >>> >>> how do I make this work: I want a division algebra over a field k, and I want to define >>> the conjugation of complex numbers, i.e. conj (C 1 2) but also the conjugation of tensors of complex numbers >>> conj (C (C 1 2) (C 1 4)) >>> >>> ghci load that stuff butt barfs on a simple >>>> conj (C 1 2) >>> >>> with >>> instance Real a => DAlgebra a a -- Defined at Clifford.hs:20:10-31 >>> instance (Real r, Num a, DAlgebra a r) => DAlgebra (Complex a) r >>> >>> >>> here's the code: >>> >>> -- for a normed division algebra we need a norm and conjugation! >>> class DAlgebra a k | a -> k where -- need functional dependence because conj doesn't refer to k >>> conj :: a -> a >>> abs2 :: a -> k >>> >>> -- real numbers are a division algebra >>> instance Real a => DAlgebra a a where >>> conj = id >>> abs2 x = x*x >>> >>> -- Complex numbers form a normed commutative division algebra >>> data Complex a = C a a deriving (Eq,Show) >>> >>> instance Num a => Num (Complex a) where >>> fromInteger a = C (fromInteger a) 0 >>> (C a b)+(C a' b') = C (a+a') (b+b') >>> (C a b)-(C a' b') = C (a-a') (b-b') >>> (C a b)*(C a' b') = C (a*a'-b*b') (a*b'+b*a') >>> >>> instance (Real r, Num a, DAlgebra a r) => DAlgebra (Complex a) r where >>> conj (C a b) = C a (conj (-b)) >>> abs2 (C a b) = (abs2 a) + (abs2 b) >>> >> >> >> What error are you getting in GHCi? It wasn't immediately clear from >> your email, but maybe I missed it. >> >> It looks like you have overlapping instances between `DAlgebra a a` >> and `DAlgebra (Complex a) r`, so if that's what you want you'll need >> to making sure you have the OverlappingInstances extension turned on. >> >> You might run in to other issues further on. >> >> Antoine >> >>> >>> Thanks for you help! >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> > >

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bizzaro Network.Curl problems
by Michael Litchard 28 Jan '11

28 Jan '11

Has anyone ever experienced a situation similar to this? Wrote toy code to learn how to make a cookie jar. Toy code succeeds, add functionality and better organization. new improved program works correcty, as far as forming correct headers and GET requests. no cookie jar.. Checked old toy code, still works. Reduces new program to bare essentials until it looks like old toy program. Still, no cookie jar. Upshot is we are starting to suspect libcurl itself here and not the Network.Curl wrapper. Anyway, please respond if this bizzaro problem sounds familiar.

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Building a shared library with FFI on x86_64
by Eric Webster 28 Jan '11

28 Jan '11

I have a project that involves building a shared library containing code generated by GHC and exposed using the foreign function interface to other C programs that link against it. I'm able to build a functioning library without issue on 32bit x86 systems using GHC 6.8.2 and 6.12.3. When I try to build on a 64bit x86 system I get a relocation error that advises I include -fPIC. Including -fPIC does not help though. I'm wondering if I'm either doing something wrong or if this could be a problem with GHC. I've created and attached a small example that illustrates this issue. When building on 64bit x86 with GHC 7.0.1 and -fPIC and -dynamic I get the following error. (Similar with GHC 6.12.3) /usr/bin/ld: CLibrary.o: relocation R_X86_64_PC32 against `CLibrary_testFunctionCWrapper_srt' can not be used when making a shared object; recompile with -fPIC /usr/bin/ld: final link failed: Bad value When building on 64bit x86 with GHC 7.0.1 and -fPIC and without -dynamic I get the following error (Similar with GHC 6.12.3 and 6.8.2) /usr/bin/ld: /home/CIRG/ew/local/lib/ghc-7.0.1/base-4.3.0.0/libHSbase-4.3.0.0.a(Base__1.o): relocation R_X86_64_32S against `a local symbol' can not be used when making a shared object; recompile with -fPIC /home/CIRG/ew/local/lib/ghc-7.0.1/base-4.3.0.0/libHSbase-4.3.0.0.a: could not read symbols: Bad value When building on 32bit x86 with GHC 6.12.3 I get no error regardless of -fPIC or -dynamic. When building on 32bit x86 with GHC 6.8.2 I get no error regardless of -fPIC and without -dynamic. Any advice would be appreciated! Thanks.

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Call for Copy: Monad.Reader Issue 18
by Brent Yorgey 28 Jan '11

28 Jan '11

Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 18 will be: **Friday, April 1, 2011** This will be a normal issue -- but don't forget to also submit your Haskell poems and short stories by February 18 for inclusion in the special poetry and fiction edition [1]! The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki-page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out. [1] http://themonadreader.wordpress.com/2011/01/19/call-for-copy-special-poetry…

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ANNOUNCE: WAI 0.3.1, Warp 0.3.1, wai-app-static 0.0.0 and warp-static 0.0.0
by Michael Snoyman 27 Jan '11

27 Jan '11

Hi everyone, Firstly, after some feedback, I'm releasing some point updates for WAI and Warp. They fix some documentation issues, add some status aliases, make it possible to handle exceptions more easily, and make things more responsive on Windows. Next, I'm releasing two new packages: wai-app-static is a static file serving application for WAI. It handles all of your basic needs in this department: * Proper 301 requests to fix trailing-slash issues for folder vs directory. * Directory listing support (if turned on), including a default listing function that uses Haskell colors and a Haskell logo in the corner. Thanks to Happstack for the basis of this code. * Directory index support (eg, index.html, index.htm), again configurable. * Mime type lookups. Thanks to Snap for this table. warp-static is simply a standalone executable that runs wai-app-static on the Warp webserver. The executable name is warp. I know I personally have needed such a light-weight web server at work a few times, so if nothing else, putting it on Hackage is a convenience for me :-). I hope people find these useful. Don't expect the performance to beat nginx, but it is still quite good. if there are any other features requests on any of these packages, let me know. Michael

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Re: [Haskell-cafe] Applicative and parsec3
by Henning Thielemann 27 Jan '11

27 Jan '11

On Thu, 27 Jan 2011, Magnus Therning wrote: > 2011/1/27 Henning Thielemann <lemming(a)henning-thielemann.de>: >> >> On Thu, 27 Jan 2011, Kazu Yamamoto (山本和彦) wrote: >> >>> I'm using parsec3 with the applicative style. Since the functions in >>> Control.Applicative and parsec3 conflicts, I need to use the "hiding" >>> keyword as follows: >>> >>> import Control.Applicative hiding (many,optional,(<|>)) >>> import Text.Parsec >> >> There are several reasons to stick to explicit or qualified imports: >> http://www.haskell.org/haskellwiki/Import_modules_properly > > What that article fails to mention is the inconvenience of using > qualified imports in combination with operators. For infix operators you would use unqualified explicit imports.

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Conditionals and case expressions in do blocks
by Edward Amsden 27 Jan '11

27 Jan '11

Inside a do block, I can very conveniently substitute let x = <pure exp> <continued monadic code> for either x <- return <pure exp> <continued monadic code> or let x = <pure exp> in do <continued monadic code> However, I can't do anything similar (that I know of) with if or case expressions. If I use if or case inside a do block, it's likely that I'm still using monadic expressions (at least in my experience). Is there some nasty semantic ambiguity that comes of this? Is it just not in the standard yet? Is there some extension that I'm not aware of? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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Re: [Haskell-cafe] Problem about exception
by oleg＠okmij.org 27 Jan '11

27 Jan '11

It seems that the problem is to follow a protocol, reporting protocol errors (unexpected responses) and permitting recovery if the `supervisor' decided that the error can be fixed by re-reading from the server. This problem -- specifically, communicating with a supervisor `out-of-band' and recovery -- is a good use case for delimited continuations. The following code implements the scenario in Magicloud's message. http://okmij.org/ftp/continuations/CCmonad/ProtocolRecovery.hs Actually, the code is interactive, letting the use play a good or a bad server. One can see for themselves the effect of expected or unexpected responses. The title comments show the examples of good or bad interactions. Here is a sample transcript of several interactions. ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 2 Client Dis-connected ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 3 Exception: Err_bad_resp Res_debug DEBUG Enter response, as a number 1..4 3 Exception: Err_bad_resp Res_debug DEBUG Enter response, as a number 1..4 2 Client Dis-connected *ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 4 Exception: Err_bad_resp Res_disconnect Aborting Client Dis-connected *ProtocolRecovery> main Client Connected sending: Req_hello Enter response, as a number 1..4 1 sending: Req_who_are_you Enter response, as a number 1..4 1 Exception: Err_bad_resp Res_hello Really bad response! Client Dis-connected

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community.haskell.org is down
by Yitzchak Gale 27 Jan '11

27 Jan '11

The community server, on which resides the domains: community.haskell.org projects.haskell.org code.haskell.org trac.haskell.org planet.haskell.org is down, due to a hacker attack. Rather than trying to repair the old machine it was on, this is the time to migrate this server (finally) to the new machine that has been allocated for it. We are now putting together a strike team to get this done. Hopefully, the server will be up again in a few days. We'll try to keep the community updated about progress.

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Haskell Weekly News: Issue 166 - January 26, 2011
by Daniel Santa Cruz 27 Jan '11

27 Jan '11

Welcome to issue 166 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 16 to 22, 2011. Announcements Sukit Tretriluxana [2]proudly announced version 1.0 of SiteBridge. He also included a short introductory video. Mark Wright [3]annouced the release of a Haskell binding to ANTLR LL(*) parser generator C runtime library. Jasper Van der Jeugt [4]inveted everyone to the 6th Ghent Functional Group Meeting to be held on Feb 17, 2011 in the Technicum Building (Sint-Pietersnieuwstraat 41, 9000 Gent) of Ghent University. I know I'm not the only one who wishing I could attend!. Quotes of the Week * Guy Steele: Although Fortress is originally designed as an object-oriented framework in which to build an array-style scientific programming language, [...] as we've experimented with it and tried to get the parallelism going we found ourselves pushed more and more in the direction of using immutable data structures and a functional style of programming. [...] If I'd known seven years ago what I know now, I would have started with Haskell and pushed it a tenth of the way toward Fortran instead of starting with Fortran and pushing it nine tenths of the way toward Haskell. * geheimdienst:ah, yes. now we have to make a lambdacat captioned "kittehs no bery gud at speling. we says leksah fur lexer and monad fur moniod in catagory of ennobunktors" * mreh:some jerk said to me over my shoulder once, if you're doing functional programming, why do you have all those assignments? * revenantphx:You don't need category theory to understand Haskell, though you may need it to understand #haskell. * tolkad:haskell isn't even turing complete without infinite memory * monochrom:C-c C-p was the secret command for sending a report to the kgb * Draconx|Laptop:Rice's theorem: Let P be an interesting problem that you want to solve with a computer. Haha, you're screwed! * monochrom:one ring to rule them all! and in your monad >>= them! Top Reddit Stories * Haskell's Own DLL Hell Domain: cdsmith.wordpress.com, Score: 51, Comments: 27 On Reddit: http://www.reddit.com/r/haskell/comments/f3lh5/haskells_own_dll_hell/ Original: http://cdsmith.wordpress.com/2011/01/16/haskells-own-dll-hell/ * [CCC] On how Haskell’s Arrows are NOT just function compositions with a fancier name Domain: ro-che.info, Score: 40, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/f3e06/ccc_on_how_haskells_arrows_a… Original: http://ro-che.info/ccc/12.html * Announcing the Warp web server (benchmarks inside) Domain: docs.yesodweb.com, Score: 36, Comments: 12 On Reddit: http://www.reddit.com/r/haskell/comments/f4bn8/announcing_the_warp_web_serv… Original: http://docs.yesodweb.com/blog/announcing-warp/ * The ABCs of semantics Domain: haskell.org, Score: 33, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/f4v8f/the_abcs_of_semantics/ Original: http://www.haskell.org/pipermail/haskell-cafe/2011-January/088315.html * Thoughts on node.js and Haskell Domain: moreindirection.blogspot.com, Score: 33, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/f6o5i/thoughts_on_nodejs_and_haske… Original: http://moreindirection.blogspot.com/2011/01/thoughts-on-nodejs.html * A low level performance comparison of simple operations on unboxed integer arrays between Haskell/GHC (6.12.1) and OCaml (3.12.0) (and also C) [originally on Proggit] Domain: lix.polytechnique.fr, Score: 32, Comments: 16 On Reddit: http://www.reddit.com/r/haskell/comments/f3yf8/a_low_level_performance_comp… Original: http://www.lix.polytechnique.fr/~kaustuv/expo/incr_uarray/ * The Butterfly Effect in Cabal Domain: cdsmith.wordpress.com, Score: 29, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/f4179/the_butterfly_effect_in_caba… Original: http://cdsmith.wordpress.com/2011/01/17/the-butterfly-effect-in-cabal/ * Painless NP-complete problems: an embedded DSL for SMT solving Domain: donsbot.wordpress.com, Score: 27, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/f3fp6/painless_npcomplete_problems… Original: http://donsbot.wordpress.com/2011/01/16/painless-np-complete-problems-an-em… * Merging SMT solvers and programming languages Domain: corp.galois.com, Score: 26, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f4r5r/merging_smt_solvers_and_prog… Original: http://corp.galois.com/blog/2011/1/18/merging-smt-solvers-and-programming-l… * Tell Haskellers: GHC's Trac now has source code browsing (courtesy of Git) Domain: hackage.haskell.org, Score: 26, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f6391/tell_haskellers_ghcs_trac_no… Original: http://hackage.haskell.org/trac/ghc/browser Top StackOverflow Questions * Criticize simple monad votes: 11, answers: 1 Read on SO: http://stackoverflow.com/questions/4765260/criticize-simple-monad * Parsing function in haskell votes: 8, answers: 1 Read on SO: http://stackoverflow.com/questions/4711893/parsing-function-in-haskell * Possible optimizations in Haskell that are not yet implemented in GHC? votes: 7, answers: 1 Read on SO: http://stackoverflow.com/questions/4720499/possible-optimizations-in-haskel… * Is there a Haskell/ML-like compiler to C? votes: 6, answers: 3 Read on SO: http://stackoverflow.com/questions/4722439/is-there-a-haskell-ml-like-compi… * How to find the longest word in list? votes: 5, answers: 4 Read on SO: http://stackoverflow.com/questions/4708028/how-to-find-the-longest-word-in-… About the Haskell Weekly News You can find an html version of this newsletter at: http://contemplatecode.blogspot.com/2011/01/haskell-weekly-news-issue-166.h… To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85348 3. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18437 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85519

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Problem about exception.
by Magicloud Magiclouds 27 Jan '11

27 Jan '11

Hi, Consider such a case: I'm making a client program. There is a process, client and server exchange some information in a strict order. So I do (persudo code): exchange = do sendHello readMsg >>= expect hello processHelloReply sendWhatsyourname readMsg >>= expect name processNameReply And expect is something like: expect c f (cmd, msg) = if c == cmd then f msg else fail "unexpected" This is OK until this situation: The server may send some special command, for example DEBUG, DISCONNECT. This breaks the process above. So I think I could throw the unexpected command to outer function to handle. Something like: main = do connect catch exchange $ \e -> do case e of UnexpectedCMD DEBUG -> -- ignore process UnexpectedCMD DISCONNECT -> -- disconnect process _ -> -- something really wrong Well, with Control.Exception, I do not know how to make this done. And with something like Data.Either, I got more problem like: 36| Couldn't match expected type `Control.Monad.Trans.State.Lazy.StateT 37| InternalState IO b' 38| with actual type `Either (Int8, ByteString) b1' 39| Expected type: (Int8, ByteString) 40| -> Control.Monad.Trans.State.Lazy.StateT InternalState IO b 41| Actual type: (Int8, ByteString) -> Either (Int8, ByteString) b1 -- 竹密岂妨流水过山高哪阻野云飞

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web-routes and forms
by Corentin Dupont 27 Jan '11

27 Jan '11

Hello, I have difficulties mixing web-routes and forms: I have put routes in all my site, except for forms which remains with the type ServerPartT IO Response. How to make them work together? I have: type NomicServer = ServerPartT IO type RoutedNomicServer = RouteT PlayerCommand NomicServer newRule :: ServerHandle -> NomicServer Response newRule sh = do methodM POST -- only accept a post method mbEntry <- getData -- get the data case mbEntry of Nothing -> error $ "error: newRule" Just (NewRule name text code pn) -> do *html <- nomicPageComm pn sh (submitRule name text code pn))* ok $ toResponse html nomicPageComm :: PlayerNumber -> ServerHandle -> Comm () -> RoutedNomicServer Html nomicPageComm pn sh comm = (..) launchWebServer :: ServerHandle -> IO () launchWebServer sh = do putStrLn "Starting web server...\nTo connect, drive your browser to \" http://localhost:8000/Login\"" d <- liftIO getDataDir simpleHTTP nullConf $ mconcat [dir "postLogin" $ postLogin, fileServe [] d, dir "Login" $ ok $ toResponse $ loginPage, dir "NewRule" $ newRule sh, dir "NewGame" $ newGameWeb sh, dir "Nomic" $ do html <- implSite " http://localhost:8000/Nomic/" "" (nomicSite sh) ok $ toResponse html ] The red line doesn't compile. I don't know how to transform a RoutedNomicServer into a NomicServer. For the future I intend to use formlets: is these some examples of programs using happstack + web-routes + formlets? Thanks, Corentin On Fri, Jan 7, 2011 at 5:10 PM, Jeremy Shaw <jeremy(a)n-heptane.com> wrote: > Hello, > > The [(String, String)] argument is for adding query parameters. > > > encodePathInfo ["foo", "bar", "baz"] [("key","value")] > > "foo/bar/baz?key=value" > > Instead of showURL you would use showURLParams. > > hope this helps!d > - jeremy > > On Fri, Jan 7, 2011 at 8:12 AM, Corentin Dupont > <corentin.dupont(a)gmail.com> wrote: > > Hello Jeremy, > > I'm using Web routes with happstack. > > I'm following this tutorial: > > http://tutorialpedia.org/tutorials/Happstack+type+safe+URLs.html > > > > But It seems out of synch with the latest version of web-routes: 0.23.2. > > The haddock documentation seems out of date also: > > > > encodePathInfo :: [String] -> [(String, String)] -> String > > > > For example: > > > > encodePathInfo [\"foo\", \"bar\", \"baz\"] > > > > "foo/bar/baz" > > > > And I can't figure out what this [(String, String)] is for ;) > > > > Thanks, > > > > Corentin > > >

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stat64 missing when loading download in GHCi
by Magnus Therning 27 Jan '11

27 Jan '11

Don, I'm having issues with loading download in GHCi: Loading package download-0.3.1.1 ... linking ... <interactive>: /usr/lib/ghc-6.12.3/site-local/download-0.3.1.1/HSdownload-0.3.1.1.o: unknown symbol `stat64' ghc: unable to load package `download-0.3.1.1' I found an old email you sent in 2008[1] on the same issue, but you don't seem to have received any replies back then. Did you ever found out why GHCi complains, but it it possible to compile and link into an executable? I'm CC-ing to Haskell Cafe as well, in case someone else has any information on this. /M [1] http://www.haskell.org/pipermail/glasgow-haskell-bugs/2008-July/014071.html -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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ANN: hmatrix-0.11
by Alberto Ruiz 27 Jan '11

27 Jan '11

Hi all, I am happy to announce hmatrix-0.11, a package for numerical computation based on LAPACK and GSL. Home page: http://code.haskell.org/hmatrix/ Hackage: http://hackage.haskell.org/package/hmatrix Recent changes: http://code.haskell.org/hmatrix/CHANGES By default this version uses Data.Vector.Storable from Roman Leshchinskiy’s vector package. This is useful for working with 1D arrays without explicit conversion (but note that Matrix t /= Vector (Vector t)). As a result, there is a different Show/Read instance for Vector, but no other changes should be observed using only hmatrix functions. (A possible issue is that empty vectors are typically not allowed in hmatrix, but can now be easily created using tools in the vector package.) I hope this is useful. In any case, this can be disabled with flag -f-vector and we can still have O(1) conversion functions. Thanks to everyone who contributed with patches, bug reports, suggestions, and installation problems. Special thanks go to Vivian McPhail for many excellent contributions, including single precision support, API reorganization, and many useful functions. Alberto PS: In relation with the license thread, I am open to change to BSD3 the source code not depending on GSL.

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Are constructors strict?
by Daryoush Mehrtash 26 Jan '11

26 Jan '11

I am having hard time understanding the following paragraph in "Purely functional Lazy non-deterministic programing" paper http://www.cs.rutgers.edu/~ccshan/rational/lazy-nondet.pdf The problem with the naive monadic encoding of non-determinism is that the arguments to a constructor must be deterministic. If these arguments are themselves results of non-deterministic computations, these computations must be performed completely before we can apply the constructor to build a non-deterministic result. Why does the argument to constructors must be deterministic? WHy is it that thunks are not used in this case? Thanks -- Daryoush Weblog: http://onfp.blogspot.com/

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