- Haskell-Cafe - Haskell.org

Unification of PolyTypeable TypeReps ?
by Ismael Figueroa Palet 30 May '12

30 May '12

Hi haskell-cafe In my current project I'm working with type representations created with PolyTypeable, and it turns out that given two type representations I need to find out if they are unifiable. I've looked at polytypeable-utils and see that they have some functions for substituting/applying/free vars, but it doesn't seem like a full solution for what I need. So my question is: is there a library to do this? is there a known way to transform the TypeRep's to make them more amenable to other unification libraries. I noticed that in GHC-api there is an unification library, and that there is also unification-fd. I wanted to ask here first in case someone can point me to the right direction. Thanks! -- Ismael

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What is the difference between runhaskell and compile?
by Magicloud Magiclouds 29 May '12

29 May '12

Hi, I am writing a small program using LDAP hackage. A weird problem occured. When the code was run by runhaskell, things were fine, worked as expected. But when ghc compiled (no any args), and ran, I got this: LDAP error: ldapSimpleBind: LDAPException LdapServerDown(-1): Can't contact LDAP server. There is sure no problem with the server. So I am confused. I thought the two supposed to be the same. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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A functional programming solution for Mr and Mrs Hollingberry
by Andreas Pauley 29 May '12

29 May '12

Hi all, I'm in the process of learning how to approach problems from a functional perspective, coming from an Object Oriented background (mostly Smalltalk). One of the general concerns/questions raised when talking to people in a similar position is: "How do I design/model a problem when I don't have my trusted classes and objects available?" With this in mind I've created a programming exercise where I imagine an OO programmer would use an object hierarchy with subtype polymorphism as part of the solution. And then I'd like to compare functional implementations of the same problem: https://github.com/apauley/HollingBerries I want to see how elegant a solution I can get in a functional language, given that the problem description is not really elegant at all. It has a few annoying exceptions to the normal rules, typical of what one might get in a real specification from some client. Currently there are 3 implementations: - one in Erlang, my attempt at implementing a functional solution - one in Ruby, my attempt to see how an object hierarchy could be used - one in Clojure, done by one of the people in our FP user group [1] I would love to include some Haskell implementations as well, if any of you are interested. Kind regards, Andreas Pauley 1. http://www.meetup.com/lambda-luminaries/ -- http://pauley.org.za/ http://twitter.com/apauley

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ANN: hledger 0.18
by Simon Michael 29 May '12

29 May '12

I'm pleased to announce version 0.18 of hledger! hledger (http://hledger.org) is a haskell library, command-line tool and web application for working with financial data (or anything that can be tracked numerically in an accounting journal). It is inspired by and compatible with John Wiegley's Ledger. hledger reads a plain text file describing transactions and lets you query account balances or transaction details by various criteria. It can also help you record new transactions, or convert CSV data from your bank. Release notes (http://hledger.org/NEWS.html) * web: hledger-web is now based on yesod 1.0 * web: fix js error breaking second use of add form (#72) * web: make `yesod devel` work * the command-line now supports a more powerful query language, consistent with the web UI * hledger now fully supports tags (aka metadata) on both transactions and postings, and querying by tag or tag value * new commands `incomestatement`, `balancesheet`, and `cashflow` provide basic financial statements under certain conditions * format conversion is now done on demand, and the convert command has been dropped. So instead of `hledger convert FILE.csv` just do `hledger -f FILE.csv print` or any other command. You can also pipe any supported format into `hledger -f- CMD` and hledger will try to do the right thing. * support for GHC 6.12 has been dropped; this release has been tested with GHC 7.0.4, 7.2.2, and 7.4.1 * unicode is now handled properly on all supported GHC versions * extensive API, test and internal cleanups Stats: - Release contributors: Simon Michael, xiaoruoruo - 87 days, 155 commits, 6 end-user features and 3 end-user bug fixes since last major release - 216 unit & functional tests (hledger-lib & hledger) - 7836 lines of code (hledger-lib, hledger & hledger-web) Install it: cabal update; cabal install hledger [hledger-web] If you have trouble, see http://hledger.org/MANUAL.html#installing . You can fund a ready-to-run binary for your platform with a donation at http://hledger.org/DOWNLOAD.html . Prosperity and clarity, -Simon

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Estágio com Haskell em SP (Haskell internship at São Paulo, Brazil)
by Felipe Almeida Lessa 28 May '12

28 May '12

(English message follows after the break.) Olá! Estamos procurando um estagiário para trabalhar conosco em São Paulo capital. Nós somos uma startup apaixonada por Haskell com uma boa participação na comunidade e grandes projetos e desafios! Se você está interessado, basta me mandar um e-mail (felipe.lessa(a)gmail.com) com um link para sua conta do GitHub (ou para projetos em que você já tenha trabalhado). Você não precisa ser experiente em Haskell nem ter nenhum código Haskell publicado, mas isso seria um diferencial =). Como a startup está em stealth mode, não posso divulgar o nome da empresa e por isso estou usando meu e-mail pessoal. Até mais! === Hey! We're looking for an intern to work with us in São Paulo, Brazil. We're a startup passionate about using Haskell with great projects and challenges! If you're interested, just send me an e-mail (felipe.lessa(a)gmail.com) with a link to your GitHub account (or to any projects you've worked at). You don't need to be an experienced Haskeller nor have any published Haskell code, but that would be a plus =). As the startup is in stealth mode, I can't disclose the company's name nor use my work e-mail (that's why I'm using my personal one). Cheers, -- Felipe.

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Building pattern and trying monads
by L Corbijn 28 May '12

28 May '12

Hello cafe, I'm working on a project where the main goal of the program is building some complex output (e.g. a Haskell function, module, etc.). In this process there is almost always some partially finished product on which to work. Currently I'm modelling this with a wrapper around StateT containing the partial product but I'm having some doubts about this approach. On one of the projects there is some rule-based transformation which needs to be done. The problem is with the matching and applying the rules. The matches need to retrieve information from the monads in the stack, and the application of a rule changes values in the stack. However when a match fails the stack should be left *untouched* and another rule should be tried. The solution I've in mind depends on the stack being pure. When the monad stack is pure a rule can be applied, returning a maybe value (or having a MaybeT wrapper) and when returning Nothing (failed rule) reverting the stack to it's point before applying the rule. As I'm not quite sure about the design (nor good at software design) I've some questions about this approach. 1. Is there a better approach then using a state monad for building the 'products'? 2. My solution with saving/reverting monad-stacks seems quite a hassle/hack, so is it a good approach or is there something better? Thanks in advance, Lars

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Re: [Haskell-cafe] Building pattern and trying monads
by wren ng thornton 27 May '12

27 May '12

On 5/27/12 8:21 AM, L Corbijn wrote: > 2. My solution with saving/reverting monad-stacks seems quite a > hassle/hack, so is it a good approach or is there something better? One good solution for backtracking is to use logict[1]. I've used it with various state-like monads and it works well (e.g., the IntBindingT monad in unification-fd[2]). [1] http://hackage.haskell.org/package/logict http://okmij.org/ftp/papers/LogicT.pdf [2] http://hackage.haskell.org/package/unification-fd http://hackage.haskell.org/packages/archive/unification-fd/0.7.0/doc/html/C… -- Live well, ~wren

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Record syntax, reopening a can of worms.
by timothyhobbs＠seznam.cz 27 May '12

27 May '12

Hello list, As my first post. I'd like to open a can of worms that I sure has been opened before. That is record syntax. As we all know, there are some type safety problems with our current record syntax. The most obvious is that this compiles without even giving you a warning: >data MyData = > A {a::Int, > b::Int} | > B {c::Int} >foo :: MyData -> Int >foo a@A{} = c a >main :: IO () >main = print $ foo (A 1 2) This compiles, then crashes at run time. Rediculus, why didn't the glorious GHC catch this? This is not some ambigious scenario... One way I found to improve type checking is this: >data MyData = A A' | B B' >data A' = A'{a::Int, > b::Int} >data B' = B'{c::Int} >foo :: MyData -> Int >foo (A a) = c a >main :: IO () >main = print $ foo (A (A' 1 2)) This doesn't compile anymore because a :: A' and c :: B' -> X It seems like the later method is simply better. Better type checking = fewer runtime errors(I actually ran into a crash in a real project because of this lack of proper type checking...). Can we have a nice sugar for doing the later method? Thank you for your time, Timothy

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Problem with packet documentation generated by cabal on windows 7
by Nicu Ionita 27 May '12

27 May '12

Hi cafe, I have a problem with haddock documentation created when installing new packages with cabal on windows. The generated html files have all links in the form j:\Users\...\doc\...\xxx.html, but firefox says, it cannot open that link. Actually all links should be prefixed by "file:///". I wonder if this is only on windows so and if there is a solution to this. Is there a cabal or haddock flag for this? Nicu

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ANN: format-0.1.0.0
by Scott Lawrence 26 May '12

26 May '12

Hey all, I've just uploaded to hackage format-0.1, a small library for handling format strings similar to those used by nginx's log_format directive. The idea is to allow easy parsing of text rendered with such a format string, to make parsing simple, automatically generated text files as easy as possible. So for example, if you want to parse lines consisting of two bits of text, separated by " -- ", then map (scanFormatString "$a -- $b") $ lines text will do so, generating associative arrays with the variable names as keys and their content as values. A `renderFormatString` function, doing the obvious, is also provided. Obligatory blog post with some more details: http://bytbox.net/blog/2012/05/format-haskell.html I hope somebody finds this useful. (This seems the sort of thing which should already have existed, but I couldn't find anything. Did I miss it?) I'm using parsec underneath, both for parsing the format string and the generated parser, and I've payed no attention to optimization. Some simple testing indicates that a reasonably fast computer takes about 1 second to parse 5000 lines with nginx's (and apache's) default format string - pretty slow. If there's demand, I'll be happy to pay more attention to that. -- Scott Lawrence

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