April 2013 - Haskell-Cafe

N-ANN: HBlog-0.1
by Tom Murphy 02 Apr '13

02 Apr '13

Hi All, Not particularly happy to announce the non-release of my latest library, HBlog 0.1. I've been working on it steadily for a long time now, but it's still not ready. The library -- when it's finished -- will be a 100% Haskell representation of blogs, from authors and posts up to the most abstract notion of an entire blog as a Haskell data type. I'd like to finish the library sometime, but I've been so busy! Any spare time I've had, I've just ended up watching TV. I've been so caught up with the final season of The Office, I didn't realize how much time had passed since I originally had the idea! (2 years.) Didn't it seem like Jim and Pam were going to break up a couple weeks ago?! Anyway, here's the code I've definitely settled on so far: data Author = Author { name :: String } deriving (Show, Eq) data Post = Post { title :: String , body :: String , authors :: [Author] } deriving (Show) data Blog = Blog [Post] hasSingleAuthor :: Post -> Bool hasSingleAuthor post = 1 == (length $ authors post) Feel free to use portions of this code in your own projects! Just be sure to put at the top of all your source files: "Portions of this code (c) 2013 Tom Murphy. All rights reserved." ROADMAP: -- Lenses sound pretty cool, so I've started writing lenses for the Author type, as well as finding out what a lens is. Maybe one day there'll be lenses for the Post type, too -- keep your fingers crossed! -- One time when I was walking home from work, I had the idea for blogs being modeled as monads. Wouldn't that be awesome?: "The Blog Monad." I haven't done it yet, though, because I haven't figured out what bind would do. If you figure out how to model blogs as monads, just be sure to include this in all of your sourcefiles: "Idea for blog monad (c) 2013 Tom Murphy. All rights reserved." Happy hacking! Tom

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cabal install pandoc
by Roger Mason 02 Apr '13

02 Apr '13

Hello, I installed ghc (7.6.2) on an Arch Linux machine. I'm trying to install pandoc via cabal but it fails: ... Configuring text-0.11.2.3... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package deepseq-1.3.0.1 requires array-0.4.0.1 package text-0.11.2.3 requires array-0.4.0.1 Building text-0.11.2.3... Preprocessing library text-0.11.2.3... <command line>: cannot satisfy -package-id array-0.4.0.1-db49bb8b0087ae85b5875d4c0cc12874 (use -v for more information) Failed to install text-0.11.2.3 ... There are then errors for other packages that depend on 'text' or 'array'. I will be grateful for any help. Thanks, Roger This electronic communication is governed by the terms and conditions at http://www.mun.ca/cc/policies/electronic_communications_disclaimer_2012.php

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User group in Stockholm?
by Sergey Bushnyak 01 Apr '13

01 Apr '13

I'll be in Stockholm next month, so I'm curious about haskellers in Stockholm. I saw attempts to organize regular meetings in old mailing threads, but can't find *up to date* information to correct my schedule and see fellow haskellers. Sorry, if this offtopic -- Best regards, Sergey Bushnyak

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Haskel let
by A Smith 01 Apr '13

01 Apr '13

Hi I think I've mastered much of functional programming with Haskell, or at least I can write programs which process as desired. However the existence of the "let" statement evades me apart from a quick way to define a function. Then there is the "in" and "where" parts. Its been suggested its to do with scope and is obvious. I think I need to understand it in a C/C++/Perl style of code to have a Eureka moment when it will be obvious. Can someone refer me to something which will assist me ? Andrew

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Assembly EDSL in Haskell
by C K Kashyap 01 Apr '13

01 Apr '13

Hi Cafe, I am trying to embed x86 assembly in Haskell. I'd like the EDSL to not allow invalid movements into registers - for example, should not allow moving into RIP. I was not able to get it to work. I ended up using DataTypeContexts - which is considered misfeature anyway. I was wondering if I could get some suggestions. {-# LANGUAGE DatatypeContexts #-} data SREG = RIP data DREG = RBX data SNDREG = RAX data (Source s, Destination d) => Instruction s d = MOV s d class Source a class Destination a instance Source SREG instance Source SNDREG instance Destination DREG instance Destination SNDREG move :: (Source s, Destination d) => s -> d -> Instruction s d move s d = MOV s d hello = [move RAX RAX, move RAX RAX] hello = [move RAX RAX, move RAX RBX] -- this is still not allowed. Regards, Kashyap

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graphical editor links ?
by Luc TAESCH 01 Apr '13

01 Apr '13

I am designing some graphical editor ( visio - dia like) and would be interested to hear any pointer to help me think on the architecture of this, in term of data structure/ EDSL/ persistence/ paradigm (FP/ FRP/...) for the different "layers of abstraction " I may see , like rendering libraries ( gtk/wx..)<-> model of the diagram<-> semantics ( this is a state diag/ archimate / mind mapping , i.e. what the glyph and lines means ) <-> persistence Any help & pointers welcomed ! ( and which place is best to discuss this ?) Luc

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Haskell SBV Package with Z3
by J. J. W. 01 Apr '13

01 Apr '13

Dear all, I have a small question regarding the installation of the SBV package. I first installed the SBV 2.10 package with cabal with the following instructions: cabal install sbv Next I installed the Z3 theorem prover and adding the path to my system variables (Windows 7 x64). Next I tested whether I could find it by opening cmd.exe and then typing z3, I get an error message of Z3, so I can safely assume the system can find it. (I added the path to the bin of Z3, I have not included the include directory, I see no reason why I should add a path to this directory, but maybe I am wrong). I ran the program: SBVUnitTests. However it had some errors in the beginning and afterwards a few failures. Having no idea how to fix this, I continued to check whether I can get the SBV to work. So I started to execute the SBV package: ghci -XScopedTypeVariables ghci> :m Data.SBV ghci> prove $ \(x::SWord8) -> x `shiftL` 2 .== 4*x Now this should return Q.E.D., however it returned the following: An error occured. Failed to complete the call to z3 Executable: "C:\\Program Files\\z3-4.3.0-x64\\bin\\z3.exe Options: /in /smt2 Exit code: 0 Solver output: =========================================================== ; :smt.mbqi ; :pp.decimal_precision =========================================================== Giving up.. It does seems like that the Z3 has a normal output, however not a result. Can someone help me to figure out what I actually did wrong? I am using Z3 version 4.3.0, SBV 2.10 and GHCi 7.4.2 Thank you for your help! Yours sincerely, Jun Jie

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