- Haskell-Cafe - Haskell.org

ANNOUNCE: smt-lib-0.0.1
by Tom Hawkins 17 Jul '10

17 Jul '10

smt-lib [1] is a library for reading and writing SMT-LIB [2] files via Haskell. SMT-LIB is a common language and benchmark suite used by most SMT solvers. Currently the library supports the full SMT-LIB version 2 syntax. However at this time, only command scripts -- not responses -- can be parsed. Comments, bug reports, or suggestions for improvements welcome. -Tom [1] http://hackage.haskell.org/package/smt-lib [2] http://goedel.cs.uiowa.edu/smtlib/

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lambda calculus and equational logic
by Patrick Browne 17 Jul '10

17 Jul '10

Hi, In Haskell what roles are played by 1)lambda calculus and 2) equational logic? Are these roles related? Hopefully this question can be answered at a level suitable for this forum. Thanks, Pat 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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is this a bug ?
by gate03＠landcroft.co.uk 17 Jul '10

17 Jul '10

http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27631 The comments in the code explain the problem. If ".Lazy" be removed from the code (occurs three times), i.e., the code is changed to strict byte strings, it works as expected. Michael Mounteney.

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passing a polymorphic function as a parameter vs using it directly in a function definition
by Pasqualino "Titto" Assini 16 Jul '10

16 Jul '10

Hi, can anyone please explain why in the following code evalAST compiles while evalAST2 doesn't?: Is that because the polymorphic function k is specialised in two different ways in evalAST while in evalAST2 it is constrained to be the same function? {-# LANGUAGE GADTs #-} test = evalAST (TxtA "abc") -- This is OK evalAST :: AST -> IO () evalAST (IntA i) = k $ Lit i evalAST (TxtA i) = k $ Lit i k :: (Show a) => Expr a -> IO () k e = print $ eval e -- This is the same thing, only the k function is passed as a parameter. -- But it won't compile. -- I would expect its type to be: -- evalAST2 :: (Expr a -> IO()) -> AST -> IO () -- But is actually: -- evalAST2 :: (Expr Int -> IO ()) -> AST -> IO () evalAST2 k (IntA i) = k $ Lit i -- evalAST2 k (TxtA i) = k $ Lit i -- The untyped expression data AST = IntA Int | TxtA String -- A typed expression. data Expr a where Lit :: a -> Expr a eval :: Expr a -> a eval (Lit i) = i Thanks titto

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lists of arbitrary depth
by vadali 16 Jul '10

16 Jul '10

hello, iam really new to haskell, i want to define a function which takes as a parameter a list which can contain other lists, eg. [1,[2,3],[4,[5,6]]] how would i define a function that can iterate through the items so (in this example) iter1 = 1 iter2 = [2,3] iter3 = [4,[5,6]] ? ( can i do that without using the Tree data type? ) THANKS! -- View this message in context: http://old.nabble.com/lists-of-arbitrary-depth-tp29139641p29139641.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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trees and pointers
by Sergey Mironov 16 Jul '10

16 Jul '10

Hi cafe! I have a question of C-to-Haskell type:) Imagine web application wich allows users to browse some shared filesystem located at the server. Application stores every users's position within that filesystem (current directory or file). In C this can be implemented with the help of following data types: struct tree_node { union item { // some file data struct file *file; // struct dir has link to another list of tree_node struct dir *dir; }; int type; // List of tree_nodes struct tree_node *next; struct tree_node *prev; }; struct user { struct tree_node *position; // List of users struct user *next; struct user *prev; }; This implementation will give us 1) O(1) time to insert to shared tree 2) O(1) time to access user's current position Is it possible to reach this requirements in haskell? For example, managing distinct tree type like data TreeNode = File | Dir [TreeNode] will lead to failure of req. 2 (have to traverse this tree to find each user's position). Also one could manage several zipper types (one for every user): data TreeNodeCtx = Top | TreeNodeCtx { left :: [TreeNode], right :: [TreeNode], up :: TreeNodeCtx } data TreeNodeZ = TreeNodeZ { ctx :: [TreeNodeCtx] pos :: TreeNode } It works for one user but not for many because of req. 1 (have to insert new item into several zippers). Any ideas? -- Thanks, Sergey

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Re: [Haskell-cafe] Hot-Swap with Haskell
by austin seipp 16 Jul '10

16 Jul '10

Sorry Andy! CC'ing to the rest of -cafe in case anybody notices (I need to stop haskelling so early in the morning...) On Fri, Jul 16, 2010 at 8:59 AM, austin seipp <as(a)0xff.ath.cx> wrote: > You also may like one project I wrote, an IRC bot that used hs-plugins > to do hot code reloading (only works on GHC 6.8.) Inspired by > lambdabot, it could swap the entire running program as well as its own > plugins, on the fly and maintain state pretty successfully. I haven't > touched the source code in a long time - it was one of my first bigger > Haskell projects after reading Dons paper about yi. I tried a bunch of > things with various versions of it. > > If I wrote the code again today (providing plugins worked) it would > likely be 1/10th the size and more manageable as I have become much > better at Haskell, but oh well. Looking over it now though, most of > the code is pretty simple. :) > > http://code.haskell.org/infinity/src/ > > On Fri, Jul 16, 2010 at 8:04 AM, Andy Stewart <lazycat.manatee(a)gmail.com> wrote: >> Bartek Ćwikłowski <paczesiowa(a)gmail.com> writes: >> >>> Hello Andy, >>> >>> 2010/7/16 Andy Stewart <lazycat.manatee(a)gmail.com>: >>> >>>> There are some problems with re-compile solution: >>>> >>>> 1) You can't save *all* state with some FFI code, such as gtk2hs, you >>>> can't save state of GTK+ widget. You will lost some state after >>>> re-launch new entry. >>> >>> For my 2008 GSOC application I wrote a demo app that used hot code >>> reloading and it maintained gtk state just fine. The code (and video) >>> is available at http://paczesiowa.dw.pl/ , it worked in ghc 6.8 and >>> needed hs-plugins, so it won't work now, but the approach should work. >> Wow, thanks for your hint, i will wait dons fix hs-plugins then try this >> cool feature.. >> >> Thanks, >> >> -- Andy >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > > -- > - Austin > -- - Austin

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Haskell Platform dependency hell
by Kevin Jardine 16 Jul '10

16 Jul '10

I have Haskell Platform 2010.1.0.0 installed under Windows and I often find that Haskell breaks if I try upgrading some of the current modules. For example, after using cabal to upgrade to the latest version of Network.CGI, I can no longer compile any code and instead get this message: cabal.exe: dependencies conflict: ghc-6.12.1 requires array ==0.3.0.1 however array-0.3.0.1 was excluded because ghc-6.12.1 requires array ==0.3.0.0 The only way I have found to fix this problem in the past has been to uninstall the Haskell Platform and start over. Can anyone tell me: a. why does this occur, and b. what can I do to stop it from occurring again? Is it the case that the Haskell Platform is simply not upgradable and that I need to wait for the next release? Kevin

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ANNOUNCE: highlighting-kate 0.2.7
by John MacFarlane 16 Jul '10

16 Jul '10

I'm pleased to announce a new release of highlighting-kate [1], a syntax highlighting library based on syntax definitions from the kate editor. !! Important note !! The new release uses new, two-letter class names in its HTML output. If you use highlighting-kate, you will need to update your css files. Three sample css files can be found in the css directory of the source. This release also incorporates improvements to the kate syntax definition for Haskell by Nicolas Wu. There are two minor API changes: 1. A new function, languagesByFilename, has been added. This will give better results than languagesByExtension for things like "Makefile" or "CMake.txt". 2. A new formatting option, OptDetailed, has been added. Use this only if you need access to detailed, language-specific lexical information, and not generic styles. [1]: http://hackage.haskell.org/package/highlighting-kate

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point-free ADT pattern matching ?
by Alexey Karakulov 16 Jul '10

16 Jul '10

I wonder if pattern matching could be less verbose. Maybe this sounds weird, but here is example of what I mean: > type A = (Int, String) > > f :: String -> A -> A > f s (i,s') = (i, s ++ s') > > data B = B Int String deriving Show > >g :: String -> B -> B >g s (B i s') = B i $ s ++ s' Types A/B and functions f/g are quite similar: (x :: A) or (x :: B) means that x contains some integer and string values, and f/g functions take some string and prepend it to the string part of x. The code for f and g has the same level of verbosity, but -- ta-dah! -- we can use arrows and define f in a highly laconic manner: > import Control.Arrow > f' :: String -> A -> A > f' = second . (++) So my queastion is how I could define (g' :: String -> B -> B) in the same way.

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