July 2010 - Haskell-Cafe

Plotting Vectors with GnuPlot Wrapper
by Maryam Moghadas 17 Jul '10

17 Jul '10

Hi When I use Vectors as a PlotStyle in Graphics.Gnuplot.Simple, the output curve.gp and curve0.csv is not generated correctly. For example when I write in ghci: plotPathStyle [] (PlotStyle Vectors (DefaultStyle 1)) [(1,1),(2,7)] the output files are: curve.gp: plot "curve0.csv" using 1:2 with vectors linestyle 1 curve0.csv: 1, 1 2, 7 But they must be: curve.gp: plot "curve0.csv" using 1:2:3:4 with vectors linestyle 1 curve0.csv: 1, 1, 2, 7 Is there any option that should I use to correct this behavior? Regards Mary

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Hot-Swap with Haskell
by Andy Stewart 17 Jul '10

17 Jul '10

Hi all, I'm research to build a hot-swap Haskell program to developing itself in Runtime, like Emacs. Essentially, Yi/Xmonad/dyre solution is "replace currently executing" technology: re-compile new code with new binary entry when re-compile success $ do save state before re-launch new entry replace current entry with new binary entry (executeFile) store state after re-launch new entry 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. 2) Sometimes re-execute is un-acceptable, example, you running some command in temrinal before you re-compile, you need re-execute command to restore state after re-launch, in this situation re-execute command is un-acceptable. I wonder have a better way that hot-swapping new code without re-compile/reboot. Thanks, -- Andy

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in-equality type constraint?
by Paul L 17 Jul '10

17 Jul '10

Does anybody know why the type families only supports equality test like a ~ b, but not its negation? -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale

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ANNOUNCE: feldspar 0.3
by Emil Axelsson 17 Jul '10

17 Jul '10

On behalf of the Feldspar team, I'm happy to announce a new release of the embedded language Feldspar and its C code generator: http://hackage.haskell.org/package/feldspar-language http://hackage.haskell.org/package/feldspar-compiler The main changes in 0.3 are: * Signed/unsigned integers of different bit widths (8, 16 and 32). * Fixed-point numbers. * Slight change of the vector library interface: All intermediate vectors are guaranteed to be removed. * A new Stream data type. * Support for user-defined types. * Tracing functions. * Limited support for code generations for the TMS320C64x chip family. * Bugfixes. For more information, see: http://feldspar.inf.elte.hu/feldspar/ / Emil

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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 16 Jul '10

16 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 16 Jul '10

16 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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