- Haskell-Cafe - Haskell.org

Data creation pattern?
by Eugene Dzhurinsky 13 May '10

13 May '10

Hello, all! I need to create objects like this data Object = MyObject { param1, param2, param3 :: String } from the input file param_1_param1=value11 param_2_param1=value21 param_2_param2=value22 param_1_param3=value13 param_2_param3=value23 param_1_param2=value12 so general pattern of recognizing parameter name and value is param_{id}_{property name}={property value} so I need to create function > parseDataFile :: [String] -> [Object] For now I can think on splitting the task into 2 functions > groupSameObjectParameters :: [String] -> [[String]] which will group the lines with same id into a list, and then apply function > createObject :: [String] -> Object so overall solution will be > parseDataFile :: [String] -> [Object] > parseDataFile = map createObject . groupSameObjectParameters however I have no neat idea about how to create instance of MyObject - I need to supply all of parameters to the constructor at once. Also I don't like the idea of rearranging list first and then create objects from another list, because the list can be relatively large. In imperative programming with mutable objects I would create an array, then for each line get the id and try to find if there is the object in the array at 'id' index. If no - create one. Then set appropriate property from value. Is it possible to do something similar in Haskell? Thank you in advance! -- Eugene N Dzhurinsky

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ANNOUNCE: yaml, data-object-yaml and data-object-json 0.3.0
by Michael Snoyman 13 May '10

13 May '10

Hi all, We're happy to announce the newest release of the three above packages. Notable changes include: * Anton Ageev wrote support for anchors and aliases into both the yaml and data-object-yaml package. * Anton also wrote support in data-object-yaml for merge keys. * The yaml package is now based on the iteratee package, replacing the previous cursor interface. This should hopefully make it easier to use, and perhaps more performant. * data-object-json API was modified to match (as much as possible) the data-object-yaml API. * In addition, there was a version bump for quite a number of the underlying libraries. Overall, data-object-yaml has not changed in any significant way for the user, and the data-object-json API changes should make it much more palatable. As always, feedback is welcome! Michael

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Major Emacs mode for reading GHC Core files
by Johan Tibell 13 May '10

13 May '10

Hi all, I wrote a minimal major mode for reading (and editing) GHC Core files. It provides syntax highlighting and removal of commonly ignored annotations, similar to what's offered by ghc-core. * Usage Dump the simplifier output in a file with a .hcr suffix: ghc -c -ddump-simpl -O2 Test.hs > /tmp/Test.hcr Open the .hcr file in Emacs. Use `ghc-core-clean-buffer` to remove commonly ignored annotations and namespace prefixes from the buffer. * Prerequisites You need to have haskell-mode installed. * Installation Download the file from http://johantibell.com/files/ghc-core.el and put it in your ~/.emacs.d directory. Add (load-file "~/.emacs.d/ghc-core.el") to ~/.emacs.d/init.el Cheers, Johan

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ANN: precis-0.3.1 - Cabal package diff tool
by Stephen Tetley 13 May '10

13 May '10

Hello all I've upload precis to Hackage - a diff tool for Cabal packages. It compares two cabal files looking at changes to the exposed and internal modules lists, then diffs the exposed modules. It only works on extant .hs files - i.e. it won't work on .hsc files. Also, I haven't tested if it works on .lhs files - parsing is left to haskell-src-exts. When it diffs new and old revisions of an exposed module it should report: - changes to the export list - changes to instance declarations - changes to exported data declarations - changes to the exported type signatures of exported functions The report it produces is not very pretty. A declaration is marked as *conflict* if its textual representation has changed - I compare the string output from pretty printing the new and old versions rather than look deeply into the AST. Though low-tech, this seems to work well in practice. I expect there are some bugs... Thanks to Malcolm Wallace for CppHs, and Niklas Broberg for haskell-src-exts. Best wishes Stephen

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corner case in Text.JSON 0.4.3
by Martin Hilbig 13 May '10

13 May '10

hi, since i got no answer from the maintainer, maybe someone else can take care of it, or at least point out, what i did wrong. so, i recently stumbled upon some error while using Text.JSON 0.4.3 [1]: Text/JSON/String.hs:(127,4)-(137,49): Non-exhaustive patterns in case indeed ghc warned: [5 of 7] Compiling Text.JSON.String ( Text/JSON/String.hs, dist/build/Text/JSON/String.o ) Text/JSON/String.hs:127:4: Warning: Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: [] from looking at the code i couldn't see how this would ever happen, but you can reproduce it be running the files from [2]: $ ./test < problem Ok (JSArray [JSString (JSONString {fromJSString = "this"}),JSString (JSONString {fromJSString = "is"}),JSString (JSONString {fromJSString = "some"}),JSString (JSONString {fromJSString = "json"}),JSObject (JSONObject {fromJSObject = [("that",JSString (JSONString {fromJSString = "works"}))]})]) test: Text/JSON/String.hs:(127,4)-(137,49): Non-exhaustive patterns in case the patch i put there fixes it (at least for me) to return an Error instead of dying: $ ./test < problem Ok (JSArray [JSString (JSONString {fromJSString = "this"}),JSString (JSONString {fromJSString = "is"}),JSString (JSONString {fromJSString = "some"}),JSString (JSONString {fromJSString = "json"}),JSObject (JSONObject {fromJSObject = [("that",JSString (JSONString {fromJSString = "works"}))]})]) Error "Unexpected end of String: does" Error "Malformed JSON: invalid token in this context not\"]" test: <stdin>: hGetLine: end of file have fun martin hilbig [1]: http://hackage.haskell.org/package/json [2]: http://friendpaste.com/3IvnChRMoczf0mIKpOtrYE

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Why is TChan GHC specific?
by Edward Amsden 12 May '10

12 May '10

I'm currently just getting into playing around with concurrency in haskell, primarily because I find STM intriguing. In looking through the docs I noticed that the transactional channels are GHC specific. http://hackage.haskell.org/packages/archive/stm/2.1.1.2/doc/html/Control-Co… I dug around the wiki and Google but I couldn't find the reason for this.

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ghc api renamed source
by Phyx 12 May '10

12 May '10

Hi all, I was wondering if it's possible to get to the renamed source before actually typechecking, I currently have parsed <- parse modName checked <- typecheckModule parsed let renamed = tm_renamed_source checked value = tm_typechecked_source checked but if typechecking failes, I get no information at all (an error), while I should be able to still get the renamed source. Any suggestions? Cheers, Phyx

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hSetEncoding on socket handles
by David Powell 12 May '10

12 May '10

Greetings, I am having trouble sending unicode characters as utf8 over a socket handle. Despite setting the encoding on the socket handle to utf8, it still seems to use some other encoding when writing to the socket. It works correctly when writing to stdout, but not to a socket handle. I am using ghc 6.12.1 and network-2.2.1.7. I can get it to work using System.IO.UTF8, but I was under the impression this was no longer necessary? I also don't seem to understand the interaction between hSetEncoding and hSetBinaryMode because if I set the binary mode to 'False' and the encoding to utf8 on the socket, then when writing to the socket the string seems to be truncated at the first non-ascii codepoint. Here is a test snippet, which can be used with netcat as a listening server (ie. nc -l 1234). > import System.IO > import Network > main = do > let a="λ" > s <- connectTo "127.0.0.1" (PortNumber 1234) > hSetEncoding s utf8 > hSetEncoding stdout utf8 > hPutStrLn s a > putStrLn a > hClose s Thanks, David

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Tsuru Capital is hiring
by Simon Cranshaw 12 May '10

12 May '10

We are looking to hire a programmer that has experience programming in a functional language, and is familiar with Haskell. There is a wide range of tools and applications that we want to build over time, including graphical interfaces and profiling tools for our trading system, parsers for new market feeds, and algorithms for trading strategies. If you have experience in any one of these areas it's a definite bonus. You will be working with a small team of developers, and from time to time will be working along side Traders to assist in tool development. Annual compensation will be in the region of USD150k with potential for growth. The team is based in Tokyo but there are projects that could be done remotely so feel free to apply for this type of work also. http://tsurucapital.com/en/jobs.html#programmer

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comprehending monad execution
by rick＠rickmurphy.org 12 May '10

12 May '10

All: I've been analyzing monads for a while now and have a question on execution. I suspect I've missed something basic. In the snippet below the ghc debugger shows that in just one single step Discriminate has been substituted for g. Even with :trace turned on in the debugger there is no evidence of an execution step to carry out the substitution. This leads me to assume a compile time operation identifies Discriminate as a substitute for g. Could someone correct my assumption or explain how this works? Note: this snippet was extracted from [1]. > {-# LANGUAGE EmptyDataDecls, Rank2Types #-} > module LEM where > import Control.Monad.Identity > > data F -- no constructors > > data Discriminate a b = L a | R b > instance Monad (Discriminate e) where > return = R > R x >>= f = f x > L x >>= f = L x > > h :: (forall m. Monad m => ((a -> m F) -> m F)) -> a > h g = case g (\x -> L x) of > R x -> error "Not reachable" > L x -> x > > -- A proof term for a -> NOT NOT a > lift :: forall a m. Monad m => a -> ((a -> m F) -> m F) > lift x = \k -> k x > > t = h $ lift True > ghci trace ... C:\haskell\lem>ghci lem.lhs GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling LEM ( lem.lhs, interpreted ) Ok, modules loaded: LEM. *LEM> :b h Breakpoint 0 activated at lem.lhs:(14,2)-(16,17) *LEM> :trace t Loading package mtl-1.1.0.1 ... linking ... done. Stopped at lem.lhs:(14,2)-(16,17) _result :: a = _ [lem.lhs:(14,2)-(16,17)] *LEM> :step Stopped at lem.lhs:(14,8)-(16,17) _result :: a = _ g :: (a -> Discriminate a F) -> Discriminate a F = _ [lem.lhs:(14,8)-(16,17)] *LEM> :hist -1 : h (lem.lhs:(14,2)-(16,17)) -2 : t (lem.lhs:22:6-18) <end of history> [lem.lhs:(14,8)-(16,17)] *LEM> My previous analysis includes running single step execution on Wadler's I monad here [2], where I am able to observe execution of the I monad. [essence.lhs:32:33-37] *Essence> :hist -1 : unitI (essence.lhs:7:34) -2 : unitI (essence.lhs:7:2-34) -3 : interp (essence.lhs:32:26-38) -4 : interp (essence.lhs:(31,2)-(39,44)) -5 : test (essence.lhs:54:41-51) -6 : showval (essence.lhs:(26,2)-(28,45)) -7 : showI (essence.lhs:9:34-42) -8 : showI (essence.lhs:9:2-42) -9 : test (essence.lhs:54:34-52) -10 : test (essence.lhs:54:2-52) <end of history> [1] http://okmij.org/ftp/Computation/lem.html [2] http://homepages.inf.ed.ac.uk/wadler/papers/essence/essence.ps Thanks much in advance for your help. Rick

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