April 2011 - Haskell-Cafe

Haskell from SML - referrential Transparency?!
by Gregory Guthrie 26 Apr '11

26 Apr '11

I am pretty new to Haskell, so need some clarification. I am porting some code from SML, and getting a result that surprises me. I basically have some functions which work like this: f1 = fa fb fc test1 = do print "test1:" f1 But I ran a few tests, and got odd results - so I ran the same test function twice, and got different results - that was my surprise. I did this: f1 = fa fb fc f2 = fa fb fc test2 = do print "test1:" f1 f2 and I get different results from the two executions (f1,f2), even though they have exactly the same definition. Reversing their order, gives the exact same results (i.e. the results are still different, and in the same original order as f2;f1). Even doing (f1;f1) gives two different results. Seems to me that by referential transparency, I should always get the same result from the function(s). So, I added some Debug.trace to the argument functions which are used, and I get a trace from the first call(s), but none from the second one(s), although I do get the result from each. It is as if because of the laziness, it someone cached some of the intermediate results, so did not re-invoke the functions. Anyway, totally confused. I must be missing something significant here. Thanks for any clarification! (The original code is a bit long, so I did not include here...)

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Data.Enumerator.Text.utf8 not constant memory?
by Skirmantas Kligys 25 Apr '11

25 Apr '11

Hi, I am learning iteratees, and as a starter project I wanted to use expat- enumerator to parse a 2 gigabyte XML file. I expected to be able to do what SAX does in Java, i.e. to avoid loading the whole 2 gigabytes into memory. For warm-up, I wrote an iteratee to count lines in the file, and it does load the whole file into memory! After profiling, I see that the problem was Data.Enumerator.Text.utf8, it allocates up to 60 megabytes when run on a 40 megabyte test file. Any suggestions how to fix Text.utf8, or what people do for parsing UTF-8 encoded text files with iteratees? Thanks! Here is my code and profiling results: http://i.imgur.com/XEI1v.png http://hpaste.org/46037/counting_lines_with_iteratees http://hpaste.org/46038/counting_lines_with_iteratees

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errors while installing yesod 0.8
by Michael Litchard 25 Apr '11

25 Apr '11

Trying to install yesod 0.8 breaks when it's time to install monad-control. Google wasn't very helpful, nor was the error message I received mlitchard@apotheosis:~/monad-control$ cabal install Resolving dependencies... Configuring monad-control-0.2.0.1... cabal: Error: some packages failed to install: monad-control-0.2.0.1 failed during the configure step. The exception was: ExitFailure 11 note: I've been trying to use the cab command to manage my packages, I get the same error as above when I use cab instead of cabal.I mention this just in case there is some unforseen problem having to do with cab/cabal interaction. Has anyone experienced this problem, or know what I can do to get more useful error messages that might reveal the cause of the breakage?

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Storing failing testcases for QuickCheck
by Sönke Hahn 25 Apr '11

25 Apr '11

Hi! I would like to have a library that would allow to use QuickCheck in the normal manner, but it would save test data for failing properties on the filesystem (maybe using the shiny new acid-state?). On consecutive test runs, the saved test data would be used first (before generating new arbitrary test data). I think, this would come in quite handy when doing test driven development. Any comments on this? Does something like this already exist? Cheers, Sönke

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Trouble with currying and uncurrying...
by Angel de Vicente 25 Apr '11

25 Apr '11

Hi, I'm reading "The Craft of Functional Programming" and I found something I don't understand in page 185. It says: "Suppose first that we want to write a curried version of a function g, which is itself uncurried and of type (a,b) -> c. curry g This funtion expects its arguments as a pair, but its curried version, curry g, will take them separately - we therefore have to form them into a pair before applying g to them: curry :: ((a,b) -> c) -> (a -> b -> c) curry g x y = g (x,y) curry multiplyUC will be exactly the same function as multiply." OK, I have tried it and it works, but I don't understand the syntax for curry. Until now I have encountered only functions that take the same number of arguments as the function definition or less (partial application), but this syntax looks a bit new to me. curry is supposed to have as its argument one function of type (a,b) -> c and produce another function, but then the second line gives three arguments to curry, the function itself and the variables x and y. What I'm missing here? Thanks a lot, Ángel de Vicente -- http://www.iac.es/galeria/angelv/ High Performance Computing Support PostDoc Instituto de Astrofísica de Canarias --------------------------------------------------------------------------------------------- ADVERTENCIA: Sobre la privacidad y cumplimiento de la Ley de Protección de Datos, acceda a http://www.iac.es/disclaimer.php WARNING: For more information on privacy and fulfilment of the Law concerning the Protection of Data, consult http://www.iac.es/disclaimer.php?lang=en

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A parallelization problem
by wren ng thornton 24 Apr '11

24 Apr '11

Hello all, I have some loopy code I'd like to make parallel, but I'm not sure the best way to go about it. Everywhere else in this program I'm using STM, but that seems pretty heavy-handed for this task, and it looks like the perfect place for `par` and friends. So the 10,000ft view of the code is something like: foldl' step (0,state0) xs step (n,stateN) x = (n+1, k stateN) where k = case lookup n stateN of Nothing -> error "oh noes" Just substateN -> insert (n+1) $ foldl' maybeInsert empty (views substateN x) maybeInsert state (k,v) | p (k,v) = state | otherwise = insert k v state That is, for each x that comes in we add a bunch of things to a map datastructure. The thing that makes the inner loop parallelizable is that we happen to know that none of the new keys will (a) conflict with old keys (as indicated by the fact that we're nesting maps), (b) conflict with the other new keys, nor (c) depend on the other new key/value pairs. Thus, we know that we can reassociate the order of insertions without changing the semantics. So the question is, how do I do it? One issue not apparent in this high-level look at the code is that I'm actually using ST for doing the inner loop efficiently, which is why I'm not just using fromList. So far I'm using an IntMap for storing the state. The STM approach would be to store the IntMap in a TMVar and fire off threads to compute and insert each one of the views. But even as lightweight as Haskell's threads are, I worry that the overhead would swamp the benefits. (Of course, it may be viable to push the ST down far enough to use (fromList . catMaybes). Then we could just construct the list and call par on all the elements. That wouldn't do much to parallelize the insertions themselves, but that shouldn't matter too much I hope. Perhaps I've just answered my own question; any other ideas?) -- Live well, ~wren

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generics and sql
by nadine.and.henry＠pobox.com 24 Apr '11

24 Apr '11

Dear Group, Greetings. I have a feeling that what I am trying to do is easy, I just don't know how to say it in Haskell. Let's start with: > {-# LANGUAGE > DeriveDataTypeable, GeneralizedNewtypeDeriving #-} > > import Database.HDBC > import Data.Typeable (Typeable) > import Data.Data > data C = C { str :: String, dbl:: Double } > deriving (Eq, Ord, Typeable, Data) > > a :: C > a = C "twelve" 12.0 > Now I load this up in ghci and I can do the following: toSql . str $ a -- result: SqlString "twelve" toSql . dbl $ a -- result: SqlDouble 12.0 but what I would really like to do is something like: gmapQ toSql $ a which results in: <interactive>:1:7: Could not deduce (Convertible d SqlValue) arising from a use of `toSql' from the context (Data d) bound by a type expected by the context: Data d => d -> SqlValue at <interactive>:1:1-11 Possible fix: add (Convertible d SqlValue) to the context of a type expected by the context: Data d => d -> SqlValue or add an instance declaration for (Convertible d SqlValue) In the first argument of `gmapQ', namely `toSql' In the expression: gmapQ toSql In other words, I'm looking for a function with a signature: (Whatever Instances I neeed here) => a -> [SqlValue] I have tried various incantations of type signatures, but thus far I can't get it right. Can someone point me in the right direction? Thanks. Henry Laxen -- Nadine & Henry Laxen Belle, Venus, Aphrodite 10580 N. McCarran Blvd. Adonis, Miss Parker & Jarod Suite 115-396 Via Alta # 6 Reno, Nevada Chapala, Jalisco, Mexico 89503-1896 CP 45900 The rest is silence. (Hamlet)

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How to use cabal's data-files feature and run in-place?
by Richard Cobbe 24 Apr '11

24 Apr '11

I'm running Haskell Platform 2011.2.0.1 on a MacOS 10.6.7 machine (the machine is 64-bit, but I'm running the 32-bit platform). I'm writing an application for personal use, and I'd like to use Cabal to package it up and handle installation. This way, when I'm working on the program, I won't break the reasonably stable installed version and can continue to use it. (I'm not committed to cabal by any means; suggestions for other ways to accomplish the same thing are more than welcome!) I've got some data files I'd like to package with the application, and Cabal's data-files field is the obvious way to do that. The immediate problem, then, is how to access the files -- I can use the generated Paths_pkg module for the "installed" copy of the app, but then I can't run the thing in-place in ghci in the development directory. I did some googling and came across a blog post (http://neilmitchell.blogspot.com/2008/02/adding-data-files-using-cabal.html) which suggested that I provide my own Paths_pkg.hs file that points to the files' location in the development copy. When I build with Cabal, the generated form of this module would override my hand-written one, and it should therefore work in the installed case as well. Unfortunately, that's not happening. Cabal is clearly generating the module; I can see it in dist/build/autogen. But my copy is overriding the autogenerated one, even for cabal builds -- at least, that's what I'm seeing when I run the binary out of dist/build/<package>/<executable>. Is there a way to be able to use data files in both contexts? FWIW, I'm running cabal as runhaskell Setup.hs configure --user runhaskell Setup.hs build Thanks much, Richard

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http://trac.haskell.org/ not redering correctly
by Erik de Castro Lopo 24 Apr '11

24 Apr '11

Hi all, Trac pages aren't rendering correctly. It seems the HTTP server can't find the CSS files. See below. Cheers, Erik erikd > wget --proxy=off http://trac.haskell.org/trac/chrome/common/css/ticket.css -O /dev/null --2011-04-17 20:11:53-- http://trac.haskell.org/trac/chrome/common/css/ticket.css Resolving trac.haskell.org... 178.63.91.44, 2a01:4f8:121:6::1:10 Connecting to trac.haskell.org|178.63.91.44|:80... connected. HTTP request sent, awaiting response... 404 Not Found 2011-04-17 20:11:56 ERROR 404: Not Found. erikd > wget --proxy=off http://trac.haskell.org/trac/chrome/common/css/trac.css -O /dev/null --2011-04-17 20:28:36-- http://trac.haskell.org/trac/chrome/common/css/trac.css Resolving trac.haskell.org... 178.63.91.44, 2a01:4f8:121:6::1:10 Connecting to trac.haskell.org|178.63.91.44|:80... connected. HTTP request sent, awaiting response... 404 Not Found 2011-04-17 20:28:40 ERROR 404: Not Found. -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Berlin Haskell Meeting
by Sönke Hahn 23 Apr '11

23 Apr '11

Hi all! The next Berlin Haskell Meeting will be next wednesday: Date: Wednesday, April 27th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin I hope, this is not on too short notice. Cheers, Sönke

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