- Haskell-Cafe - Haskell.org

Generating test data in QuickCheck
by Janek S. 15 Oct '12

15 Oct '12

I'm writing tests using QuickCheck to test data types that are not an instance of QC's Arbitrary. To prevent orphan instances I use forAll function instead of instantiating these types as Arbitrary. The problems arise when I need to generate several different variables for one test. To do this I either nest calls to forAll or create a pair generator like this: genPair = liftM2 (,) (myGenerator1) (myGenerator2) Running time of such tests is very long (both with nesting forAll and using pair generator) and it gets shorter when I resize myGenerator1 and myGenerator2. This leads me to conslusion that, due to nesting of generators QC is generating 100*100*100 test cases instead of 100. How do I "flatten" my generators to always get 100 test cases (or whatever the default number of test is)? Janek

1 0

NYC Haskell Developer Wanted
by Doug Beardsley 15 Oct '12

15 Oct '12

Karamaan Group, a principal investment firm based in Manhattan is looking for an outstanding software developer to develop tools for financial analysis and knowledge management. Candidates should have at least a bachelor's degree in computer science and a good sense of coding style. This is a high-impact, high-visibility job opportunity where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. Karamaan Group is a small hedge fund, so the next hire will have a very direct influence on our software development and the firm as a whole. We are a growth oriented firm that values people who take a craftsman's pride in their work. We like to take a contrarian view of things, value experimenting, and use often overlooked technologies. We are looking for a candidate who shows strong analytical, organizational, and communication skills. We have a legacy Java code base that may need occasional maintenance, but for the past few years, most of our new development has been done with Haskell (with bits and pieces of a few other languages here and there). The ideal candidate would have at least some experience with Java and the related technologies used in our existing code, but since Haskell is our language of choice, we're looking for strong Haskell skills. We have a knowledge management system built on Snap, so experience with Snap or Haskell web development in general would help. Unlike most finance companies, we don't require previous experience in finance or business, but knowledge in those areas is definitely a plus. Please send your CV and cover letter to recruitment at karamaan.com.

2 1

Interleaving type variables in a tree, has this a name ?
by Eric Dedieu 14 Oct '12

14 Oct '12

Hi Haskell Helpers, For a hobby projet to learn haskell, I'm trying to use this kind of interleaved tree structure (simplified): > data ANode a b = ANode a [BNode a b] [BNode a b] > data BNode a b = BNode b [ANode a b] [ANode a b] The first list of each node is for links "up", the second for "down". I can navigate with functions like: > type UpRank = Int -- index in the list of nodes > type DownRank = Int > > upA :: UpRank -> ANode a b -> Maybe (BNode a b, DownRank) > downB :: DownRank -> BNode a b -> Maybe (ANode a b, UpRank) Such as going first up then down leads me to the same point (actually this simplified structure is not enough to do this, but is enough to show my problem simply). Thee are also of course upB and downA. >From this idea I have devised a kind of zipper to navigate the tree, add nodes, and be able to step back. However, with this pattern of interleaving a and b values, I could not help but duplicating in a way every data or function declaration, to apply from an A or a B, and often code is very similar (like for upA and downB). Also, I want to go through the tree and modify it keeping the same structure, applying different functions to A and B nodes, like a "map" involving a pair of functions that will be applied alternatively. I also want a kind of "fold". This leads very often to pairs or Either: > (a -> a, b -> b) > (ANode a b -> ANode a b, BNode a b -> BNode a b) > Either (ANode a b) (BNode a b) Out of curiosity, is all this (duplicating functions for alternating type variables, using pairs) a known pattern ? Does it have a name ? Are there well known (and less tedious) alternatives, or higher-level workarounds ? Thanks for any hint, Eric

2 2

Tutorial: Haskell for the Evil Genius
by Andrew Pennebaker 14 Oct '12

14 Oct '12

I've gotten mixed feedback from Reddit for my tutorial. It provides an overview of how functional and declarative programming in Haskell empower baddies, increasing the accuracy and efficiency of their atomic superweapons. What do you guys think of my tutorial, Haskell for the Evil Genius <http://www.yellosoft.us/evilgenius/>? Cheers, Andrew Pennebaker www.yellosoft.us

16 30

Re: [Haskell-cafe] character encoding problems with hxt (I think)
by Uwe Schmidt (FH Wedel) 14 Oct '12

14 Oct '12

Hi Henry, it's not an encoding error but an error concerning the validation of the document. The document "/usr/share/xml/iso-codes/iso_3166_2.xml" is not valid with respect to its internal DTD. The opening tag <iso_3166_country code="GH"> on line 3711 does not have a closing tag. That closing tag should be inserted directly in front of  <iso_3166_country code="GL" /> on line 3735. Further the "/" for an empty element for the "GL" code must be removed. So it's not an error in HXT, but HXT has found an error in the input file. The input file is not valid XML. Solution: Turn off validation and live with the wrong structure of that document, or correct the XML file (and give the maintainers of that file a bug report). But with the error message HXT gave you, there is some room for improvements. Cheers, Uwe -- Uwe Schmidt FH Wedel

1 0

2 line finite (monadic) state machine
by Izaak Meckler 13 Oct '12

13 Oct '12

Hi all, I just wrote some nice code: A finite state machine with monadic state, allowing both DFAs and NFAs to be expressed with the same functions. I've only used it in [], Set, and Maybe, but I think it would be interesting in several others (a probability monad comes to mind). Also, if anyone has any sensible interpretation of this in the continuation monad, let me know. import Control.Monad import Data.Maybe type State = String type Map a b = [(a, b)] -- In Map State (Map Char (m State)), the monad m determines the kind of FSM that is being run. -- If m = [] (or Set), these functions work as a NFA. If m = Maybe, we essentially have a DFA. transition :: (MonadPlus m) => Map State (Map Char (m State)) -> State -> Char -> m State transition transMap q c = fromMaybe mzero $ lookup q transMap >>= lookup c toFSM :: (MonadPlus m) => Map State (Map Char (m State)) -> State -> (String -> m State) toFSM transMap q0 = foldM (transition transMap) q0 egMachine = toFSM [("p", [ ('0', ["p"]) , ('1', ["p", "q"])])] "p"

1 0

character encoding problems with hxt (I think)
by nadine.and.henry＠pobox.com 13 Oct '12

13 Oct '12

Dear Haskellers, I'm trying to write some code that grabs countries and provinces from the iso_3166 files on Linux systems. I seem to be running into some kind of character encoding problem. file says iso_3166_2.xml is a utf8 file, and isutf8 agrees, but when I run the following code, it crashes. uft8Copy makes a byte for byte copy as expected. noCrash read and writes the document without crashing, but the accented characters in the strings show up garbled. Just search for "DE" and you'll see what I mean. crash (on my system, (Debian testing)) produces the error message below. Can anyone enlighten me on what is going on? Thanks in advance. Henry Laxen ------------------------------------------------------------------------ {-# LANGUAGE Arrows #-} import Text.XML.HXT.Core import Data.List import qualified System.IO.UTF8 as U isoFile = "/usr/share/xml/iso-codes/iso_3166_2.xml" countZerosInLines = length . filter (\x -> x == '0') . concat utf8Copy = do xml <- U.readFile isoFile let doc = readString [withValidate no] xml U.writeFile "iso_written.xml" xml putStrLn "Ran utf8Copy" noCrash :: IO () noCrash = do lines <- runX (readDocument [withValidate no] isoFile >>> writeDocumentToString [withShowTree yes] ) -- mapM_ putStrLn lines print $ countZerosInLines lines putStrLn "Ran noCrash" crash :: IO () crash = do xml <- U.readFile isoFile let doc = readString [withValidate no] xml src <- runX $ doc //> writeDocumentToString [withShowTree yes] print $ countZerosInLines src putStrLn "Ran crash (didn't)" {- Ran utf8Copy 1236 Ran noCrash error: UTF-8 encoding error at input position 2640: InvalidLaterByte 1 error: UTF-8 encoding error at input position 2646: InvalidLaterByte 1 *** Exception: Enum.toEnum{Word8}: tag (363) is outside of bounds (0,255) -} main = do utf8Copy noCrash crash ------------------------------------------------------------------------

1 0

Type of scramblings
by Roman Cheplyaka 12 Oct '12

12 Oct '12

I am reading through Oleg's "Eliminating translucent existentials"[1]. [1]: http://okmij.org/ftp/Computation/Existentials.html#eliminating-translucent He draws a distinction between forall a . [a] -> [a] and forall a . [a]^n -> [a] as types of "scramblings". This is something I'm struggling to understand. First of all, I think here we're talking about total functions, otherwise there's no point in introducing dependent types. There are of course more total functions of type `[a]^n -> [a]` than of type `[a] -> [a]`, in the sense that any term of the latter type can be assigned the former type. But, on the other hand, any total function `f :: [a]^n -> [a]` has an "equivalent" total function g :: [a] -> [a] g xs | length xs == n = f xs | otherwise = xs (The condition `length xs == n` can be replaced by a similar condition that also works for infinite lists.) The functions `f` and `g` are equivalent in the sense that for any list `xs` of length `n` `f xs === g xs`. Thus, even though it seems that we allow more total functions by replacing `[a]` with `[a]^n`, that doesn't buy us any additional expressiveness. What am I missing? Roman

3 2

Using CatchIO on StateT
by Dmitry Vyal 12 Oct '12

12 Oct '12

Greetings, I found The MonadCatchIO-mtl package while looking for a way to catch exceptions in my custom StateT s (ReaderT r IO) monad. CatchIO worked flawlessly when handlers used the environment r from Reader, but things broken when I tried to use state. According to source, the handler is run with the same state which was passed to runStateT. instance MonadCatchIO m => MonadCatchIO (StateT s m) where m `catch` f = StateT $ \s -> (runStateT m s) `Control.Monad.CatchIO.catch` (\e -> runStateT (f e) s) And my finalizing action depends on the current state. Can you give an advice how to get the behavior I need? Best regards, Dmitry Vyal

2 1

Hackage Documentation issue
by Jonas Almström Duregård 12 Oct '12

12 Oct '12

Hi, Short question: can I or someone else make the Hackage build bot rebuild a package with a new dependency (even though the initial build was successful)? Long question: Does anyone know what would cause the documentation of a successful Hackage build not to appear on the web? Specifically I'm trying to get docs working for: http://hackage.haskell.org/package/BNFC-meta-0.4 Doing cabal haddock works well for me, and I managed to find the log of the Hackage build: http://hackage.haskell.org/packages/archive/BNFC-meta/0.4/logs/success/ghc-… The problem seems to be related to one of the dependencies (Alex-meta) which had a similar problem: the documentation step failed (but not the build). I was able to fix it (or at least work around it) so Alex-meta has working documentation now. Can I or someone else prompt the BNFC-meta documentation to rebuild with the new dependency (even though the initial build was successful)? I wouldn't want to upload a new version just to get docs rebuilt, and i'm not even sure it solves the issue. Regards, Jonas

1 0