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parsec-3 problem
by Christian Maeder 19 Feb '11

19 Feb '11

Hi, the following code goes through parsec-2 (and parsec1) but is rejected by parsec-3 import Text.ParserCombinators.Parsec gqrrel = many1 (alphaNum <|> char '_') myparser = sepBy gqrrel (char ',') The error message is: No instance for (parsec-3.1.1:Text.Parsec.Prim.Stream s m Char) arising from a use of `char' at P3.hs:3:25-32 Possible fix: add an instance declaration for (parsec-3.1.1:Text.Parsec.Prim.Stream s m Char) In the second argument of `sepBy', namely `(char ',')' In the expression: sepBy gqrrel (char ',') In the definition of `myparser': myparser = sepBy gqrrel (char ',') Please review your decision to put parsec-3 into the HP. Cheers Christian

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Data.HashMap: Strict or lazy by default?
by Johan Tibell 19 Feb '11

19 Feb '11

Hi all, I'm about to release my new unordered-containers library, which includes a Data.HashMap module. I have one remaining design issue I'd like to get some feedback on. Here are the constraints: * There are a number of functions that need both lazy and strict versions. Example: insertWith * There are a number of functions that don't need a strict version, but where having a strict version would help avoid space leaks and support the common strict use case. Example: insert * A particular use case for a map rarely uses both the lazy and strict versions (suggesting a module split). * Keeping both the strict and lazy versions in the same module makes the module very large and users typically want to see only half of the functions in there (the ones that fit their current use case). My current thinking is to provide lazy and strict versions in different modules. The two modules could still share the same data type. The question is, what should be the default? Haskell is a lazy language but the most common use cases for maps are strict (e.g. a map from strings to integer counters). I intend to to use one of three possible module layouts: Option 1: Data.HashMap (strict) Data.HashMap.Lazy Option 2: Data.HashMap (lazy) Data.HashMap.Strict Option 3: Data.HashMap (re-export one of the two modules below) Data.HashMap.Lazy Data.HashMap.Strict I'd like to see some arguments for and against each of the two possible defaults (lazy or strict). Also, real life examples of cases where lazy maps are useful would be appreciated. Cheers, Johan

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Moving Foldable.foldl' inside the type class
by Johan Tibell 18 Feb '11

18 Feb '11

Hi all, It would be nice if foldl' was moved inside the Foldable type class. Right now you get terrible performance if you use foldl' from Foldable. Ironically it'll also use O(n) space. Someone should move the function inside the type class. Johan

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Proposal: Add MonadFix instance to STM
by Sebastiaan Visser 17 Feb '11

17 Feb '11

Hi all, The STM monad currently has no MonadFix instance, but wonderful things are possible when it has one. I propose adding the 'MonadFix STM' instance provided by Antoine Latter on the Haskell-Cafe[1] list to the STM package: > {-# LANGUAGE MagicHash, UnboxedTuples, DoRec #-} > > import GHC.Exts > import GHC.Conc > import Control.Monad.Fix > > data STMret a = STMret (State# RealWorld) a > > liftSTM :: STM a -> State# RealWorld -> STMret a > liftSTM (STM m) = \s -> case m s of (# s', r #) -> STMret s' r > > instance MonadFix STM where > mfix k = STM $ \s -> > let ans = liftSTM (k r) s > STMret _ r = ans > in case ans of STMret s' x -> (# s', x #) Discussion Period: 1 week -Sebastiaan Visser [1] http://www.haskell.org/pipermail/haskell-cafe/2011-February/089226.html

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ANN: bytestring-trie 0.2.3
by wren ng thornton 12 Feb '11

12 Feb '11

-------------------------------------------- -- bytestring-trie 0.2.3 -------------------------------------------- A long-awaited release for efficient finite maps from (byte)strings to values. This version adds a number of new functions for taking advantage of the trie structure. At the Haskell Symposium 2010, Milan Straka presented a wonderful paper comparing the state of the @containers@ library pre-GHC7. In it he compared the new @hashmap@ against the buggy old @bytestring-trie@ 0.1.4 (the major bug was fixed in 0.2.2 released 2010-06-10), and I've been meaning to update the package ever since. I haven't had a chance to do an extensive performance analysis yet, but it's worth clearing up some misconceptions--- which is what this release is all about. If you are only interested in being able to associate strings to values, then you may prefer the @hashmap@ package which is faster for those only needing a map-like structure. This package is intended for those who need the extra capabilities that a trie-like structure can offer (e.g., structure sharing to reduce memory costs for highly redundant keys, taking the submap of all keys with a given prefix, contextual mapping, extracting the minimum and maximum keys, etc.) While hacking on another project I noticed that @bytestring-trie@ didn't actually export many functions taking advantage of the trie structure. So let's fix that! -------------------------------------------- -- Changes (available in 0.2.2, but not announced previously) -------------------------------------------- * Added some functions for treating tries like priority queues (minAssoc, maxAssoc, updateMinViewBy, updateMaxViewBy). Currently these are only exported by Data.Trie.Internal though future releases may re-export them from Data.Trie too. -------------------------------------------- -- Changes (since 0.2.2) -------------------------------------------- * Added Data.Trie.Internal.alterBy_: A variant of 'alterBy' which also allows modifying the sub-trie. This is useful for cases where you want to alter a number of values which are each prefixes of one another (e.g., "a", "ab", "abc",...). * Added a number of "contextual" mapping functions (contextualMap, contextualMap', contextualFilterMap, contextualMapBy) which give access to the sub-trie rooted at each value. Currently these are only exported by Data.Trie.Internal though future releases may re-export them from Data.Trie too. * Added strict variants of a number of functions in Data.Trie.Convenience (fromListWith', insertWith', insertWithKey', unionWith'). * Added some additional functions for building tries from association lists (fromListWith', fromListWithL, fromListWithL') as suggested by Ian Taylor. * Converted the definitions for fmap, foldMap, traverse, and filterMap to use the worker/wrapper transform. This appears to be an optimization, though I don't have a decent benchmarking suite for verifying this. * Lots of tweaks and improvements to the documentation. -------------------------------------------- -- Future work -------------------------------------------- * I'm still not leased with the situation of having so many different variants of fromList. There must be a nicer solution which generalizes over all of them and doesn't have the various pessimal corner cases... * The priority queue functions still need some work to make them general as well as comprehensive like the other functions in Data.Trie.Internal are. * I still need to do a thorough performance analysis. The code for Data.Trie was based very closely on the code for Data.IntMap. Milan Straka found a number of ways to optimize the Data.IntMap code, so I need to incorporate those into the Data.Trie code. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/bytestring-trie Darcs: http://community.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://community.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring… -- Live well, ~wren

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Data.Text for parsec3
by Kazu Yamamoto 10 Feb '11

10 Feb '11

Hello, I would like to add Data.Text support to parsec3. The following is a sample implementation. https://github.com/kazu-yamamoto/parsec3/commit/58c268c37e54a470c841c3ef5ec… If there is no problem, please merge it to parsec3. Regards, --Kazu

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Proposal: Add chop function to Data.List
by Lennart Augustsson 09 Feb '11

09 Feb '11

I would like to propose the following function for inclusion in Data.List chop :: (a -> (b, [a]) -> [a] -> [b] chop _ [] = [] chop f as = b : chop f as' where (b, as') = f as It's commonly occuring recursion pattern. Typically chop is called with some function that will consume an initial prefix of the list and produce a value and the rest of the list. The function is clearly related to unfoldr, but I find it more convenient to use in a lot of cases. Some examples ------------- -- From Data.List group :: (Eq a) => [a] -> [[a]] group = chop (\ xs@(x:_) -> span (==x) xs) -- From Data.List words :: String -> [String] words = filter (not . null) . chop (span (not . isSpace) . dropWhile isSpace) -- From Data.List lines :: String -> [String] lines = chop ((id *** dropNL) . span (/= '\n')) where dropNL ('\n':s) = s; dropNL s = s -- From Data.List tails :: [a] -> [[a]] tails = (++ [[]]) . chop (\ xs@(_:xs') -> (xs, xs')) -- From Data.List map f = chop (\ (x:xs) -> (f x, xs)) -- Split a list into a list of list with length n. splitEveryN n = chop (splitAt n) -- Simple Haskell tokenizer tokenize = chop (head . lex) History ------- I first encountered this function around 1981 when I was talking to Sören Holmström about this recursion pattern and he said that he had also observed it and he called the function chopList. Ever since then I've used chopList a lot, but unfortunately I always have to make my own definition of this common function.

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Proposal: Deprecate Control.Exception.blocked in favour of getMaskingState
by Bas van Dijk 08 Feb '11

08 Feb '11

Dear all, I would like to propose deprecating 'Control.Exception.blocked' in favour of 'getMaskingState'. The term "blocked" refers to the old way of dealing with asynchronous exceptions (block/unblock). It can also be a bit confusing to which "blocked" property it refers: blocked asynchronous exceptions, or a blocked thread. Note that 'blocked' is already listed in the section "(deprecated) Asynchronous exception control": http://hackage.haskell.org/packages/archive/base/4.3.0.0/doc/html/Control-E… We could add an equivalent function with an unambiguous and more consistent name: masked :: IO Bool masked = fmap (/= Unmasked) getMaskingState However, I don't think it will be used enough to warrant its addition. I believe the proposal is clear enough, so I will make a patch when consensus is reached. Discussion deadline: 2 weeks (19 February). Bas

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Proposal: Applicative => Monad: Is there any consensus?
by John Smith 08 Feb '11

08 Feb '11

There has been much discussion on this proposal, much of it orthogonal to the actual patch. The discussion deadline has passed, but it is difficult to gauge whether consensus has been reached or not. Can we have a final yay or nay? The patch is attached to http://hackage.haskell.org/trac/ghc/ticket/4834

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Cabal problem: cannot satisfy package-id ...
by Alistair Bayley 08 Feb '11

08 Feb '11

(Is this the best place for cabal help?) I'm trying to install Takusen and build the tests on an Ubuntu 10.4 machine, with haskell-platform 2010.1.0.0.1. Takusen itself built and installed correctly. When building the tests I get this failure: Building Takusen-0.8.7... Building executable takusen_tests... Creating dist/build/takusen_tests (and its parents) Creating dist/build/takusen_tests/takusen_tests-tmp (and its parents) /usr/bin/ghc --make -o dist/build/takusen_tests/takusen_tests -hide-all-packages -fbuilding-cabal-package -no-user-package-conf -package-conf dist/package.conf.inplace -i -idist/build/takusen_tests/takusen_tests-tmp -i. -idist/build/autogen -Idist/build/autogen -Idist/build/takusen_tests/takusen_tests-tmp -optP-DUSE_NEW_EXCEPTIONS -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build/takusen_tests/takusen_tests-tmp -hidir dist/build/takusen_tests/takusen_tests-tmp -stubdir dist/build/takusen_tests/takusen_tests-tmp -package-id QuickCheck-2.1.0.3-c608a0a2c555bdf292cfea5b3273ac6b -package-id base-4.2.0.0-2cc27b7e43511c4ca001642a7f77a8f6 -package-id mtl-1.1.0.2-23ca4de7c573fd4df309067199a40634 -package-id old-time-1.0.0.3-9efceb2bb905784e503d6cf147359f62 -package-id random-1.0.0.2-cf650c34bae72c01e165a0d26c11236b -package-id time-1.1.4-1ced433acd576e314843b083f147bc38 -package-id Takusen-0.8.7 -O -XCPP -XDeriveDataTypeable -XScopedTypeVariables ./Main.lhs <command line>: cannot satisfy -package-id Takusen-0.8.7 (use -v for more information) (and that was the -v output) There's no clue as to why "-package-id Takusen-0.8.7" cannot be satisfied. "ghc-pkg list" shows the package (in the global database), as does "cabal list Takusen". Thanks, Alistair

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