- Haskell - Haskell.org

ANN: hdbc-missingh
by John Goerzen 29 Dec '05

29 Dec '05

Hello, I'm pleased to announce the first release of HDBC-MissingH, a library to add database features to MissingH. At the present time, this allows any HDBC database to act as a backend for the MissingH.AnyDBM architecture. As such, you can use a SQL database as storage for a simple DBM-like key/value interface. This is particularly interesting with Sqlite3, which is an embedded SQL engine that stores its data in a single file (similar to *dbm). Download from: http://quux.org/devel/hdbc Darcs repo: darcs get --partial http://darcs.complete.org/hdbc-missingh -- John

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A few newbie questions about tracing/debugging and order of execution
by Hunter Kelly 29 Dec '05

29 Dec '05

Heya, I decided to play around with Haskell and see what it's like. I used a small problem to explore it. Basically, given two words, find the least number of 1 letter changes that will go from one word to the other (e.g. for "fig" and "dog" either fig -> fog -> dog or fig -> dig -> dog). I came up with a solution, but I have to say it was quite difficult to get any debugging information, and when I did, the result was fairly surprising! Here's the out put I got: dagger:~/stuff$ ./figt /usr/share/dic/twords fig dog Word: dog visited: ["bit","bin","bid","bib","bug","bog","beg","bag","fix","fit","fir","fin","fie","fib","fog","fag","wig","rig","pig","jig","gig","dig","big","fig"] Word: dig visited: ["big","fig"] Word: fig visited: [] fig -> dig -> dog The first thing I found surprising was that the debug output comes in the "reverse" order - I would have expected it to be the output for fig, then dig, then dog. Secondly, I'm not entirely sure why I only got output for the statements along the "solution" path. If I try a combination that has no solution, I get no output at all. I got the program working, and I'm quite intrigued, but I figure it's probably important to figure out why the above things happen... I'm using GHC 6.4.1, but I get the same thing if I use runhugs. Thanks! --------- figt.hs -------------- import System import System.IO import Data.Set import Debug.Trace type Word = String data Path = None | Path Word Path instance Show Path where showsPrec _ path = showsPath path where showsPath :: Path -> ShowS showsPath None = ("<None>"++) showsPath (Path w None) = (w ++) showsPath (Path w p) = (showsPath p) . ((" -> " ++ w)++) findPath :: Word -> Word -> Set String -> Path findPath orig target allWords = let gen = singleton orig in iteratePath [(Path orig None)] gen [] where iteratePath :: [Path] -> Set String -> [String] -> Path iteratePath [] _ _ = None iteratePath (None: _) _ _ = None iteratePath (fullPath@(Path w _):remain) generated visited = let allPossibilities = findPossibleWords w existant = Prelude.filter (\p -> member p allWords) allPossibilities new = Prelude.filter (\e -> not (member e generated)) existant marked = foldl (\x y -> insert y x) generated new thingum = trace ("Word: " ++ w ++ "\n\tvisited: " ++ (show visited) ++ "\n") fullPath todo = remain ++ Prelude.map (\u -> (Path u thingum)) new in if target == w then thingum else if length todo > 0 then iteratePath todo marked (w:visited) else None findPossibleWords :: String -> [String] findPossibleWords word = concatMap enumChars (splits) where enumChars (prefix, _:cs) = Prelude.map (\c -> prefix ++ [c] ++ cs) ['a'..'z'] splits = Prelude.map (\x -> splitAt x word) [0..((length word) -1)] findAllWordsSizeN :: String -> Int -> Set String findAllWordsSizeN content wordLength = fromDistinctAscList (Prelude.filter (\x -> wordLength == (length x)) (lines content)) main :: IO () main = do [wordsFile, orig, target] <- getArgs contents <- readFile wordsFile allWordsSizeN <- (return (findAllWordsSizeN contents (length orig))) putStrLn (show (findPath orig target allWordsSizeN ))

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more newbie questions regarding do syntax, mondic context & Data.HashTable
by Hunter Kelly 29 Dec '05

29 Dec '05

Hi there, I'm having some trouble trying to get information out of a HashTable. I'm using it to represent a set (I'm using a hash table for O(1) speed reasons to compare against Data.Sets O(lgN) operations). I'm trying to write a simple function, member, which returns whether or not a key is in the HashTable. I've tried the following: member :: HashTable String Int -> String -> Bool member hash x = let r = do return (Data.HashTable.lookup hash x ) in case r of Nothing -> False (Just v) -> True But this seems to always return True. I _think_ the type of r here is IO (Maybe Int), but I'm trying to unwrap the IO part. However, if I try: member :: HashTable String Int -> String -> Bool member hash x = let r = do Data.HashTable.lookup hash x in case r of Nothing -> False (Just v) -> True I get the following error: figt.hs:46:31: Couldn't match `IO (Maybe Int)' against `Maybe a' Expected type: IO (Maybe Int) Inferred type: Maybe a When checking the pattern: Nothing In a case alternative: Nothing -> False How can I get at the underlying value? Can I only access it from within a "do" construct? Is there anyway to get at this function to return just True or False? Or has using something that uses an IO monad "polluted" everything else that depends on the answer? H

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RE: [Haskell] A question about fundeps <-> GADT interaction
by Simon Peyton-Jones 29 Dec '05

29 Dec '05

Tomasz Intriguing! I'm afraid it'll be some time before your code works, though. First I have to get GADTs and type classes to play together nicely, which I am hoping to do during Jan/Feb. Then I'll have to think about the interaction between GADTs and fundeps. As of today, if it works at all, it's quite amazing. Simon | -----Original Message----- | From: haskell-bounces(a)haskell.org [mailto:haskell-bounces@haskell.org] On Behalf Of Tomasz | Zielonka | Sent: 22 December 2005 14:16 | To: Haskell Mailing List | Subject: [Haskell] A question about fundeps <-> GADT interaction | | Hello! | | (This message is a literate Haskell module) | | > {-# OPTIONS -fglasgow-exts -fno-warn-missing-methods #-} | > module Term where | | The papers on GADTs have an example showing how you can transform, | traverse and evaluate ASTs (or terms) with more type safety. I've | used such an approach in one of my applications and it works remarkably | well. | | However, I would like to be able to "turn off" that type-safety | in some parts of code, for example to separate parsing from typing. | I thought I found a way to do this, because I was able to create Typed | (with all consistency checking) and Untyped (without consistency | checking) terms. Unfortunately I seem to be unable to write any useful | function on such terms - GHC complains that there are type errors. | | I think this is because of the known problem with current GADT | implementation in GHC, namely - bad GADT / type-classes interaction. I | would be very happy to hear that it will be solved in GHC 6.6. | | I use an additional type parameter for Term that determines whether | the Term is used as Typed or Untyped. Typed terms will have types | (Term Typed Int), (Term Typed Bool), ..., but Untyped terms only | one type: (Term Untyped ()) | | I use a multiparameter type-class with fundeps to describe a relation | between the Typed/Untyped tag used, the types actually used in data | constructors and the resulting "phantom" type. | | > class F f a b | f a -> b where | > -- these methods are just for experiments in GHCi | > a2b :: f -> a -> b | > b2a :: f -> b -> a | | > data Typed = Typed | > instance F Typed a a where | | > data Untyped = Untyped | > instance F Untyped a () where | | > data Term f a where | > Lit :: (F f Int int) => | > Int -> Term f int | > Succ :: (F f Int int) => | > Term f int -> Term f int | > IsZero :: (F f Int int, F f Bool bool) => | > Term f int -> Term f bool | > If :: (F f Bool bool, F f a a') => | > Term f bool -> Term f a' -> Term f a' -> Term f a' | | Some examples: | | a typeable term | | > ex1 :: Term Untyped () | > ex1 = | > If (IsZero (Succ (Lit 0))) | > (Lit 1) | > (Lit 2) | | the same as ex1, but Typed | | > ex1' :: Term Typed Int | > ex1' = | > If (IsZero (Succ (Lit 0))) | > (Lit 1) | > (Lit 2) | | a term that has type bug, but will be accepted as Untyped | | > ex2 :: Term Untyped () | > ex2 = | > If (IsZero (Succ (Lit 0))) | > (Lit 1) | > (IsZero (Lit 2)) | | Here is the function that doesn't type check (you can comment it out to | get the rest of code to compile). | | > -- A simple cast from typed terms to untyped terms | > untype :: Term Typed a -> Term Untyped () | > untype (Lit x) = Lit x | > untype (IsZero t) = IsZero (untype t) | > untype (Succ t) = Succ (untype t) | > untype (If c t e) = If (untype c) (untype t) (untype e) | | The error given by GHC (for the commented "untype" function) is: | | T.hs:52:8: | Couldn't match the rigid variable `a' against `Int' | `a' is bound by the type signature for `untype' | Expected type: Int | Inferred type: a | When using functional dependencies to combine | F Typed a a, arising from the instance declaration at T.hs:13:0 | F Typed Int a, | arising from the pattern for `Lit' at T.hs:52:8-12 at T.hs:52:8-12 | In the definition of `untype': untype (Lit x) = Lit x | Failed, modules loaded: none. | | So it seems it can't infer that the 'a' in (F Typed Int a) is Int. | At the same time... | | *T> :t a2b | a2b :: (F f a b) => f -> a -> b | *T> :t a2b Untyped | a2b Untyped :: a -> () | *T> :t a2b Typed | a2b Typed :: b -> b -- see? it knows a and b are equal! | *T> :t b2a Typed | b2a Typed :: b -> b | | If there is another way to do this right now (conveniently, Oleg! ;-), I | would be more than happy to hear about it. | | If this worked, it would be a cool trick and a nice example for GADT | use. Let me know if it was proposed before. | | Best regards | Tomasz | | -- | I am searching for a programmer who is good at least in some of | [Haskell, ML, C++, Linux, FreeBSD, math] for work in Warsaw, Poland | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell

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RE: [Haskell] GADTs and GHC
by Simon Peyton-Jones 29 Dec '05

29 Dec '05

| My current uses of GADTs belong to the company I work for, so it would | be easier for me if *I* could check from time to time. Is the code | publicly available? Not yet, I'm afraid. But it will be soon. Simon

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RE: [Haskell] Making Haskell more open
by Simon Peyton-Jones 28 Dec '05

28 Dec '05

Dear Haskell folk, A month or so ago I sent a message inviting suggestions about how to make Haskell more open, and in particular how to make it easier for Haskell users to contribute. There was quite a bit of traffic for a while, which has died down now. Here's a quick summary of what I learned. * Gour suggested using a Content Management System (e.g. Drupal http://drupal.org/) for haskell.org's front page. To me this sounds like a good plan. (e.g. "Drupal is software that allows an individual or a community of users to easily publish, manage and organize a great variety of content on a website." That sounds like what we are trying to do.) A CMS may offer more than we need, but it'd be more open and dynamic than the current setup. * There were suggestions of newsgroups and web forums. Different people seem to like different things. That's ok -- maybe a good content management system would support a lot of things, and people could choose. * Some people suggested using MediaWiki instead of MoinMoin for Hawiki. I have no idea about the issues here. Maybe it's just a matter of taste. * We don't have a plausible way of annotating GHC's user manual. One suggestion is a tree of Wiki pages, each linked from the corresponding section of the manual. We'd need an automated way to generate such a tree, and it's not clear what to do when moving from one release of GHC to the next. * #haskell has way more simultaneous logins than I realised. Good stuff! My sense is that the main action item is how to make haskell.org a better web site (This is with no disrespect to John and Olaf, who have done a great job. But I know they would be only too happy to share the load.) I'm no expert, but it sounds to me as if some content management system might well be just the thing. However, the thread didn't have many concrete offers of help. I'm hoping that is perhaps because you're all too shy to put yourselves forward! So: - is a CMS a plausible way forward? - which one? - is anyone, or small group, willing to set one up and look after it? Maybe not, in which case things are not at all terrible. But I thought it was worth asking. Simon PS: GHC is now using Trac as its bug tracker, and has its own Wiki as well. Please improve it! (Anyone can edit.) http://hackage.haskell.org/trac/ghc

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Re: A question about fundeps <-> GADT interaction
by oleg＠pobox.com 28 Dec '05

28 Dec '05

[Sorry for possible duplication, our DNS server seems to be broken, and the sysadm is on vacation] I don't think that is the problem with GADTs. The following works > untype :: Term f a -> Term Untyped () > untype (Lit x) = Lit x > untype (Succ t) = Succ (untype t) > untype (IsZero t) = IsZero ((untype t)::Term Untyped ()) > untype (If c t e) = If ((untype c)::Term Untyped ()) > ((untype t)::Term Untyped ()) > ((untype e)::Term Untyped ()) Here's a more extensive example: > tlit :: Int->Term Typed Int > tlit = Lit > > [l0,l1,l2] = map tlit [0..] > > --ex2 :: Term Untyped () > ex2' = > If (IsZero (Succ (Lit 0))) > (untype l1) > (IsZero (untype l2)) ex2' of course won't typecheck if `untype' are removed.

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Re: [Haskell] Announcing initial release of Shaskell
by David Mercer 27 Dec '05

27 Dec '05

On Tue, 27 Dec 2005 15:17:50 -0500, Dave Roundy wrote: >On Tue, Dec 27, 2005 at 01:36:53AM -0700, David Mercer wrote: >> We are pleased to announce Version 0.21a, the initial functioning >> release of Shaskell, our SHA2 library for (so far) ghc6. It supports >> sha256 and sha512 hashes in this version, and is "pure": no monads or >> C libraries are involved! > >You state in your abstract that the speed is comparable to a C >implementation. Do you have numbers comparing speed with the openssl >implementation, or any other implementations? > >David Roundy Doh! I meant to tone down that sentence, and insert the word "hopefully", as I haven't done speed testing in any depth yet, but a trip to the hospital this month left a few loose ends dangling, that being one of them! New abstract with toned down claim inserted to be in the future tense! Thanks for catching that. Performance of the sha512 implementation on 32 bit machines is currently pretty horrid, as would be expected, but the sha256 is much better. Speaking of testing, is anyone aware of any good test harnesses for benchmark comparison testing, it's been quite a while since I cared about such things and am out of touch with that corner of computing! Testing other sha2 libraries for comparison is going to be a bit of a bear, as most of them don't seem to have convenient command line versions/wrappers available, and I loath touching C, except with the traditional "make; make install" ritual of package installation. Thanks, -David Mercer Tucson, AZ

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ANN: HDBC collection 0.99.0
by John Goerzen 27 Dec '05

27 Dec '05

I'm pleased to announce version 0.99.0 of HDBC, HDCB-postgresql, and HDBC-sqlite3. Tar.gz downloads are available at htp://quux.org/devel/hdbc. Darcs repositories are available under http://darcs.complete.org/. I have uploaded these three packages to Debian (sid). This release is intended as a beta release before 1.0. Changes since 0.6.0: * Make run and execute return more useful information * Added quickQuery * Added getColumnNames * Added fetchRowsMap, fetchRowsAL, etc. They return a map from column name to value. * Fixes for Hugs compatibility * Added getTables * Sqlite3 driver now supports useful return values from run/execute * Many new test cases -- John

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GADTs and GHC
by Simon Peyton-Jones 27 Dec '05

27 Dec '05

Dear Haskell folk One of the interesting things about ICFP was the number of times GADTs came up, either in papers or in discussions in the breaks. Francois's invited talk was very inspiring, and that was just the beginning. Stephanie and Dimitrios and I are now busy revising our wobbly-types GADT paper (which is implemented in GHC, but never published http://research.microsoft.com/%7Esimonpj/papers/gadt/index.htm) We have a *far* simpler story now... but that means it'll reject some programs that would be accepted by the earlier version. We don't think this loss of expressiveness is important, but we'd like to check. Hence this message: If you use GADTs, would you to send us your GADT code, so we can check that we can still compile it? (A secondary reason for asking is that it'd be interesting just to see the way you are using GADTs.) If you send your code just to me, I'll collect it. You could a) send a complete Haskell program, which should compile and run b) send a Haskell module, or collection of modules, which should compile, but which isn't a complete program c) send a Haskell module using GADTs, but lacking the support modules that would enable it to compile d) send Omega code, which GHC won't understand, but which demonstrates some cunning use of GADTs All of these would be useful, even (c) and (d) --- we can eyeball the code. As a bonus, in the case of (a) or (b) I'll add your code to GHC's regression suite, so that all future releases will guarantee to run your code. [Or at least if not it'll be deliberate!] Tell me if you'd like this; it'll become visible in GHC's CVS repository. Thanks Simon

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