June 2011 - Haskell-Cafe

Generating simple histograms in png format?
by Dmitri O.Kondratiev 12 Jun '11

12 Jun '11

I am looking for platform-independent library to generate simple histograms in png format. Does such thing exist? Thanks!

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Instances and multiple inheritance
by Patrick Browne 12 Jun '11

12 Jun '11

How do I make an instance of the Vehicles class below? Thanks, Pat class Containers x y where insert :: y -> x y -> x y remove :: y -> x y -> x y whatsIn :: x y -> [y] instance Containers [] Char where insert y [] = y:[] insert y m = y:m remove _ [] = [] remove x (y:ys) | x == y = remove x ys | otherwise = y : remove x ys whatsIn = id instance Containers [] Integer where insert y [] = y:[] insert y m = y:m remove _ [] = [] remove x (y:ys) | x == y = remove x ys | otherwise = y : remove x ys whatsIn = id instance Containers [] [a] where insert x y = x:y -- lists of lists -- insert [1] [[3,6],[45]] class Surfaces a b where put :: b -> a b -> a b takeOff :: b -> a b -> a b whatsOn :: a b -> [b] instance Surfaces [] Integer where put y [] = y:[] put y m = y:m takeOff _ [] = [] takeOff x (y:ys) | x == y = takeOff x ys | otherwise = y : takeOff x ys whatsOn = id class Contacts a b where attach :: b -> a b -> a b detach :: b -> a b -> a b whatsAt :: a b -> [b] instance Contacts [] Integer where attach y [] = y:[] attach y m = y:m detach _ [] = [] detach x (y:ys) | x == y = detach x ys | otherwise = y : takeOff x ys whatsAt = id class Paths a b c where move :: c -> a b c -> a b c origin, destination :: a b c -> b whereIs :: a b c -> c -> b instance Paths (,) Integer Integer where -- `Integer -> (Integer, Integer) -> (Integer, Integer)' move c (a,b) = (a+1,b+1) origin (a,b) = a destination (a,b) = b whereIs (a, b) c = b -- Test -- move (1::Integer) (3::Integer,5::Integer) -- origin (3::Integer,5::Integer) class People p where class HeavyLoads l where class Surfaces w o => WaterBodies w o where instance WaterBodies [] Integer where class Containers h o => Houses h o where class (Surfaces v o, Paths a b (v o)) => Vehicles v o a b where -- I do not know how to make an instance of Vehicles -- What seems to be the *obvious* does not work -- instance Vehicles [] Integer (,) Integer Integer where -- Kind error: `Vehicles' is applied to too many type arguments -- In the instance declaration for -- `Vehicles [] Integer (,) Integer Integer' This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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pool: Why doesn't it block?
by Ertugrul Soeylemez 12 Jun '11

12 Jun '11

Hello Michael, hello fellow haskellers, there is something, which has bothered me for quite a while, but now it has become a serious problem for me, because I see it as a bug, and there is no elegant way to work around it. I wonder if it's the right semantics for Data.Pool to simply fail with an exception, if the pool is exhausted. It would be much more appropriate, if it would just block, until a resource becomes available. Otherwise it's just /safe/ for multi-threading, but not really /useful/ for it. I noticed this when I launched 512 worker threads, but my pool had only 16 database connections. I need the pool to block, until a resource is available. It's also common that my Yesod site just returns an internal server error, when the pool is exhausted, so you can only handle as many connections successfully as there are database connections. I would expect Yesod to wait for a connection to become available instead of simply blowing the request. Blocking should at least be an option and be somehow reachable from Yesod/persistent. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Yesod: How safe are forms?
by Ertugrul Soeylemez 12 Jun '11

12 Jun '11

Hello fellow haskellers, how far does Yesod.Form protect you from invalid input? I'm particularly interested in what happens, when you submit invalid data to select fields or radio groups. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Toy implementation of the STG machine
by Florian Weimer 11 Jun '11

11 Jun '11

I'm looking for a simple implementation of the STG machine to do some experiments, preferably implemented in something with memory safety. Performance is totally secondary. I'm also not interested in garbage collection details, but I do want to look at the contents of the various stacks.

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Fixed points
by Alexander Solla 11 Jun '11

11 Jun '11

Hi Everybody, I have a function which computes a fixed point in terms of iterate: equivalenceClosure :: (Ord a) => Relation a -> Relation a equivalenceClosure = fst . List.head -- "guaranteed" to exist . List.dropWhile (uncurry (/=)) -- removes pairs that are not equal . U.List.pairwise (,) -- applies (,) to adjacent list elements . iterate ( reflexivity . symmetry . transitivity ) Can this function be written in terms of fix? It seems like there should be a transformation from this scheme to something with fix in it, but I don't see it.

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How do you test a parser?
by Andrew Coppin 11 Jun '11

11 Jun '11

OK, so suppose you sit down and write a complicated string parser. Now how do you test that it works correctly? One way is to run the parser over a large corpus of "real world" samples. This has a number of problems: * If the parser is for a grammar you just invented yourself, presumably no "real world" corpus exists yet. * This tests that the most commonly used features work OK, but might not test whether more obscure features work right. * There's no really obvious way to check that what the parser returns is /correct/. You've got the input, but there's nothing to compare with. * This probably doesn't test whether the parser /fails/ for invalid input, since a real world corpus would presumably consist entirely of valid input. Most people's library of choice appears to be QuickCheck. But while it's not hard to have QuickCheck generate random strings and confirm that the returned parse tree (if any) doesn't violate any invariants, it's not so easy to check that the parse tree is actually what it should be. On top of that, depending on what the grammar is, the vast majority of random strings are probably just parse failures. Even if they aren't, there are probably specific constructs that are special-cased in the parser, which are very unlikely to appear by chance. (Things like keyword names, special combinations of punctuation, etc.) You can try to write your own text generator that constructs text more closely approximating what your parser is supposed to parse. But then how do you tell whether your generator is right? If you have a function that turns a parse tree back into text again, you can try verifying that a round-trip is the identity function. Except perhaps sometimes it isn't. Perhaps a given expression has more than one equivalent representation. A round-trip from string and back again is even less likely to be stable. So what's the best way to attack this problem?

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Announcing: hashtables-1.0.0.0
by Gregory Collins 11 Jun '11

11 Jun '11

Hi all, I’m very pleased to announce today the release of the first version of hashtables, a Haskell library for fast mutable hash tables. The hashtables library contains three different mutable hash table implementations in the ST monad, as well as a type class abstracting out the functions common to each and a set of wrapper functions to use the hash tables in the IO monad. It's also substantially faster than Data.HashTable. For more information, including performance benchmarks vs. some of the other common Haskell associative datatypes, please see my blog post: http://gregorycollins.net/posts/2011/06/11/announcing-hashtables Although I’ve made substantial efforts to test this code prior to release, it is a "version 1.0". Please send bug reports to the hashtables github issues page: https://github.com/gregorycollins/hashtables/issues. Thanks! G -- Gregory Collins <greg(a)gregorycollins.net>

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In ML books; functors only consume & produce structures; whereas, functions C&P values.
by KC 11 Jun '11

11 Jun '11

I've never seen such an easy description in Haskell books. Is there more going on with Haskell functors? -- -- Regards, KC

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Class and Instance
by Patrick Browne 11 Jun '11

11 Jun '11

Hi Below is a class that I wish to create some instances of. I do not wish to change the class definition. It is supposed to represent containers of type x that contain things of type y. My attempt at the insert function seems ok for Char and lists, but not ok for Integer. How do I instantiate this class for integers and lists? Does this class definition permit y to be a list and hence x to be a list of lists? Something like: instance Containers [] [] where .. Is this a constructor class? Thanks, Pat class Containers x y where insert :: y -> x y -> x y remove :: y -> x y -> x y whatsIn :: x y -> [y] instance Containers [] Char where insert y [] = y:[] insert y m = y:m instance Containers [] Integer where insert y [] = y:[] insert y m = y:m -- OK -- insert '1' ['u','u','l','i'] -- Not OK -- insert 2 [9,2] This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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