- Haskell-Cafe - Haskell.org

ANN: clckwrks - a Haskell blogging and CMS framework
by Jeremy Shaw 13 Jun '12

13 Jun '12

Hello! I am pleased to announce the launch of clckwrks.com. clckwrks (pronounced, 'clockworks') is a new Haskell-based content management (CMS) and blogging framework: http://www.clckwrks.com/ It is freely available under the BSD3 license. clckwrks is still in early alpha development. We are looking for developers who want to contribute or just like to try bleeding edge software. clckwrks is already being used to power happstack.com and clckwrks.com -- so it is already useful. There are a lot of things we are not happy with yet ranging from code formatting to design issues. But, at the same time, there are a lot of things we are very happy with. If you would like to help get us to clckwrks 1.0, we'd love to have your help! For more information, check out the website and consider joining the mailing list. Thanks! - jeremy

1 0

Troubles understanding Parsec Error Handling
by Matthias Hörmann 12 Jun '12

12 Jun '12

I recently started writing my first application at work in Haskell and it deals with a lot of parsing. Among other things I often have to check for a lot of alternatives for fixed strings (parsing natural language text and people have a lot of ways to abbreviate the same thing in labels). So far I have been doing this basically via choice $ map (try . string) [ "foo", "bar", ... ] This works fine but has two disadvantages, it isn't very fast, in particular when many of the strings start with the same prefix and it also is a bit error prone since it breaks when you place a prefix of another string earlier in the list. My attempt at a solution was to use the bytestring-trie package for a little utility function that basically parses one character at a time, checks if the string parsed so far is in the trie and then calls itself recursively with the trie starting with that string. My attempt at that so far looks like this: (dependencies bytestring-trie, utf8-string and parsec 3) import qualified Data.ByteString.UTF8 as UTF8 import qualified Data.Trie as Trie import Text.Parsec import Text.Parsec.Text (GenParser) anyOf :: [String] -> GenParser u String anyOf l = try $ anyOf' t "" where t = Trie.fromList $ zip (map UTF8.fromString l) (repeat ()) anyOf' :: Trie.Trie () -> String -> GenParser u String anyOf' t s = try $ do c <- lookAhead $ anyChar let newS = s ++ [ c ] in case Trie.submap (UTF8.fromString newS) t of emptyT | Trie.null emptyT -> case Trie.member (UTF8.fromString s) t of True -> return s False -> unexpected $ show newS++", expecting one of "++show l restT -> do _ <- anyChar try $ anyOf' restT newS A successful example usage would be: parseTest (do; r1 <- anyOf ["Hello", "Hallo", "Foo", "HallofFame"]; r2 <- string "bla"; return (r1, r2)) "Hallobla" which results in ("Hallo","bla") (the extra string parser is there so errors in parsing too much are not hidden). An error would result .e.g. from parseTest (do; r1 <- anyOf ["Hello", "Hallo", "Foo", "HallofFame"]; r2 <- string "bla"; return (r1, r2)) "Hallofbla" which prints this: parse error at (line 1, column 8):unknown parse error And my question about this is made up of two parts 1. Why doesn't it print my "unexpected" message but instead says unknown parse error 2. Why is the location in the text off (I would expect it to fail at column 6 (first character beyond the result it could return) or 7 (first character that makes the string no prefix of any acceptable string) I am afraid my knowledge of Parsec internals is a bit too limited, some Google queries showed no similar problems and no obvious places in the Parsec source code to check for the answer to the first question in particular and I suspect the second is closely related to the first. Thanks for reading through my question and I hope someone knows the answer or at least some clues on where i might find it. Matthias Hoermann P.S.: I am hoping this time this works, last time it was rejected because google sends with @googlemail.com instead of @gmail.com for some reason.

4 8

vector operations
by Evan Laforge 12 Jun '12

12 Jun '12

So I wanted to find the first index in a vector whose running sum is greater than a given number. The straightforward way is to create the running sum and then search: Vector.findIndex (>=target) (Vector.scanl' (+) 0 vector) But vectors are strict so it could do extra work, and what if I don't want to generate garbage? I could do it with a fold, but it would have to have the ability to abort early. Of course I could write such a fold myself using indexing: import qualified Data.Vector.Generic as Vector fold_abort :: (Vector.Vector v a) => (accum -> a -> Maybe accum) -> accum -> v a -> accum fold_abort f accum vec = go 0 accum where go i accum = maybe accum (go (i+1)) $ f accum =<< vec Vector.!? i find_before :: (Vector.Vector v a, Num a, Ord a) => a -> v a -> Int find_before n = fst . fold_abort go (0, 0) where go (i, total) a | total + a >= n = Nothing | otherwise = Just (i+1, total+a) So it's bigger and clunkier, but I would think it would be much more efficient (provided using Data.Vector.Generic won't inhibit inlining and unboxing). But I'm a bit surprised there isn't already something like fold_abort... or is there?

4 7

GHCi Loop Detection
by John Van Enk 11 Jun '12

11 Jun '12

Hi Cafe, Is there a reason that the GHCi interpreter doesn't detect and report infinite loops in statements like this (like compiled programs do) even though no CPU time appears to be used? My (admittedly weak) searching for an answer didn't turn much up. let s | not $ null s = [] in s GHCi v7.0.4 on OSX Thanks, John

2 2

Re: [Haskell-cafe] attoparsec double precision, quickCheck and aeson
by Doug McIlroy 11 Jun '12

11 Jun '12

> Date: Tue, 5 Jun 2012 10:25:26 -0700 > From: Warren Harris <warrensomebody(a)gmail.com> > > On Jun 5, 2012, at 9:57 AM, Johan Tibell wrote: > >> I don't think applying == to something that contains floating point >> values at the leaves makes much sense. You want some approxEq function >> that uses approximate equality on floating point value or you want to >> equality function that ignores the floating point values. Probably not >> the answer you like, but I don't know how to define Eq in a robust way >> for types that include floating point values. > > I buy that in general for comparing floats (those that result from arithmetic operations), but this is a case where attoparsec's parser is munging the value. I would like to have a law that says "parse . print == id" ... which is why this seems more like a bug than the usual floating point concerns. This law seems to hold for haskell's double parser: quickCheck (\d -> read (show d) == d) > > Date: Tue, 5 Jun 2012 10:51:08 -0700 > From: "Bryan O'Sullivan" <bos(a)serpentine.com> > > If you need the full precision, use rational instead. The double parser is > there because parsing floating point numbers is often a bottleneck, and > double intentionally trades speed for precision. If I understand the intended meaning of "parse" correctly, what's at issue is decimal-to-binary conversion. It is hard to defend sloppy answers for such a fundamental operation. The recommendation of rational calculation makes little sense for most floating-point computation, which approximates irrationals. The necessary imprecision, though, does not justify sloppiness--and especially not sloppy tests. It's worth noting that a "law" of input being inverse to output must fail, though rarely. Different granularity of the two bases means that there must exist cases where adjacent values in one base convert to the same value in the other. (Conceivably the specialization to parse.print could hold for some hardware. Does anybody know whether the wish is actually hopeless?) Last I looked (admittedly quite a while ago), the state of the art was strtod in http://www.netlib.org/fp/dtoa.c. (Alas, dtoa.c achieves calculational perfection via a murmuration of #ifdefs.) It's disheartening to hear that important Haskell code has needlessly fallen from perfection--perhaps even deliberately. Doug McIlroy

3 4

High memory usage with 1.4 Million records?
by Andrew Myers 11 Jun '12

11 Jun '12

Hi Cafe, I'm working on inspecting some data that I'm trying to represent as records in Haskell and seeing about twice the memory footprint than I was expecting. I've got roughly 1.4 million records in a CSV file (400M on disk) that I parse in using bytestring-csv. bytestring-csv returns a [[ByteString]] (wrapped in `type`s) which I then convert into a list of records that have the following structure: > 3 Int > 1 Text Length 3 > 1 Text Length 11 > 12 Float > 1 UTCTime All fields are marked strict and have {-# UNPACK #-} pragmas (I'm guessing that doesn't do anything for non primitives). (Side note, is there a way to check if things are actually being unpacked?) My back of the napkin memory estimates based on the assumption that nothing is being unpacked (and my very spotty understanding of Haskell data structures): Platform: 64 Bit Linux # Type (Sizeof type (occasionally a guess)) 3 * Int (8) 14 * Char (4) -- Text is some kind of bytestring but I'm guessing it can't be worse than the same number of Char? 12 * Float (4) 18 * sizeOf (ptr) (8) UTC: -- From what I can gather through :info in ghci 4 * (ptr) (8) 2 * Integer (16) -- Shouldn't be overly large, times are within 2012 List: (Pointer to element and next cons cell) 1408113 * 8 * 2 = 2513G + 21.5M So even if the original bytestring file is being kept entirely in memory somehow that's not more than 3G. I've written a small driver test program that just parses the CSV, finds the minimum value for a couple of the Float fields, and exits. In the process monitor the memory usage is 6.9G before the program exits. I've tried profiling with +RTS -hc but it ran for >3 hours without finishing, it normally finishes within 4 minutes. Anyone have any ideas for me? Things to try? Thanks, Andrew

4 5

How to select n random words from a file ...
by Noon Silk 11 Jun '12

11 Jun '12

Hi, I'm clearly new to haskell, and I suppose those is a basic question, but after a bit of searching I've been unable to figure out the "best" way to do this. I was first trying to find out how to, say, get a random element from a list, but I'm starting to think that may not be the best way (because list access isn't constant time, among other reasons). So, I wonder if there is a more direct way to just get a random word from a wordfile (i.e. one word per line) ... can anyone suggest a method? Ideally I'd like to be able to adapt this to get n random words from several files (i.e. randomly select n words from all files; i.e. just read them into one merged array and choose from there?) Thanks for any advice ... -- Noon Silk Fancy a quantum lunch? https://sites.google.com/site/quantumlunch/ "Every morning when I wake up, I experience an exquisite joy — the joy of being this signature."

6 5

Re: [Haskell-cafe] converting functional dependencies to type families
by Henning Thielemann 11 Jun '12

11 Jun '12

On Thu, 7 Jun 2012, Simon Peyton-Jones wrote: > Very useful! Maybe worth turning into a page on the Haskell wiki? I created one: http://www.haskell.org/haskellwiki/Functional_dependencies_vs._type_families

2 2

Haskell User Group Hamburg (Germany) - Meeting Thursday June 14th 2012
by Stefan Schmidt 10 Jun '12

10 Jun '12

Hello cafe, there's an upcomming meeting of the Haskell user group Hamburg next Thursday (June 14th). We'll start 19:00 at the Xing AG in Hamburg (Gänsemarkt 43, 20354 Hamburg). Falko Peters will give a short talk about hedis ( http://hackage.haskell.org/package/hedis) a Redis client written in Haskell. Although this announcement is written in english the talk will be given in german. So if you are interested to come, have a look at our doodle (also in german) and give us a short note if you want to join us. http://www.doodle.com/3fz45uqupp8pzutb Further announcements will be made via Twitter, so you may want to follow https://twitter.com/hug_hh. Stefan Schmidt

1 0

Two parallel libraries from Japan
by Kazu Yamamoto 10 Jun '12

10 Jun '12

Hello cafe, I would like to announce two parallel libraries from Japan. - Paraiso (http://hackage.haskell.org/package/Paraiso) The purpose of this library is to design a high-level language for implementing explicit partial-differential equations solvers on supercomputers as well as today's advanced personal computers. Muranushi-san is also known as the author of Monadius and one of the two translators of "Learn you". - GTALib (http://hackage.haskell.org/package/GTALib) This package provides the core functionalities of the GTA (Generate, Test, and Aggregate) programming framework on Haskell See "Test, and Aggregate - A Calculation-based Framework for Systematic Parallel Programming with MapReduce" for more information. Regards, --Kazu

3 2

ANN: scrobble-0.1.0.1: Scrobbling server
by Christopher Done 10 Jun '12

10 Jun '12

Heyo, If you use Last.fm/Libre.fm you might be interested in running your own local scrobbling server in Haskell. $ cabal update $ cabal install scrobble $ scrobble-server 8910 Source is here with further instructions for audio players and such: https://github.com/chrisdone/scrobble Exactly what the server does with the scrobbles it receives is purely up to you, use it as a library and do as you please. Personally I might work on a Last.fm replacement, at least as far as the statistics/reporting generation goes. Ciao!

1 0

Fundeps and overlapping instances
by Simon Peyton-Jones 10 Jun '12

10 Jun '12

Oleg points out, and Martin also mentions, that functional dependencies appear to interact OK with overlapping instances, but type families do not. I this impression is mistaken, and I'll try to explain why in this message, in the hope of exposing any flaws in my reasoning. We can't permit overlap for type families because it is unsound to do so (ie you can break "well typed programs don't go wrong"). But if it's unsound for type families, it would not be surprising if it was unsound for fundeps too. (I don't think anyone has done a soundness proof for fundeps + local constraints + overlapping instances, have they?) And indeed I think it is. So the short summary of this message is: if it works for fundeps it works for type families, and vice versa. (NB this equivalence is not true about GHC's current implementation, however. GHC doesn't support the combination of fundeps and local constraints at all.) Such an equivalence doesn't argue against fundeps; I'm only suggesting the that the two really are very closely equivalent. I much prefer type families from a programming-style point of view, but that's a subjective opinion. Simon Imagine a system "FDL" that has functional dependencies and local type constraints. The big deal about this is that you get to exploit type equalities in *given* constraints. Consider Oleg's example, cut down a bit: class C a b | a -> b instance C Int Bool newtype N2 a = N2 (forall b. C a b => b) t2 :: N2 Int t2 = N2 True We end up type-checking (True :: forall b. C Int b => b). From the functional dependency we know that (b~Bool), so the function should typecheck. GHC rejects this program; FDL would not. But making use of these extra equalities in "given" constraints is quite tricky. To see why look first at Example 1: module X where class C a b | a -> b data T a where MkT :: C a b => b -> T a module M1 where import X instance C Int Char where ... f :: Char -> T Int f c = MkT c module M2 where import X instance C Int Bool g :: T Int -> Bool g (MkT x) = x module Bad where import M1 import M2 bad :: Char -> Bool bad = g . f This program is unsound: it lets you cast an Int to a Bool; result is a seg-fault. You may say that the problem is the inconsistent functional dependencies in M1 and M2. But GHC won't spot that. For type families, to avoid this we "eagerly" check for conflicts in type-family instances. In this case the conflict would be reported when compiling module Bad, because that is the first time when both instances are visible together. So any FDL system should also make this eager check for conflicts. What about overlap? Here's Example 2: {-# LANGUAGE IncoherentInstances #-} module Bad where import X -- Overlapping instances instance C Int Bool -- Instance 1 instance C a [a] -- Instance 2 f :: Char -> T Int f c = MkT c -- Uses Instance 1 g :: T a -> a g (MkT x) = x -- Uses Instance 2 bad :: Char -> Int bad = g . f Again, a seg fault if it typechecks. But will it? When typechecking 'g', we get a constraint (C a ?), where 'a' is a skolem constant. Without IncoherentInstances GHC would reject the program on the grounds that it does not know what instance to choose. But *with* IncoherentInstances it would probably go through, which is unsound. So IncoherentInstances has moved from causing varying dynamic behaviour to causing seg faults. Very well, so FDL must get rid of IncoherentInstances altogether, at least for classes that have functional dependencies (or that have superclasses that do). But at the moment GHC makes an exception for *existentials*. Consider Example 3: class C a b | a -> b -- Overlapping instances instance C Int Bool -- Instance 1 instance C a [a] -- Instance 2 data T where MkT :: C a b => a -> b -> T f :: Bool -> T f x = MkT (3::Int) x -- Uses Instance 1 g :: T -> T g (MkT n x) = MkT n (reverse x) -- Uses Instance 2 bad :: Bool -> T bad = g . f In the pattern match for MkT in g we have the constraint (C a b), where 'a' is existentially bound. So under GHC's current rules it'll choose the (C a [a]) instance, and conclude that (b ~ [a]). So it's ok to reverse x. But it isn't; see function bad! So to avoid unsoundness we must not choose a particular instance from an overlapping set unless we know, absolutely positively, that the other cases cannot match. (GHC's exception for existentials was introduced in response to user demand. Usually, overlapping instances are somehow semantically coherent, and with an existential we are *never* going to learn more about the instantiating type, so choosing the best available seems like a good thing to do.) But even nuking IncoherentInstances altogether is not enough. Consider this variant of Example 3, call it Example 4: module M where class C a b | a -> b instance C a [a] -- Instance 2 data T where MkT :: C a b => a -> b -> T g :: T -> T g (MkT n x) = MkT n (reverse x) -- Uses Instance 2 module Bad where import M instance C Int Bool -- Instance 1 f :: Bool -> T f x = MkT (3::Int) x -- Uses Instance 1 bad :: Bool -> T bad = g . f This is nasty. In module M we have only one instance declaration for C, so there's no overlap, and function g typechecks fine. Now module Bad adds an overlapping instance declaration and 'f' uses it; moreover, it's clear that the new instance declaration is the right one to use. It's not clear how to fix this. The only way I can think of is to insist that all the instances appear in a single module, so that you cannot add more "later". But that loses part of the point of overlap in the first place, namely to provide a generic instance and then later override it. So FDL must * embody eager checking for inconsistent instances, across modules * never resolve overlap until only a unique instance can possibly match * put all overlapping instances in a single module Type families already implement the first of these. I believe that if we added the second and third, then overlap of type families would be fine. (I may live to eat my words here.) I'd be interested to know what Chameleon does on these examples.

6 18

Re: [Haskell-cafe] Have you seen this functor/contrafunctor combo?
by Edward Kmett 10 Jun '12

10 Jun '12

Here is a considerably longer worked example using the analogy to J, borrowing heavily from Wadler: As J, this doesn't really add any power, but perhaps when used with non-representable functors like Equivalence/Comparison you can do something more interesting. -- Used for Hilbert {-# LANGUAGE DefaultSignatures, TypeOperators #-} -- Used for Representable {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleContexts, FlexibleInstances #-} module Search where import Control.Applicative import Data.Function (on) import Data.Functor.Contravariant import GHC.Generics -- for Hilbert newtype Search f a = Search { optimum :: f a -> a } instance Contravariant f => Functor (Search f) where fmap f (Search g) = Search $ f . g . contramap f instance Contravariant f => Applicative (Search f) where pure a = Search $ \_ -> a Search fs <*> Search as = Search $ \k -> let go f = f (as (contramap f k)) in go (fs (contramap go k)) instance Contravariant f => Monad (Search f) where return a = Search $ \_ -> a Search ma >>= f = Search $ \k -> optimum (f (ma (contramap (\a -> optimum (f a) k) k))) k class Contravariant f => Union f where union :: Search f a -> Search f a -> Search f a instance Union Predicate where union (Search ma) (Search mb) = Search $ \ p -> case ma p of a | getPredicate p a -> a | otherwise -> mb p instance Ord r => Union (Op r) where union (Search ma) (Search mb) = Search $ \ f -> let a = ma f b = mb f in if getOp f a >= getOp f b then a else b both :: Union f => a -> a -> Search f a both = on union pure fromList :: Union f => [a] -> Search f a fromList = foldr1 union . map return class Contravariant f => Neg f where neg :: f a -> f a instance Neg Predicate where neg (Predicate p) = Predicate (not . p) instance Num r => Neg (Op r) where neg (Op f) = Op (negate . f) pessimum :: Neg f => Search f a -> f a -> a pessimum m p = optimum m (neg p) forsome :: Search Predicate a -> (a -> Bool) -> Bool forsome m p = p (optimum m (Predicate p)) forevery :: Search Predicate a -> (a -> Bool) -> Bool forevery m p = p (pessimum m (Predicate p)) member :: Eq a => a -> Search Predicate a -> Bool member a x = forsome x (== a) each :: (Union f, Bounded a, Enum a) => Search f a each = fromList [minBound..maxBound] bit :: Union f => Search f Bool bit = fromList [False,True] cantor :: Union f => Search f [Bool] cantor = sequence (repeat bit) least :: (Int -> Bool) -> Int least p = head [ i | i <- [0..], p i ] infixl 4 --> (-->) :: Bool -> Bool -> Bool p --> q = not p || q fan :: Eq r => ([Bool] -> r) -> Int fan f = least $ \ n -> forevery cantor $ \x -> forevery cantor $ \y -> (take n x == take n y) --> (f x == f y) -- a length check that can handle infinite lists compareLength :: [a] -> Int -> Ordering compareLength xs n = case drop (n - 1) xs of [] -> LT [_] -> EQ _ -> GT -- Now, lets leave Haskell 98 behind -- Using the new GHC generics to derive versions of Hilbert's epsilon class GHilbert t where gepsilon :: Union f => Search f (t a) class Hilbert a where -- http://en.wikipedia.org/wiki/Epsilon_calculus epsilon :: Union f => Search f a default epsilon :: (Union f, GHilbert (Rep a), Generic a) => Search f a epsilon = fmap to gepsilon instance GHilbert U1 where gepsilon = return U1 instance (GHilbert f, GHilbert g) => GHilbert (f :*: g) where gepsilon = liftA2 (:*:) gepsilon gepsilon instance (GHilbert f, GHilbert g) => GHilbert (f :+: g) where gepsilon = fmap L1 gepsilon `union` fmap R1 gepsilon instance GHilbert a => GHilbert (M1 i c a) where gepsilon = fmap M1 gepsilon instance Hilbert a => GHilbert (K1 i a) where gepsilon = fmap K1 epsilon instance Hilbert () instance (Hilbert a, Hilbert b) => Hilbert (a, b) instance (Hilbert a, Hilbert b, Hilbert c) => Hilbert (a, b, c) instance (Hilbert a, Hilbert b, Hilbert c, Hilbert d) => Hilbert (a, b, c, d) instance (Hilbert a, Hilbert b, Hilbert c, Hilbert d, Hilbert e) => Hilbert (a, b, c, d, e) instance Hilbert Bool instance Hilbert Ordering instance Hilbert a => Hilbert [a] instance Hilbert a => Hilbert (Maybe a) instance (Hilbert a, Hilbert b) => Hilbert (Either a b) instance Hilbert Char where epsilon = each instance (Union f, Hilbert a) => Hilbert (Search f a) where epsilon = fmap fromList epsilon search :: (Union f, Hilbert a) => f a -> a search = optimum epsilon find :: Hilbert a => (a -> Bool) -> a find = optimum epsilon . Predicate every :: Hilbert a => (a -> Bool) -> Bool every = forevery epsilon exists :: Hilbert a => (a -> Bool) -> Bool exists = forsome epsilon -- and MPTCs/Fundeps to define representable contravariant functors: class Contravariant f => Representable f r | f -> r where represent :: f a -> a -> r tally :: (a -> r) -> f a instance Representable (Op r) r where represent (Op f) = f tally = Op instance Representable Predicate Bool where represent (Predicate p) = p tally = Predicate supremum :: Representable f r => Search f a -> (a -> r) -> r supremum m p = p (optimum m (tally p)) infimum :: (Representable f r, Neg f) => Search f a -> (a -> r) -> r infimum m p = p (pessimum m (tally p)) A few toy examples: ghci> supremum (fromList [1..10] :: Search (Op Int) Int) id 10 ghci> find (=='a') 'a' ghci> fan (!!4) 5 ghci> find (\xs -> compareLength xs 10 == EQ && (xs !! 4) == 'a') "\NUL\NUL\NUL\NULa\NUL\NUL\NUL\NUL\NUL" -Edward

1 1

ANNOUNCE: optparse-applicative 0.0.1
by Paolo Capriotti 09 Jun '12

09 Jun '12

I'm pleased to announce the release of version 0.0.1 of [optparse-applicative][1], a library for writing command line option parsers in Applicative style. You can find an introduction and tutorial on the [github page][2], and an explanation of the internals on [my blog][3]. [1]: http://hackage.haskell.org/package/optparse-applicative [2]: https://github.com/pcapriotti/optparse-applicative [3]: http://paolocapriotti.com/blog/2012/04/27/applicative-option-parser/ BR, Paolo

1 0

ANN: lucienne-0.0.1
by Alexander Bau 09 Jun '12

09 Jun '12

Hi, some time ago I wrote lucienne[1] using the happstack framework: > Lucienne is a simple server side feed aggregator/reader that helps > you managing your subscribed feeds. It provides multi user support > using basic access authentication. A running mongoDB serves as > database backend. For more information, check [1] and [2]. Best regards, Alex [1] http://hackage.haskell.org/package/lucienne [2] http://www.imn.htwk-leipzig.de/~abau/lucienne

1 0

I don't understand how ST works
by Nicu Ionita 09 Jun '12

09 Jun '12

Hi, After trying the whole afternoon to make a program work using ST and mutable vectors, I must give up and ask for some help. I have a pure function which generates a list of moves. But the whole thing should live in the ST monad, so: > genMoves ... = runST $ do ... Now, as I understand, I have a private universe (under runST) in which I can run "impure code", from which nothing escapes to the outside. Now in that universe I prepare succesively (and use later) a data structure which contains pure and impure values, for example: > data MList = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe MList) } Now comes my question: in the impure values there is always that "s". I was thinking that the whole structure should have s as a parameter: > data MList s = MList { mlVec :: MVector s Move, mlNextPh :: MList -> ST s (Maybe (MList s)) } but then, when I define functions like: > splitMove :: MList s -> ST s (Maybe (Move, MList s)) > splitMove ml = do > m <- unsafeRead (mvVec ml) 0 > ... I get this message: Moves\MoveList.hs:217:28: Couldn't match type `s' with `PrimState (ST s)' `s' is a rigid type variable bound by the type signature for splitMove :: MList s -> ST s (Maybe (Move, MList s)) at Moves\MoveList.hs:210:1 Expected type: U.MVector (PrimState (ST s)) Move Actual type: U.MVector s Move In the return type of a call of `mlVec' In the first argument of `M.unsafeRead', namely `(mlVec ml)' which really doesn't make sense, as the package primitive defines the instance: instance PrimMonad (ST s) where type PrimState (ST s) = s primitive = ST internal (ST p) = p Should I do the structure agnostic of that s-state? (forall s. ...) This seems really unintuitive to me... Anybody some hint? Thanks, Nicu

3 4

link to section heading in Haddock
by Edward Amsden 09 Jun '12

09 Jun '12

Hi all After glancing through the haddock documentation and some googling, I can't tell if there's a supported way to link to a section heading in haddock documentation. Is there? Thanks, Ed

2 1

Lambda Calculus and Differential Geometry and Big Iron
by Barak A. Pearlmutter 09 Jun '12

09 Jun '12

Seeking PhD students and postdocs interested in an elegant combination of functional programming and big-iron style numeric computing. Functional Programming and Automatic Differentiation PhD Studentships Postdoctoral Positions We are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination in a production-quality optimising compiler. Our research prototype compiler generates object code competitive with the fastest current systems, which are based on FORTRAN. We are seeking PhD students and postdocs with interest and experience in relevant areas, such as programming language theory, numeric computing, machine learning, numeric linear algebra, differential geometry; and a burning drive to help lift big iron numeric computing out of the 1960s and into a newer higher order. Specific sub-projects include: compiler and numeric programming environment construction; writing, simplifying, and generalising numeric and machine learning algorithms through the use of type theory and AD operators; and associated type/lambda calculus/PLT/real computation issues. Project headquarters: Hamilton Institute, NUI Maynooth, Ireland, http://www.hamilton.ie/. Applications and queries to: "Barak A. Pearlmutter" <barak+ad-fp-job(a)cs.nuim.ie>

1 0

Introducing FP Complete
by Bartosz Milewski 08 Jun '12

08 Jun '12

You might have seen a few post by me mentioning FP Complete and asked yourself the question: Who is this guy and what is FP Complete? I haven't been active in the Haskell community, as I'm a relative newcomer to Haskell. I am better known in the C++ community where I've been promoting functional-style programming and the use of Haskell for modelling difficult aspects of template metaprogramming. I've been involved in discussing the support for concurrency and parallelism in C++; which, by the way, is ages behind what Haskell has to offer. It was therefore natural for me to join a newly founded company, FP Complete, whose goal is to commercialize Haskell. I believe that now is the right time for Haskell to become a strong software industry player, especially that functional programming is being widely recognized as the answer to the recent multicore and GPU explosion. I can't be much more specific about what the role of FP Complete will be in making Haskell an industrial success because our plans are still evolving. We should be ready to makes some announcement shortly. I should mention that the support from the part of the Haskell community that we have so far contacted has been overwhelmingly positive. We have Simon P-J's blessing, we are in close collaboration with Well Typed, we have friends at Galois, Parallel Scientific, Amgen, and many other Haskell shops. Hopefully you will hear from and about us more often in the future. -- [:Bartosz Milewski:]

5 4

XDR library ?
by ARJANEN Loïc Jean David 08 Jun '12

08 Jun '12

Hello, I would like to know if there is an Haskell XDR library, I didn't find one on Hackage. Regards, -- ARJANEN Loïc Jean David http://luigiscorner.wordpress.com --- "Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do." Michael R. Fellows and Ian Parberry

2 1

Using promoted lists
by Yves Parès 08 Jun '12

08 Jun '12

The doc page http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/kind-polymorphism-an… that lists are now usable as types. So I'm trying to make a type level function to test if a type list contains a type. Unless I'm wrong, that calls to the use of a type family. {-# LANGUAGE DataKinds, TypeOperators, KindSignatures, TypeFamilies #-} data HBool = HTrue | HFalse -- Mandatory as Bool type is not currently promoted to a kind type family Member x (l :: [*]) :: HBool type instance Member x (x ': xs) = HTrue type instance Member x (y ': xs) = Member x xs type instance Member x (y ': '[]) = HFalse But the compiler complains about my instance conflicting. Is what I'm trying to do feasible? Second question: how can type level tuples (also mentioned in the doc page) be exploited? Aren't they redundant with type-level lists?

3 4

Using promoted lists
by oleg＠okmij.org 08 Jun '12

08 Jun '12

Yves Pare`s wrote: > So I'm trying to make a type level function to test if a type list contains > a type. Unless I'm wrong, that calls to the use of a type family. More crucially, list membership also calls for an equality predicate. Recall, List.elem has the Eq constraint; so the type-level membership test should have something similar, a similar equality predicate. As you have discovered, type equality is quite non-trivial. All is not lost however. If we `register' all the types or type constructors, we can indeed write a type-level membership function. In fact, http://okmij.org/ftp/Haskell/TTypeable/ShowListNO.hs does exactly that. It uses the type-level (and higher-order) function Member to test if a given type is a one of the chosen types. If it is, a special instance is selected. Please see http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable http://okmij.org/ftp/Haskell/typeEQ.html#without-over for explanations.

1 0

Have you seen this functor/contrafunctor combo?
by Conal Elliott 07 Jun '12

07 Jun '12

> newtype Q p a = Q (p a -> a) > > instance ContraFunctor p => Functor (Q p) where > fmap h (Q w) = Q (h . w . cmap h) using "cmap" for contravariant map. For instance, p a = u -> a. > instance ContraFunctor p => Applicative (Q p) where > pure a = Q (pure a) > Q fs <*> Q as = Q (\ r -> > let > f = fs (cmap ($ a) r) > a = as (cmap (f $) r) > in > f a) I've checked the functor laws but not the applicative laws, and I haven't looked for a Monad instance. Or extend to a more symmetric definition adding a (covariant) functor f to the contravariant functor p. > newtype Q' p f a = Q' (p a -> f a) A (law-abiding) Functor instance is easy, but I don't know about an Applicative instance. Have you seen Q or Q' before? They look like they ought to be something familiar & useful. -- Conal

2 3

Data.Bitmap question
by Anders Lyckegaard 07 Jun '12

07 Jun '12

Hi, I have a short question on using Data.Bitmap. I have imported an image using STB-image. img <- loadImage "filename" Now I get img which is a Bitmap data structure. How do I access individual pixels? Thank you in advance, Anders

2 2

Inheritance constraints
by Miguel Mitrofanov 07 Jun '12

07 Jun '12

Hi cafe! The ConstraintKinds extension makes it possible to create classes of classes, like this: class F c where cfmap :: c f => (a -> b) -> f a -> f b instance F Functor where cfmap = fmap instance F Monad where cfmap = liftM So, basically we are saying that "c" is a class that has some special properties. Is it possible to express the property "c is a subclass of a specific class d"? Like class F c where Functor f => c f

3 2

Mac and Gtk2hs problem
by Tanja Piechnick 07 Jun '12

07 Jun '12

Hey Haskell-Cafe, I would like to run GTK2HS on my Mac 10.6.8 but after the successful installation with Homebrew of all modules (GTK+, GTK2HS) I get this failure message: Prelude> :l guitest.hs guitest.hs:1:8: Could not find module `Graphics.UI.Gtk': Use -v to see a list of the files searched for. Failed, modules loaded: none. I always changed my PATH. This is the content of my PATH: /usr/bin /bin /usr/sbin /sbin /usr/local/bin /Developer/usr/bin /Users/tanjapiechnick/.cabal/bin $HOME/.cabal/bin Is that correct? Where is the mistake? Thank you for any help! Greetings from Namibia

5 8

ANN: stylish-haskell 0.2
by Jasper Van der Jeugt 07 Jun '12

07 Jun '12

Hello all, I've just released stylish-haskell 0.2 [1]. This release adds a flexible configuration file, which already provides some options for the different processing steps, and it will also make future enhancements easy. You can use a per-project configuration file, as documented in the README [2]. A sample configuration file looks like [3]. Feedback is very welcome, issues and enhancement requests can filed here [4]. [1]: http://hackage.haskell.org/package/stylish-haskell [2]: https://github.com/jaspervdj/stylish-haskell/blob/master/README.markdown [3]: https://github.com/jaspervdj/stylish-haskell/blob/master/.stylish-haskell.y… [4]: https://github.com/jaspervdj/stylish-haskell/issues Peace, Jasper

2 1

Status of Haskell on Android? Possible ideas to make this happen?
by Alp Mestanogullari 07 Jun '12

07 Jun '12

Hi Cafe! I know, this topic comes back on the list from time to time. I know, there probably isn't anything new since the last time it was asked here, on reddit or on SO. However, I remember seeing that iPwn Studios were thinking about getting GHC to build applications for android, in addition to their ghc-iphone project. Is this correct? Is there anything new on that front? Is anyone else working/thinking-to-work on this? Aside from these questions, I'm wondering about the "easiest" way to get a ghc-android compiler up and running. I have found a few things, but they all look really, really painful and cumbersome, like http://stackoverflow.com/a/5152910/193424 or http://stackoverflow.com/a/7301024/193424 . Recently, I heard about Renderscript: http://developer.android.com/guide/topics/renderscript/index.html Of course, we could write a binding or some kind of EDSL à-la Atom to have some funny apps/games running on Android by generating Renderscript code under the hood. However, something in the "Renderscript Runtime Layer" section caught my attention: > You define your Renderscript code in .rs and .rsh files in the src/ directory of your Android project. The code is compiled to intermediate bytecode by the llvm compiler > that runs as part of an Android build. When your application runs on a device, the bytecode is then compiled (just-in-time) to machine code by another llvm compiler that > resides on the device. The machine code is optimized for the device and also cached, so subsequent uses of the Renderscript enabled application does not recompile the > bytecode. But hey, GHC *does* have an LLVM backend! So can't we somehow break that chain of compilers so that we can plug GHC's LLVM codegen on top of it and then use their llvm-on-the-device thing? I know I am skipping over many, many technical details like how we could have an analog to their "reflected layer" on the java side but I would really be interested in hearing your opinions/objections/thoughts on this. -- Alp Mestanogullari

1 0

GHC 7.4 and TypeSynonymInstances
by Ivan Lazar Miljenovic 07 Jun '12

07 Jun '12

Has there been a change in the behaviour/requirement of TypeSynonymInstances as of GHC-7.4.1? (Not sure if this behaviour occurs with 7.2.1 as I don't have it installed) I had an instance for String for a class which ghc accepted whilst using FlexibleInstances; however, when trying to load it in 7.0.* it stated that TypeSynonymInstances was needed (which I forgot to add since it didn't seem to need it). All I could find in the release notes was that as of 7.2.1, both extensions were needed, not that one implied the other: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-2-1.html#i… Is this behaviour deliberate, or a bug? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

3 4

Haskell Weekly News: Issue 230
by Daniel Santa Cruz 07 Jun '12

07 Jun '12

Welcome to issue 230 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 27 to June 02, 2012. Announcements Mark Lentczner had great news for us this week, announcing the release of the Haskell Platform 2012.2.0.0. Many many thanks to the platform team for getting this release out the door! [1] http://goo.gl/XX9P6 Top Reddit Stories * Conduit bugs Domain: haskellforall.com, Score: 45, Comments: 25 On Reddit: [2] http://goo.gl/hQCbs Original: [3] http://goo.gl/3wR5W * Response to "Conduit Bugs" Domain: yesodweb.com, Score: 43, Comments: 53 On Reddit: [4] http://goo.gl/TV5i3 Original: [5] http://goo.gl/N8uMM * Haskell fully async Chan/TChan vs. Go's channels Domain: plus.google.com, Score: 42, Comments: 14 On Reddit: [6] http://goo.gl/UJBXN Original: [7] http://goo.gl/K0zzA * Most Important GHC extensions to learn/use? Domain: permalink.gmane.org, Score: 38, Comments: 11 On Reddit: [8] http://goo.gl/i6ofz Original: [9] http://goo.gl/x7Mxg * What's so bad about Template Haskell? Domain: stackoverflow.com, Score: 35, Comments: 8 On Reddit: [10] http://goo.gl/fdiUm Original: [11] http://goo.gl/Vg4AT * stylish-haskell: A simple tool to clean up Haskell code Domain: github.com, Score: 32, Comments: 74 On Reddit: [12] http://goo.gl/QT9br Original: [13] http://goo.gl/TlZum * Understanding Haskell's monomorphism restriction Domain: self.haskell, Score: 27, Comments: 38 On Reddit: [14] http://goo.gl/tb1uC Original: [15] http://goo.gl/tb1uC * Pipes 2.0 vs pipes-core Domain: paolocapriotti.com, Score: 26, Comments: 40 On Reddit: [16] http://goo.gl/lmNeD Original: [17] http://goo.gl/najMM * FP Complete and Well-Typed delivered a two-day hands-on Haskell training at Qualcomm Domain: haskell.org, Score: 26, Comments: 2 On Reddit: [18] http://goo.gl/R3Ufl Original: [19] http://goo.gl/sC3O3 * ICFP'12 Accepted Papers (with links, where available!) Domain: github.com, Score: 26, Comments: 2 On Reddit: [20] http://goo.gl/361wt Original: [21] http://goo.gl/3ACvz Top StackOverflow Questions * What's so bad about Template Haskell? votes: 67, answers: 3 Read on SO: [22] http://goo.gl/Vg4AT * Which Haskell (GHC) extensions should users use/avoid? votes: 23, answers: 1 Read on SO: [23] http://goo.gl/s0TNH * Space leak with redundant use of seq in GHC interpreter votes: 17, answers: 2 Read on SO: [24] http://goo.gl/no97P * Is there a list of GHC extensions that are considered 'safe'? votes: 15, answers: 1 Read on SO: [25] http://goo.gl/10SJc * What are lenses used/useful for? votes: 14, answers: 4 Read on SO: [26] http://goo.gl/kn8uj * Haskell: Can I perform several folds over the same lazy list without keeping list in memory? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/VuWQ7 * What are the advantages and disadvantages of using lenses? votes: 11, answers: 2 Read on SO: [28] http://goo.gl/kGX4B * Calculating work done by f x = (x,x) votes: 11, answers: 1 Read on SO: [29] http://goo.gl/838C2 * Why is function definition for all types at once not allowed in Haskell? votes: 10, answers: 5 Read on SO: [30] http://goo.gl/eoDXp * Generic variant of bi f a b = (f a, f b) votes: 9, answers: 4 Read on SO: [31] http://goo.gl/NPDMU * Unicode console I/O in Haskell on Windows votes: 9, answers: 1 Read on SO: [32] http://goo.gl/AFCQh Until next time, Daniel Santa Cruz References 1. http://comments.gmane.org/gmane.comp.lang.haskell.platform/1964 2. http://www.haskellforall.com/2012/05/conduit-bugs.html 3. http://www.reddit.com/r/haskell/comments/u7vyx/conduit_bugs/ 4. http://www.yesodweb.com/blog/2012/05/response-conduit-bugs 5. http://www.reddit.com/r/haskell/comments/u8fe6/response_to_conduit_bugs/ 6. https://plus.google.com/109566665911385859313/posts/FAmNTExSLtz 7. http://www.reddit.com/r/haskell/comments/ubmz7/haskell_fully_async_chantcha… 8. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/98574 9. http://www.reddit.com/r/haskell/comments/ufenx/most_important_ghc_extension… 10. http://stackoverflow.com/questions/10857030/whats-so-bad-about-template-has… 11. http://www.reddit.com/r/haskell/comments/ugd94/whats_so_bad_about_template_… 12. https://github.com/jaspervdj/stylish-haskell/blob/master/README.markdown 13. http://www.reddit.com/r/haskell/comments/u8u1y/stylishhaskell_a_simple_tool… 14. http://www.reddit.com/r/haskell/comments/udnx2/understanding_haskells_monom… 15. http://www.reddit.com/r/haskell/comments/udnx2/understanding_haskells_monom… 16. http://paolocapriotti.com/blog/2012/05/29/pipes-2.0-vs-pipes-core/ 17. http://www.reddit.com/r/haskell/comments/uav9d/pipes_20_vs_pipescore/ 18. http://www.haskell.org/pipermail/haskell-cafe/2012-June/101558.html 19. http://www.reddit.com/r/haskell/comments/ug4wq/fp_complete_and_welltyped_de… 20. https://github.com/technogeeky/icfp12-paper-links/blob/master/README.md 21. http://www.reddit.com/r/haskell/comments/uhhd2/icfp12_accepted_papers_with_… 22. http://stackoverflow.com/questions/10857030/whats-so-bad-about-template-has… 23. http://stackoverflow.com/questions/10845179/which-haskell-ghc-extensions-sh… 24. http://stackoverflow.com/questions/10793371/space-leak-with-redundant-use-o… 25. http://stackoverflow.com/questions/10830757/is-there-a-list-of-ghc-extensio… 26. http://stackoverflow.com/questions/10788261/what-are-lenses-used-useful-for 27. http://stackoverflow.com/questions/10803221/haskell-can-i-perform-several-f… 28. http://stackoverflow.com/questions/10774614/what-are-the-advantages-and-dis… 29. http://stackoverflow.com/questions/10825933/calculating-work-done-by-f-x-x-x 30. http://stackoverflow.com/questions/10811657/why-is-function-definition-for-… 31. http://stackoverflow.com/questions/10772121/generic-variant-of-bi-f-a-b-f-a… 32. http://stackoverflow.com/questions/10779149/unicode-console-i-o-in-haskell-…

1 0

HaL7: Haskell-Treffen in Halle/Saale, Freitag 2012-07-13
by Henning Thielemann 06 Jun '12

06 Jun '12

For our German readers: Es ist wieder so weit: Am Freitag, 13. Juli laden wir zum 7. Haskell-Treffen HaL ein, diesmal nach Halle an der Saale. Wir bieten eine bunte Mischung aus Tutorien zum Mitmachen, Vortraegen zum Anhoeren und Gelegenheiten zum Fachsimpeln, theoretisches wie praktisches, ernstes wie heiteres. Das Programm im Schnelldurchlauf ist: vormittags 6 Tutorien 09:30 - 11:00 snap repa schule 11:30 - 13:00 reactive generics liveseq nachmittags 4 Vortraege 14:30 - 15:15 Dorn: Schule 15:15 - 16:00 Voigtlaender: ADP 16:30 - 17:15 Goergens: XenClient 17:15 - 18:00 Eijkel: Hommage sonstiges: 09:00 - 11:30 Installationshilfen 11:00 - 11:30 Kaffeepause 13:00 - 14:30 Mittagspause 16:00 - 16:30 Kaffeepause 19:00 - Ende Abendessen und Fachsimpelei im Restaurant Die Details finden Sie hier: http://iba-cg.de/hal7.html und die Anmeldung da: http://sim.mathematik.uni-halle.de:8080/hal7/ Bitte melden Sie sich bis zum 4. Juli an. Wir freuen uns auf Ihr zahlreiches Erscheinen! für das Programmkomitee Henning Thielemann

2 1

Typing Judgments for Haskell2010++
by Shayan Najd Javadipour 06 Jun '12

06 Jun '12

Hi Haskell Cafe! To develop Haskell-Type-Exts (HTE) [1,2], I need to have typing judgments for each language construct in Haskell 2010 with two extensions: RankNTypes and local assumptions (GADTs, Type Functions and etc). Since I could not find any academic paper describing a type system supporting both local assumptions [3] and RankNTypes [4], we tried to combine the two systems. We also tried to extend the type system to support Patterns and some other constructs. The result (in a very sketchy state) is posted at: http://cleantypecheck.wordpress.com/2012/06/06/typing-judgment-rules-14/ Now before I start to implement these rules in HTE, I need to make sure of their validity. Therefore, I'd highly appreciate if experts would take a closer look at these judgement rules and comment. Considering the fact that HTE is a Google SoC project and me being just a motivated student (limited knowledge/experience), the success of this project strongly relies on Haskell community's support. Thanks! /Shayan [1] http://www.google-melange.com/gsoc/project/google/gsoc2012/shayannajd/18001 [2] http://cleantypecheck.wordpress.com/2012/05/03/kickoff/ [3] D. Vytiniotis, S. Peyton Jones, T. Schrijvers, M. Sulzmann. OutsideIn(X) – Modular type inference with local assumptions [4] S. Peyton Jones, D. Vytiniotis, S. Weirich, and M. Shields. Practical type inference for arbitrary-rank types.

1 0

[ANNOUNCE] cereal-0.3.5.2
by Trevor Elliott 06 Jun '12

06 Jun '12

Hi Everyone, I'm happy to announce version 0.3.5.2[1] of the cereal serialization library. This is a bugfix release, mostly improving performance characteristics of the library. One noteable change is with runGetLazy, which will now perform as expected when a parser attempts to consume more input than is available. Happy hacking! --trevor [1] http://hackage.haskell.org/package/cereal-0.3.5.2

1 0

converting functional dependencies to type families
by Henning Thielemann 06 Jun '12

06 Jun '12

Hi all, when I reported a typechecker performance problem related to functional dependencies http://hackage.haskell.org/trac/ghc/ticket/5970 I promised to try to convert from functional dependencies to type families. Thus I converted my code and the llvm package to type-families: http://code.haskell.org/~thielema/llvm-tf/ Here are some of my experiences: == Advantages of TypeFamilies == * Speed For what I did the type families solution was considerably faster than the functional dependencies code at least in GHC-7.4.1. Thus the bug in ticket 5970 does no longer hurt me. (In GHC-6.12.3 the conversion to type families made the compilation even slower.) * Anonymous type function values One of the most annoying type classes of the llvm package was the IsSized class: class (LLVM.IsType a, IsPositive size) => IsSized a size | a -> size where size is a type-level decimal natural number. Many llvm functions require that an LLVM type has a size where the particular size is not important. However, I always have to name the size type. I also cannot get rid of it using a subclass, like class (IsSized a size) => IsAnonymouslySized a where The 'size' type is somehow sticky. The conversion of this type class to type families is straightforward: class (IsType a, PositiveT (SizeOf a)) => IsSized a where type SizeOf a :: * Now I have to use SizeOf only if needed. I can also easily define sub-classes like class (IsSized a) => C a where * No TypeSynonymInstances At the right hand side of a 'type instance' I can use type synonyms like type instance F T = String without the TypeSynonymInstance extension. This feels somehow more correct than refering to a type synonym in a class instance head like in instance C T String where The compiler does not need to analyze String in order to find the correct instance. * No FlexibleInstances The same applies to instance C (T a) (A (B a)) which is a flexible instance that is not required for type instance F (T a) = A (B a) * No MultiParamTypeClass, No UndecidableInstances I have some type classes that convert a type to another type and a tuple of types to another tuple of types where the element types are converted accordingly. With functional dependencies: class MakeValueTuple haskellTuple llvmTuple | haskellTuple -> llvmTuple where instance (MakeValueTuple ha la, MakeValueTuple hb lb) => MakeValueTuple (ha,hb) (la,lb) The class is a multi-parameter type class and the instance is undecidable. This is much simpler with type families: class MakeValueTuple haskellTuple where type ValueTuple haskellTuple :: * instance (MakeValueTuple ha, MakeValueTuple hb) => MakeValueTuple (ha,hb) where type ValueTuple (ha,hb) = (ValueTuple ha, ValueTuple hb) Thus summarized: Type families may replace several other type extensions. If I ignore the associated type functions then many classes become Haskell 98 with Haskell 98 instances. This is good because those instances prevent instance conflicts with other non-orphan instances. == Disadvantage of TypeFamilies == * Redundant instance arguments I have to write the type arguments both in the instance head and in the function argument. This is especially annoying in the presence of multi-parameter type classes with bidirectional dependencies. E.g. class (a ~ Input parameter b, b ~ Output parameter a) => C parameter a b where type Input parameter b :: * type Output parameter a :: * process :: Causal p (parameter, a) b instance (...) => C (FilterParam a) v (FilterResult v) where type Input (FilterParam a) (FilterResult v) = v type Output (FilterParam a) v = FilterResult v With functional dependencies it was: class C parameter a b | parameter a -> b, parameter b -> a where process :: Causal p (parameter, a) b instance (...) => C (FilterParam a) v (FilterResult v) where * Bidirectional dependencies In GHC-6.12.3 it was not possible to write class (a ~ Back b, b ~ Forth a) => C a b where Fortunately, this is now allowed in GHC-7. But bidirectional dependencies are still cumbersome to work with as shown in the example above. * Equality constraints are not supported for newtype deriving Not so important, just for completeness: http://hackage.haskell.org/trac/ghc/ticket/6088 == Confusions == * Upper case type function names Why are type function names upper case, not lower case? They are not constructors after all. Maybe this is one reason, why I forget from time to time that type functions are not injective. Sure, lower-case type variables are implicitly forall quantified in Haskell 98. In the presence of lower-case type functions we would need explicit forall quantification. * Why can associated types not be exported by C(AssocType) syntax? Why must they be exported independently from the associated class? * FlexibleContexts The context (Class (TypeFun a)) requires FlexibleContexts extension, whereas the equivalent (TypeFun a ~ b, Class b) does not require FlexibleContexts. Best, Henning

1 0

Re: [Haskell-cafe] Tests by properties: origin?
by Janis Voigtländer 06 Jun '12

06 Jun '12

Am 01.06.2012 12:00, schrieb Yves: > Out of curiosity, does someone know if QuickCheck was the first test > framework working through test by properties associated with random > generation or if it drew the idea from something else? > > Because the idea has be retaken by a lot of frameworks in several languages > (seehttp://en.wikipedia.org/wiki/Quickcheck) but I can't find what was > QuickCheck inspiration. How about reading the original paper introducing QuickCheck? If the authors drew inspiration from elsewhere, the paper is for sure where they would tell you, first hand. :-) Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/

5 5

attoparsec double precision, quickCheck and aeson
by Warren Harris 05 Jun '12

05 Jun '12

The double parser provided by Data.Attoparsec.ByteString.Char8 looses precision around the 13-15th decimal place (http://hackage.haskell.org/packages/archive/attoparsec/0.10.2.0/doc/html/Da…) Unfortunately this reeks havoc with my attempts to write a quickCheck test that validates print-read equivalence for a program that uses Aeson. I have tried compensating for this round-off error in my quickCheck generator using a function like this: roundDouble :: Double -> Double roundDouble d = let Right v = A8.parseOnly A8.double (C.pack (show d)) in v which helps in many cases, but for some the parsing seems bi-stable, alternating between two imprecise double values and causing the test to fail. I was wondering if anyone could suggest a better work-around for this problem, or explain why Attoparsec's double parser can't be isomorphic to haskell's. Thanks, Warren

3 7

Extending constraints
by Bas van Dijk 05 Jun '12

05 Jun '12

Hello, I have the following program: -------------------------------------------------------------------------- {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-} import Data.Proxy (Proxy) import Data.Typeable (Typeable, TypeRep, typeOf) data ProxyWrapper constraint = forall a. constraint a => ProxyWrapper (Proxy a) typeOfInnerProxy :: ProxyWrapper constraint -> TypeRep typeOfInnerProxy (ProxyWrapper p) = typeOfArg p typeOfArg :: forall t a. Typeable a => t a -> TypeRep typeOfArg _ = typeOf (undefined :: a) -------------------------------------------------------------------------- Type checking this gives the following expected type error: ProxyWrapper.hs:12:37: Could not deduce (Typeable a) arising from a use of `typeOfArg' from the context (constraint a) bound by a pattern with constructor ProxyWrapper :: forall (constraint :: * -> Constraint) a. (constraint a) => Proxy a -> ProxyWrapper constraint, in an equation for `typeOfInnerProxy' Is there a way to extend the 'constraint' with the 'Typeable' constraint in the type signature of 'typeOfInnerProxy'? Regards, Bas

3 6

Install a script with Cabal?
by Ketil Malde 05 Jun '12

05 Jun '12

Hi, I have a small project that installs a couple of Haskell tools and a script that uses these. Cabal will of course build and install the Haskell programs, but how can I get Cabal to install the script as well? There's a host of UserHooks available¹, but it'd probably be better to have an example than to try to experiment with the right configuration. -k ¹ http://www.haskell.org/cabal/release/cabal-latest/doc/API/Cabal/Distributio… -- If I haven't seen further, it is by standing in the footprints of giants

3 4

Installation of the New Haskell Platform
by Victor Miller 05 Jun '12

05 Jun '12

Before I install it (on on Mac OS X Lion) is there anything, manually, that I need to do in order to keep all of the cabal packages that I've installed, or are the reinstalled automatically? Victor

4 3

ANNOUNCE: hquantlib-0.0.2.3
by Pavel Ryzhov 05 Jun '12

05 Jun '12

Hi Cafe, I'm pleased to announce that HQuantLib has been released recently after a year of sleep. This is mainly a technical release to clean up dependencies and refresh build infrastructure. The library is intended to be a functional port of QuantLib (http://quantlib.org) a free/open-source library for quantitative finance. Also I'd like to thank Nicholas Pezolano for contributing copulas to the library. Business changes highlights: ------------------------------ 1. Copulas in math methods. (CDS and CDO are expected in the next release) 2. Priceable interface for Instruments. (Option pricing to be done in further releases) ============================== Technical changes highlights: ------------------------------ 1. Version upgrade to recent GHC 7.4.1. 2. Dependency clean-ups. 3. Unit-test infrastructure. (HUnit + QuickCheck 2) ============================== SCM: https://github.com/paulrzcz/hquantlib Hackage: http://hackage.haskell.org/package/hquantlib Best regards, Pavel Ryzhov http://paulrz.cz

1 0

Arithmetic expressions with GADTs: parsing
by j.romildo＠gmail.com 05 Jun '12

05 Jun '12

Hello. In order to learn GADTs, I have written the attached program, which defines a type for arithmetic expressions using GADTs, a parser for them, and an evaluation function. But my parser does not typecheck. ghc-7.4.1 gives me the error message: Expr.hs:25:28: Couldn't match expected type `Double' with actual type `Bool' Expected type: Bool -> Expr Double Actual type: Bool -> Expr Bool In the first argument of `(<$>)', namely `B' In the first argument of `(<|>)', namely `B <$> pBool' Any clues on how to fix that? Romildo

5 5

Building gtk on OS/X with platform 2012.2.0.0
by Stuart Hungerford 05 Jun '12

05 Jun '12

Hi, Has anyone successfully cabal installed gtk (from gtk2hs) on OS/X Lion with platform 2012.2.0.0? I setup a new Lion instance with latest XCode and command line tools and Homebrew. I installed gtk+, Cairo, pango etc successfully with Homebrew. The following install log for gtk2hs-buildtools and gtk shows a compilation error (line 5431): http://hpaste.org/69521 I'm probably doing something wrong, so any advice much appreciated. Stu

2 2

Re: [Haskell-cafe] Importing modules in GHCi
by Artyom Kazak 05 Jun '12

05 Jun '12

Oh, I’m sorry. I have Haskell Platform 2011.2.0.1, not 2012.2.0.1. So I guess it was a bug in GHCi…

1 0

Haddock problems with Haskell-Platform2012.2.0.0
by cutsea110 04 Jun '12

04 Jun '12

Hi all. I installed ghc-7.4.1 and haskell-platform-2.12.2.0.0 on debian linux. I'm getting lots of complaints from 'ghc-pkg check', of the following form: ---- Warning: haddock-interfaces: /usr/pkg/share/doc/haskell- platform-2012.2.0.0/html/haskell-platform.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/pkg/share/doc/haskell-platform-2012.2.0.0/ html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/pkg/share/doc/zlib-0.5.3.3/html/ zlib.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/pkg/share/doc/zlib-0.5.3.3/html doesn't exist or isn't a directory ... ---- Any advice?

1 0

Importing modules in GHCi
by Artyom Kazak 04 Jun '12

04 Jun '12

Hi Café, I would greatly appreciate your help. Entering the following in GHCi produces an error message… --> import Data.Time --> getCurrentTime >>= print <interactive>:1:20: No instance for (Show UTCTime) arising from a use of `print' Possible fix: add an instance declaration for (Show UTCTime) In the second argument of `(>>=)', namely `print' In the expression: getCurrentTime >>= print In an equation for `it': it = getCurrentTime >>= print …but when I create a file with `import Data.Time` in it, and load the file in GHCi, everything works as expected. Is it a bug in GHCi, or a feature? I’m using Haskell Platform 2012.2.0.1.

1 0

Restricted Monad question
by Ismael Figueroa Palet 04 Jun '12

04 Jun '12

Hi, I was wondering about defining a restricted monad, and found out this blog post: http://blog.omega-prime.co.uk/?p=127 In GHC 7.4.1 still the recommended way is using the rmonad package, right? I guess the tricks with ConstraintKinds explained in the blog are used for internal implementation of RMonad? Thanks -- Ismael

2 1

Edmonton Functional Programming Users Group meets June 5
by Kevin Charter 04 Jun '12

04 Jun '12

Hi Cafe, The Edmonton Functional Programming Users Group (EFPUG) is meeting tomorrow, June 5, at 6:00pm in room CS 2-49 of the Computing Science building on the University of Alberta north campus. We'll be having a hack night, exploring combinator parsing with a variety of combinator libraries, including Parsec, attoparsec, uu-parsinglib, and parsatron, a Clojure library inspired by Parsec. Everyone is welcome. https://groups.google.com/forum/?fromgroups&hl=en#!forum/edmonton-functiona… Kevin -- Kevin Charter kevin.charter(a)acm.org

1 0

An attempt at an EDSL for platform independent 3D animation
by C K Kashyap 04 Jun '12

04 Jun '12

Hi All, I've written a Haskell program that allows you to describe "dance movements" and it spits out javascript that does animation on an HTML 5 canvas (that's the platform independent bit). https://github.com/ckkashyap/Dancer Please do check it out and let me know what you think - all you'd need is Haskell platform. It's really preliminary at this point - The only animation that I've built using it is "walk" - this makes a matchstick man walk (you have to stretch your imagination a bit). Regards, Kashyap

2 1

FP freelancers group on linkedin
by Ivan Perez 04 Jun '12

04 Jun '12

Hi, A while ago somebody posed the question "What if you get hit by a bus", regarding the apparent lack of Haskell developers that could continue somebody's work if something happened to them. I just created a linkedin group for FP programmers available for hire on a per-project basis ([1]). I hope this enables new collaborations among us, and also that it helps potential clients understand that there are many Haskell professionals available and that they need not fear this language. I also think this may complement the great listing at haskellers.com created by snoyberg, as well as other linkedin groups related to FP. I'd be happy to add links to other "directories" on the group's description page. I'm open to suggestions. Regards, Ivan. [1] http://www.linkedin.com/groups/Functional-programming-freelancers-consultan…

1 0

mapping a concept to a type
by Corentin Dupont 04 Jun '12

04 Jun '12

Hi everybody, I'm still working on implementing a nomic game in Haskell. Although the game is pretty advanced, I'm still confused by one fundamental question: A nomic game is composed of rules. A Rule is a sort of little program submitted by the player during the game. They come in two fashions: - a Normal rule, when executed, can change the state of the Game. - a Meta rule, when executed on another rule, can assess the legality of that rule and change the state of the Game. In those definitions, the Game can be seen as a data structure containing all the current state of the game. Sorry if it's a bit abstract, but as it's fairly complex, I'd prefer to keep it like this for the moment. Thus, I found that those definitions can be translated to two possible type definitions: type NormalRule = State Game () type MetaRule = Rule -> State Game Bool data Rule = MR MetaRule | NR NormalRule *** or *** newtype Rule = Rule {rule :: Maybe Rule -> (State Game Bool)} All rules submitted by the user must be of the type Rule. It is thus important that it is very clean. Which type do you prefer? I find both heavy and redundant. The first forces me to specify if I want an argument of not (with the constructors MR and NR), the second forces me to pass the Nothing argument in the case of a Normal rule, and to return a dummy value (for ex. return True). Do you know of a construction/abstraction that allows having or not an argument (a variable number of arguments, here zero or one)? Thanks! Corentin

3 5