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Haskell meetups near Lund, Sweden?
by Carlo Hamalainen 24 Aug '13

24 Aug '13

Hi, Are there any Haskell or FP meetups near Lund, Sweden? I will be living there from October. Cheers, -- Carlo Hamalainen http://carlo-hamalainen.net

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 120, Issue 35
by althainz 24 Aug '13

24 Aug '13

Von Samsung Mobile gesendet -------- Ursprüngliche Nachricht -------- Von: haskell-cafe-request(a)haskell.org Datum: 23.08.2013 11:36 (GMT+01:00) An: haskell-cafe(a)haskell.org Betreff: Haskell-Cafe Digest, Vol 120, Issue 35 Send Haskell-Cafe mailing list submissions to haskell-cafe(a)haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/haskell-cafe or, via email, send a message with subject or body 'help' to haskell-cafe-request(a)haskell.org You can reach the person managing the list at haskell-cafe-owner(a)haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Haskell-Cafe digest..." Today's Topics: 1. Re: Yet Another Forkable Class (John ExFalso) 2. Re: Yet Another Forkable Class (suhorng Y) 3. Lifting strictness to types (Thiago Negri) 4. Re: Lifting strictness to types (Tom Ellis) 5. Re: Lifting strictness to types (Thiago Negri) 6. instance Alternative ZipList (Stefan Mehner) 7. Re: Lifting strictness to types (Bardur Arantsson) 8. Hoogle vs Hayoo (jabolopes(a)google.com) 9. Re: Hoogle vs Hayoo (Mateusz Kowalczyk) 10. Re: monoids induced by Applicative/Alternative/Monad/MonadPlus? (Petr Pudl?k) 11. Re: Hoogle vs Hayoo (Johannes Waldmann) 12. Re: Yet Another Forkable Class (oleg(a)okmij.org) 13. Re: Hoogle vs Hayoo (Erik Hesselink) 14. Conduit : is it possible to write this function? (Erik de Castro Lopo) 15. Re: Hoogle vs Hayoo (Daniel Trstenjak) 16. Re: Conduit : is it possible to write this function? (Michael Snoyman) 17. typeclass constraints (TP) 18. Re: typeclass constraints (Adam Gundry) ---------------------------------------------------------------------- Message: 1 Date: Thu, 22 Aug 2013 15:50:23 +0100 From: John ExFalso <0slemi0(a)gmail.com> To: "Alberto G. Corona" <agocorona(a)gmail.com> Cc: "oleg(a)okmij.org" <oleg(a)okmij.org>, haskell-cafe <Haskell-cafe(a)haskell.org> Subject: Re: [Haskell-cafe] Yet Another Forkable Class Message-ID: <CAJEmqMj8Y7KoNVPS7x6zvvtkAmYheax9oigTPLLHDchfHde_pA(a)mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" To be honest I'm not so sure about these "effects"... Simply the fact that the Member class needs -XOverlappingInstances means that we cannot have duplicate or polymorphic effects. It will arbitrarily pick the first match in the former and fail to compile in the latter case. Furthermore I don't really understand the way open sums are implemented. These unions should be disjoint, but the way they're implemented in the paper they try to be "true" unions which cannot be done as that would need type equality (-XOverlappingInstances is a hack around this) A correct disjoint open sum would behave well with duplicate and polymorphic types in the type list. For example we should be able to project the open sum equivalent of Either String String into the second String but we cannot with the implementation in the paper. This means we need to ~index~ the type list instead of picking the result type and "trying for equality" with each entry. Something like this: http://lpaste.net/92069 Of course this is very inconvenient and simply replaces the monad transformers' lifts with a static index into the "effect" list. In general I think there is no convenient way of stacking effects that is also type safe. At some point we have to disambiguate which effect we are trying to use one way or the other. The implementation in the paper simply picks a heuristic and chooses the first effect that seems to match and discards the others. On 22 August 2013 12:15, Alberto G. Corona <agocorona(a)gmail.com> wrote: > The paper is very interesting: > > http://www.cs.indiana.edu/~sabry/papers/exteff.pdf > > It seems that the approach is mature enough and it is better in every way > than monad transformers, while at the same time the syntax may become > almost identical to MTL for many uses. > > I only expect to see the library in Hackage with all the blessings, and > with all the instances of the MTL classes in order to make the transition > form monad transformers to ExtEff as transparent as possible > > > 2013/8/22 <oleg(a)okmij.org> > > >> Perhaps effect libraries (there are several to choose from) could be a >> better answer to Fork effects than monad transformers. One lesson from >> the recent research in effects is that we should start thinking what >> effect we want to achieve rather than which monad transformer to >> use. Using ReaderT or StateT or something else is an implementation >> detail. Once we know what effect to achieve we can write a handler, or >> interpreter, to implement the desired operation on the World, obeying >> the desired equations. And we are done. >> >> For example, with ExtEff library with which I'm more familiar, the >> Fork effect would take as an argument a computation that cannot throw >> any requests. That means that the parent has to provide interpreters >> for all child effects. It becomes trivially to implement: >> >> > Another example would be a child that should not be able to throw >> errors as >> > opposed to the parent thread. >> It is possible to specify which errors will be allowed for the child >> thread (the ones that the parent will be willing to reflect and >> interpret). The rest of errors will be statically prohibited then. >> >> > instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) >> where >> > fork (ReaderT f) = liftIO . forkIO . f =<< getSink >> >> This is a good illustration of too much implementation detail. Why do we >> need to know of (Sink p) as a Reader layer? Would it be clearer to >> define an Effect of sending to the socket? Computation's type will >> make it patent the computation is sending to the socket. >> The parent thread, before forking, has to provide a handler for that >> effect (and the handler will probably need a socket). >> >> Defining a new class for each effect is possible but not needed at >> all. With monad transformers, a class per effect is meant to hide the >> ordering of transformer layers in a monad transformer stack. Effect >> libraries abstract over the implementation details out of the >> box. Crutches -- extra classes -- are unnecessary. We can start by >> writing handlers on a case-by-case basis. Generalization, if any, >> we'll be easier to see. From my experience, generalizing from concrete >> cases is easier than trying to write a (too) general code at the >> outset. Way too often, as I read and saw, code that is meant to be >> reusable ends up hardly usable. >> >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > > -- > Alberto. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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monoids induced by Applicative/Alternative/Monad/MonadPlus?
by Petr Pudlák 23 Aug '13

23 Aug '13

Dear Haskellers, are these monoids defined somewhere? import Control.Applicativeimport Data.Monoid newtype AppMonoid m a = AppMonoid (m a)instance (Monoid a, Applicative m) => Monoid (AppMonoid m a) where mempty = AppMonoid $ pure mempty mappend (AppMonoid x) (AppMonoid y) = AppMonoid $ mappend <$> x <*> y-- With the () monoid for `a` this becames the monoid of effects. newtype AltMonoid m a = AltMonoid (m a)instance Alternative m => Monoid (AltMonoid m a) where mempty = AltMonoid empty mappend (AltMonoid x) (AltMonoid y) = AltMonoid $ x <|> y (and similarly for Monad/MonadPlus, until they become subclasses of Applicative?) Best regards, Petr

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Munich Haskell Meeting, August 26th
by Christian Neukirchen 23 Aug '13

23 Aug '13

Dear all, our monthly Haskell Meeting will take place next week, Monday the 26th of August, at 19h30 in Munich. Please note that we will meet this time at the Max-Emanuel-Brauerei[1]. Last time, much more people showed up than registered and our table was pretty cramped: thus, if you plan to join, *please* add yourself to the Doodle at http://www.doodle.com/wfgkscwhkakf9uqa We also apologize that our website http://www.haskell-munich.de/ is currently unavailable. Please post to high-order-munich(a)fs.lmu.de or myself directly if you have any questions. Have a nice weekend and see you soon, chris [1] http://www.max-emanuel-brauerei.de/ -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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typeclass constraints
by TP 23 Aug '13

23 Aug '13

Hi everybody, There is something I do not understand in the way typeclass constraints are inferred. 1/ Take the following function definition: sum' [] = [] sum' (x:xs) = x + sum' xs GHCI correctly gives: > :t sum' sum' :: Num [a] => [[a]] -> [a] So it has inferred that the type list has to be an instance of Num for sum' to be able to work. It will give an error if we try to use sum' without implementing the instance. 2/ Now, take the following definition: ------------------------ {-# LANGUAGE TemplateHaskell #-} import Language.Haskell.TH import Language.Haskell.TH.Syntax p :: a -> ExpQ p n = [| show n |] ------------------------ We obtain an error if we try to load it in GHCI: No instance for (Lift a) arising from a use of `n' Possible fix: add (Lift a) to the context of the type signature for p :: a -> ExpQ In the first argument of `show', namely `n' In the Template Haskell quotation [| show n |] In the expression: [| show n |] And indeed, if we use instead: ------------------------ {-# LANGUAGE TemplateHaskell #-} import Language.Haskell.TH import Language.Haskell.TH.Syntax p :: Lift a => a -> ExpQ p n = [| show n |] ------------------------ it works correctly. Why GHC is able to infer the typeclass constraint (Num a) in 1/, but not (Lift a) in 2/? Thanks in advance, TP

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Lifting strictness to types
by Thiago Negri 22 Aug '13

22 Aug '13

I've just read the post "Destroying Performance with Strictness" by Neil Mitchell [1]. One of the comments from an Anonymous says: How hard would it be to lift strictness annotations to type-level? E.g. instead of f :: Int -> Int f !x = x + 1 write f :: !Int -> Int f x = x + 1 which would have the same effect. At least it would be transparent to the developer using a particular function. The problem I see with this approach is on type classes, as it would be impossible to declare a type instance with strict implementation to a type class that used lazy types. Is this a real problem? Is it the only one? [1] http://neilmitchell.blogspot.ru/2013/08/destroying-performance-with-strictn…

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instance Alternative ZipList
by Stefan Mehner 22 Aug '13

22 Aug '13

I had an idea for |instance Alternative ZipList|, which doesn't seem to exist so far. Maybe there just is no need for it. Please tell me what you think. After giving the instance definition I will add some intuition on why this might be useful. Then some words on laws and other conceivable instances. This appeared on stackoverflow before to make sure it is new ( http://stackoverflow.com/questions/18210765/instance-alternative-ziplist-in… ). J. Abrahamson proposed to move this to the list, so here it is. The only thing I found so far is by AndrewC on stackoverflow: There are two sensible choices for Zip [1,3,4] <|> Zip [10,20,30,40]: Zip [1,3,4] because it's first - consistent with Maybe Zip [10,20,30,40] because it's longest - consistent with Zip [] being discarded (Here Zip is basically ZipList with the known Applicative instance.) Proposed instance ================= I think the answer should be Zip [1,3,4,40]. Let's see an instance: > instance Aternative Zip where > empty = Zip [] > Zip xs <|> Zip ys = Zip (go xs ys) where > go [] ys = ys > go xs [] = xs > go (x:xs) (_:ys) = x : go xs ys The only Zip a we can produce without knowing the type argument a is Zip [] :: Zip a, so there is little choice for empty. If the empty list is the neutral element of the monoid, we might be tempted to use list concatenation as the monoid operation. However, go is not (++), since every time we use one entry of the first argument list, we drop one of the second. Thus we have a kind of overlay: The left argument list hides the beginning of the right one (or all of it). [ 1, 3, 4,40] [10,20,30,40] [ 1, 3, 4] [ 1, 3, 4] ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ | | | | | | | | | | | | | | [ 1, 3, 4] | [10,20,30,40] []| | | [ 1, 3, 4] [10,20,30,40] [ 1, 3, 4] [ 1, 3, 4] [] (use monospace for ascii-art) For the some/many methods I'd guess > some (Zip z) = Zip (map repeat z) > many (Zip z) = Zip (map repeat z ++ repeat []) where some takes a ziplist and replaces every entry x by repeat x and many does the same but additionally extends the ziplist with empty lists. Probably not particularly usefull, but that's what the recursive definition of some and many gives. What is it good for? ==================== One intuition behind ziplists is processes: A finite or infinite stream of results. When zipping, we combine streams, which is reflected by the Applicative instance. When the end of the list is reached, the stream doesn't produce further elements. This is where the Alternative instance comes in handy: We can name a replacement, taking over as soon as the default process terminates. For example we could write < fmap Just foo <|> pure Nothing to wrap every element of the ziplist foo into a Just and continue with Nothing afterwards. The resulting ziplist is infinite, reverting to a default value after all (actual) values have been used up. This could of course be done by hand by appending an infinite list inside the Zip constructor. Yet the above is more elegant and does not assume knowledge of constructors, leading to higher code reusability. Another intuition one might have of zipLists is partial functions on naturals. Using this analogy, <|> behaves like the Monoid instance of Map and IntMap. Lawfulness ========== The definition of <|> given above is associative and the empty list really is the empty element. We also have < Zip [] <*> xs = fs <*> Zip [] = Zip [] < (fs <|> gs) <*> xs = fs <*> xs <|> gs <*> xs < fs <*> (xs <|> ys) = fs <*> xs <|> fs <*> ys so all the laws you could ask for are satisfied (which is not true for list concatenation by the way). This instance is consistent with the one for Maybe: Choice is biased to the left, yet when the left argument is unable to produce a value, the right argument takes over. The functions > zipToMaybe :: Zip a -> Maybe a > zipToMaybe (Zip []) = Nothing > zipToMaybe (Zip (x:_)) = Just x > maybeToZip :: Maybe a -> Zip a > maybeToZip Nothing = Zip [] > maybeToZip (Just x) = pure x are morphisms of alternatives (meaning psi x <|> psi y = psi (x <|> y) and psi x <*> psi y = psi (x <*> y)). Other options ============= Before putting this up to discussion I have to say this was conceived in an armchair: Until now I don't really know of any concrete uses for this instance. Might be there are none. Some words on AndrewC's instances (none of which has been put forward as a serious suggestion). Picking the longer list has a number of problems to it. When it comes to infinite lists (which are introduced by pure), we get undefined values. Also it's not very lazy: We have to evaluate both arguments until the shorter list ends just to get the first entry of the result. When both lists are of equal length we probably pick the left one, which defies the laws (distributivity in particular). Finally, you could just write > maximumBy (compare `on` length) [ys,xs] which is surprisingly readable (and biassed to the right). Picking the first nonempty list also feels strange: The 100-th entry of xs <|> ys should only depend on the 100-th entry of xs and ys or their absence. This is reasonable since processes shouldn't care to much about what happend 100 steps earlier. Yet if we only check for empty lists at the very beginning, the choice is permanent. Again, if you insist on doing so, just use > maximumBy (compare `on` not . null) [ys,xs] The overlaying instance given above introduces some non-trivial behaviour by allowing to mix lists together (instead of just returning one of the arguments). I can't think of any way to build this by combining four existing functions (avoiding length and drop). It satisfies all the laws and I can image it might at least be of some use.

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Re: [Haskell-cafe] haskore -> lilypond -> typesetting?
by Stephen Tetley 22 Aug '13

22 Aug '13

"Here's one I did earlier..." http://www.flickr.com/photos/44929957@N03/4459628487/lightbox/ This is Haskore implementation of Chick Corea's "Child Song 6" rendered to LilyPond - I don't imagine Mr. Corea's publishers will be sending me a takedown request any time soon. There's a lot missing from Haskore that is needed to make good scores - the renderer of the above took a lot of effort with metrical grouping but the result is still abysmal. I doubt mathematics can help (common practice) music typesetting much - Western notation has had a thousand years to develop without the constraint of a regular syntax; so if Lilypond is horrible it is mostly the fault of what it tries to typeset (it does make some unwarranted mistakes like over-restricting the characters it can use for "variable" names and its parenthesizing is horrible). On 21 August 2013 14:05, Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de>wrote: > I tried using lilypond ( http://www.lilypond.org/ ) > for typesetting of sheet music. > > While the output looks nice, the input language IMHO is quite horrible, > because the underlying data/execution model is underspecified. > For some parts, it tries to describe the logical structure of the score; > but for others, the layout; and in addition it has several non-obvious > context-dependencies (but see below), preventing modularity. > > Is there a better option? E.g., starting from a clear mathematical model, > as in Haskore, and use lilypond only as a PDF rendering engine? > > Do I want hly / hts perhaps? http://rd.slavepianos.org/?t=hly > > > As I see it, the main high-level design problem > is that the source language needs partial evaluation annotations > for abstractions applications: sometimes they should be expanded > (for MIDI rendering, always) and sometimes not (in typesetting, > to create repetition marks instead of actually repeating notes). > > > PS: I agree that some of lilypond's context dependencies > (relative pitch, implicit note length) do really save > large amounts of tedious typing: "c4 e g a c1" is much more economical > than "[c 1 qn, e 1 qn, g 1 qn , a 1 qn, c 2 fn]" > which I guess is the Haskore equivalent. > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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A question about laziness and performance in document serialization.
by Kyle Hanson 22 Aug '13

22 Aug '13

So I am not entirely clear on how to optimize for performance for lazy bytestrings. Currently I have a (Lazy) Map that contains large BSON values (more than 1mb when serialized each). I can serialize BSON documents to Lazy ByteStrings using Data.Binary.runPut. I then write this bytestring to a socket using Network.Socket.ByteString.Lazy. My question is this, if the Map object doesn't change (no updates) when it serializes the same document to the socket 2x in a row, does it re-evaluate the whole BSON value and convert it to a bytestring each time? Lets say I wanted to have a cache of bytestings so I have another Map object that has the serialized bytestrings that I populate it with every time the original BSON Map changes. Should the map be strict or lazy? Should the bytestrings it stores be strict or lazy? Any help in understanding laziness would be appreciated. -- Kyle Hanson

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Retrieving Haddock comments with haskell-src-exts
by Mateusz Kowalczyk 22 Aug '13

22 Aug '13

I'm writing a small tool to help to analyse Haddock comments in Haskell source files to help me to indicate any potential breakage to documentation in existing source files. Currently I'm doing the parsing with the GHC's ‘parser’ function with Opt_Haddock set and I filter out everything I don't need. There are problems with this approach: determining any extensions and options used is vital for valid parsing of the files and a large amount of source files outright fails to parse without this information. Fortunately, haskell-src-exts exists and it can deal with all (or most) of this for me. Unfortunately, there doesn't seem to be any way to get it to recognise Haddock comments and the only option available is all comments or no comments at all. It's not easily possible to stitch these together without analysing the whole parse result. Currently I'm thinking of parsing out extensions and pragmas used using haskell-src-exts and then feeding those to GHC, effectively parsing the second time. Is there a way to avoid this? There's ‘lexTokenStream’ but I believe it has similar problems to ‘parser’, that is, needing to know the extensions beforehand. -- Mateusz K.

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Haskell Weekly News: Issue 277
by Daniel Santa Cruz 22 Aug '13

22 Aug '13

Welcome to issue 277 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 11 to 17, 2013. Quotes of the Week * psygnisfive: Seminearring is the task of giving seminars. * Taneb: You know you've made it when SPJ asks you for help with Haskell. * byorgey: . o O ( "For my next trick, I will CPS-encode this rabbit!" -- edwardk ) * shachaf: I prefer the cogolden rule: When CPS-transforming, doubt. * edwardk: "You see these scars? I got them back in 7.7." * danharaj: parsing is hard, let's go shopping Top Reddit Stories * Mio: A High-Performance Multicore IO Manager for GHC [pdf] Domain: haskell.cs.yale.edu, Score: 147, Comments: 31 On Reddit: [1] http://goo.gl/ZTy1Yl Original: [2] http://goo.gl/6foNWC * Destroying Performance with Strictness Domain: neilmitchell.blogspot.ru, Score: 92, Comments: 45 On Reddit: [3] http://goo.gl/1rkx3a Original: [4] http://goo.gl/rb5UnM * A great series of posts on the power of monoids Domain: self.haskell, Score: 78, Comments: 10 On Reddit: [5] http://goo.gl/TmAAwT Original: [6] http://goo.gl/TmAAwT * Cellular Automata by Edward Kmett Domain: fpcomplete.com, Score: 69, Comments: 7 On Reddit: [7] http://goo.gl/EIFK5A Original: [8] http://goo.gl/03mNhV * Elm 0.9 - New type checker, much better error messages Domain: elm-lang.org, Score: 50, Comments: 14 On Reddit: [9] http://goo.gl/ngSBPX Original: [10] http://goo.gl/4Jp3l6 * Revisiting Matrix Multiplication, Part I by Edward Kmett Domain: fpcomplete.com, Score: 45, Comments: 4 On Reddit: [11] http://goo.gl/gYfm5S Original: [12] http://goo.gl/bg75ZX * Revisiting Matrix Multiplication, Part II by Edward Kmett Domain: fpcomplete.com, Score: 37, Comments: 9 On Reddit: [13] http://goo.gl/qU2r69 Original: [14] http://goo.gl/5cRq72 * Roles: a new feature of GHC (patch GND + type families) Domain: typesandkinds.wordpress.com, Score: 34, Comments: 28 On Reddit: [15] http://goo.gl/HhVOTu Original: [16] http://goo.gl/Twx5YB * Type Inference, Haskell and Dependent Types :: PDF Domain: personal.cis.strath.ac.uk, Score: 33, Comments: 0 On Reddit: [17] http://goo.gl/MbtYGO Original: [18] http://goo.gl/uOx01F * Embedded Languages for High-Performance Computing [Chakravarty, PDF 70 slides] Domain: speakerdeck.com, Score: 31, Comments: 4 On Reddit: [19] http://goo.gl/mb2frN Original: [20] http://goo.gl/EEG0Wk * Graph drawing with gloss Domain: mazzo.li, Score: 30, Comments: 7 On Reddit: [21] http://goo.gl/j0CZmo Original: [22] http://goo.gl/5ZtFuu * Benchmarking sqlite-simple against Python and C Domain: nurpax.github.io, Score: 28, Comments: 9 On Reddit: [23] http://goo.gl/CNyjWa Original: [24] http://goo.gl/gFmWgT * What is "lifting"? Explained on stackoverflow Domain: stackoverflow.com, Score: 25, Comments: 7 On Reddit: [25] http://goo.gl/HzXWb9 Original: [26] http://goo.gl/hQxTce * GHC API: Interpreted, compiled and package modules Domain: parenz.wordpress.com, Score: 23, Comments: 0 On Reddit: [27] http://goo.gl/xUeUYq Original: [28] http://goo.gl/llQsyE * Time to start writing on the new binding: SDL 2.0 release. Domain: lists.libsdl.org, Score: 22, Comments: 15 On Reddit: [29] http://goo.gl/G0ut36 Original: [30] http://goo.gl/MuO1uB * FP Complete's Haskell programming competition now accepting entries Domain: fpcomplete.com, Score: 21, Comments: 7 On Reddit: [31] http://goo.gl/tbEBKG Original: [32] http://goo.gl/ADsrJ * Pretext by experiments and guesses Domain: fpcomplete.com, Score: 18, Comments: 15 On Reddit: [33] http://goo.gl/oVpKIe Original: [34] http://goo.gl/2PFthz Top StackOverflow Questions * Haskell random numbers suddenly start to “converge” after months of running votes: 30, answers: 1 Read on SO: [35] http://goo.gl/oxUAQ5 * Why does ghc generate so many useless-looking nested case expressions? votes: 10, answers: 1 Read on SO: [36] http://goo.gl/6cdTd1 Until next time, [37]+Daniel Santa Cruz References 1. http://haskell.cs.yale.edu/wp-content/uploads/2013/08/hask035-voellmy.pdf 2. http://www.reddit.com/r/haskell/comments/1k6fsl/mio_a_highperformance_multi… 3. http://neilmitchell.blogspot.ru/2013/08/destroying-performance-with-strictn… 4. http://www.reddit.com/r/haskell/comments/1ker84/destroying_performance_with… 5. http://www.reddit.com/r/haskell/comments/1ka4q6/a_great_series_of_posts_on_… 6. http://www.reddit.com/r/haskell/comments/1ka4q6/a_great_series_of_posts_on_… 7. https://www.fpcomplete.com/user/edwardk/cellular-automata 8. http://www.reddit.com/r/haskell/comments/1kj5zh/cellular_automata_by_edward… 9. http://elm-lang.org/blog/announce/version-0.9.elm 10. http://www.reddit.com/r/haskell/comments/1ka0ld/elm_09_new_type_checker_muc… 11. https://www.fpcomplete.com/user/edwardk/revisiting-matrix-multiplication-pa… 12. http://www.reddit.com/r/haskell/comments/1kecqt/revisiting_matrix_multiplic… 13. https://www.fpcomplete.com/user/edwardk/revisiting-matrix-multiplication-pa… 14. http://www.reddit.com/r/haskell/comments/1khu61/revisiting_matrix_multiplic… 15. http://typesandkinds.wordpress.com/2013/08/15/roles-a-new-feature-of-ghc/ 16. http://www.reddit.com/r/haskell/comments/1kgivs/roles_a_new_feature_of_ghc_… 17. https://personal.cis.strath.ac.uk/adam.gundry/thesis/ 18. http://www.reddit.com/r/haskell/comments/1k6g0w/type_inference_haskell_and_… 19. https://speakerdeck.com/mchakravarty/embedded-languages-for-high-performanc… 20. http://www.reddit.com/r/haskell/comments/1kc9j1/embedded_languages_for_high… 21. http://mazzo.li/posts/graph-drawing.html 22. http://www.reddit.com/r/haskell/comments/1kb36j/graph_drawing_with_gloss/ 23. http://nurpax.github.io/posts/2013-08-17-sqlite-simple-benchmarking.html 24. http://www.reddit.com/r/haskell/comments/1kjpwz/benchmarking_sqlitesimple_a… 25. http://stackoverflow.com/questions/2395697/haskell-newbie-question-what-is-… 26. http://www.reddit.com/r/haskell/comments/1k7i4c/what_is_lifting_explained_o… 27. http://parenz.wordpress.com/2013/08/17/ghc-api-interpreted-compiled-and-pac… 28. http://www.reddit.com/r/haskell/comments/1kjl3h/ghc_api_interpreted_compile… 29. http://lists.libsdl.org/pipermail/sdl-libsdl.org/2013-August/089854.html 30. http://www.reddit.com/r/haskell/comments/1k9wh8/time_to_start_writing_on_th… 31. https://www.fpcomplete.com/business/competition/competition-overview 32. http://www.reddit.com/r/haskell/comments/1k8ox7/fp_completes_haskell_progra… 33. https://www.fpcomplete.com/user/tel/pretext-by-experiments-and-guesses 34. http://www.reddit.com/r/haskell/comments/1kkoup/pretext_by_experiments_and_… 35. http://stackoverflow.com/questions/18200283/haskell-random-numbers-suddenly… 36. http://stackoverflow.com/questions/18192184/why-does-ghc-generate-so-many-u… 37. https://plus.google.com/105107667630152149014/about

1 0

Yet Another Forkable Class
by John ExFalso 22 Aug '13

22 Aug '13

TLDR: New forkable monad/transformer suggestion http://pastebin.com/QNUVL12v(hpaste is down) Hi, There are a dozen packages on hackage defining a class for monads that can be forked, however none of these are modular enough to be useful in my opinion. In particular the following are not addressed: 1. Cases when the child thread's monad is different from the parent's 2. Monad transformers (this is somewhat addressed with Control.Monad.Trans.Control) I will try to demonstrate both issues with an example. 1. WebSockets WebSockets is a monad that cannot itself be forked. This is because at any given time there should only be a single thread listening on a websocket. However there is a reasonable monad that can be forked off, namely one that can send to the websocket - one that has access to the Sink. So first off a "Forkable" class should not look like this: class (MonadIO m, MonadIO n) => Forkable m where fork :: m () -> m ThreadId But rather like this: class Forkable m n where fork :: n () -> m ThreadId For our example the instance would be instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) where fork (ReaderT f) = liftIO . forkIO . f =<< getSink Another example would be a child that should not be able to throw errors as opposed to the parent thread. 2. ReaderT Continuing from the previous example to demonstrate the need to distinguish forkable transformers. Say we have some shared state S that both parent and child should have access to: type Parent p = ReaderT (TVar S) (WebSockets p) type Child p = ReaderT (TVar S) (ReaderT (Sink p) IO) The "forkability" of Child from Parent should be implied, however with Forkable we have to write a separate instance. So what I suggest is a second class: class ForkableT t where forkT :: (Forkable m n) => t n () -> t m ThreadId And then: instance ForkableT (ReaderT r) where forkT (ReaderT f) = ReaderT $ fork . f We can also introduce a default for Forkable that uses a ForkableT instance: class (MonadIO m, MonadIO n) => Forkable m n where fork :: n () -> m ThreadId default fork :: ForkableT t => t n () -> t m ThreadId fork = forkT instance (Forkable m n) => Forkable (ReaderT r m) (ReaderT r n) This means Child is automatically Forkable from Parent, no need to write a specific case for our specific monads (and if we newtype it we can use -XGeneralizedNewtypeDeriving) Note how MonadTransControl already solves the specific problem of lifting a forking operation into ReaderT. However consider ResourceT from Control.Monad.Resource: it is basically a ReaderT, however in order to safely deallocate resources when sharing reference counting is needed. This means a simple lift would not suffice. We can nevertheless provide a default ForkableT based on MonadTransControl: class ForkableT t where forkT :: (Forkable m n) => t n () -> t m ThreadId default forkT :: (MonadTransControl t, Forkable m n) => t n () -> t m ThreadId forkT t = liftWith $ \run -> fork $ run t >> return () Actually resourcet's reference counting resourceForkIO also nicely demonstrates the first problem: type Parent p = ResourceT (WebSockets p) type Child p = ResourceT (ReaderT (Sink p) IO) Note how we cannot use resourceForkIO without touching the underlying monads. What do you think? Is there already an established way of modular forking? I wouldn't like to litter hackage with another unusable Forkable class:)

1 0

ANN: hspec-test-framework - Run test-framework tests with Hspec
by Simon Hengel 21 Aug '13

21 Aug '13

Hi, I just released hspec-test-framework[1] and hspec-test-framework-th[2] to Hackage. They can be used to run test-framework tests with Hspec unmodified. This can also be used to work around test-framework's incompatibility with QuickCheck-2.6 and base-4.7.0 ;) Have a look at the README for usage instructions: https://github.com/sol/hspec-test-framework#readme Cheers, Simon [1] http://hackage.haskell.org/package/hspec-test-framework [2] http://hackage.haskell.org/package/hspec-test-framework-th

4 4

haskore -> lilypond -> typesetting?
by Johannes Waldmann 21 Aug '13

21 Aug '13

I tried using lilypond ( http://www.lilypond.org/ ) for typesetting of sheet music. While the output looks nice, the input language IMHO is quite horrible, because the underlying data/execution model is underspecified. For some parts, it tries to describe the logical structure of the score; but for others, the layout; and in addition it has several non-obvious context-dependencies (but see below), preventing modularity. Is there a better option? E.g., starting from a clear mathematical model, as in Haskore, and use lilypond only as a PDF rendering engine? Do I want hly / hts perhaps? http://rd.slavepianos.org/?t=hly As I see it, the main high-level design problem is that the source language needs partial evaluation annotations for abstractions applications: sometimes they should be expanded (for MIDI rendering, always) and sometimes not (in typesetting, to create repetition marks instead of actually repeating notes). PS: I agree that some of lilypond's context dependencies (relative pitch, implicit note length) do really save large amounts of tedious typing: "c4 e g a c1" is much more economical than "[c 1 qn, e 1 qn, g 1 qn , a 1 qn, c 2 fn]" which I guess is the Haskore equivalent.

3 2

What am I missing? Cycle in type synonym declarations
by David Fox 21 Aug '13

21 Aug '13

This file gives me the error "Cycle in type synonym declarations" Can anyone tell me why? I'm just trying to write a function to create a type that is a FooT with the type parameter fixed. {-# LANGUAGE TemplateHaskell #-} import Language.Haskell.TH (Q, Dec, TypeQ) data FooT a = FooT a foo :: TypeQ -> Q [Dec] foo t = [d| type Bar = FooT $t |]

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abs minBound < (0 :: Int) && negate minBound == (minBound :: Int)
by Nicolas Frisby 21 Aug '13

21 Aug '13

The docs at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:g… give a NB mentioning that (abs minBound == minBound) is possible for fixed-width types. This holds, for example, at Int. It is also the case that (negate minBound == minBound). Two questions: 1) This behavior surprised me. Does it surprise enough people to include a warning in the Haddock for abs and negate? IE Here. http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:N… 2) Is this a common behavior in other languages? My tinkering with gcc suggests it does not support the value -2^63, but instead bottoms out at (-2^63+1). Thanks.

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ANN: Google co-sponsoring ZuriHac 2013
by Bas van Dijk 21 Aug '13

21 Aug '13

Dear Haskellers, I would like to remind you that the Zurich FP Afternoon (with a keynote by Simon Marlow) is taking place next week (13:00, Thursday, 29 August) and is directly followed by the ZuriHac 2013 Haskell Hackathon [1]. There are still some places available at both events -- you're welcome to register at: http://bit.ly/ZuriHac2013Reg Google Switzerland will co-sponsor the FP Afternoon and ZuriHac 2013 together with ETH Zurich and Erudify. Their sponsorship means we can give each attendee a free t-shirt (picture on the wikipage [2]) and free food and drinks during the FP Afternoon. Google is also offering a quiz during the FP Afternoon with prizes including a Galaxy S4 mini. I hope to see you there! Bas [1] http://www.haskell.org/haskellwiki/ZuriHac2013 [2] http://www.haskell.org/haskellwiki/ZuriHac2013#News

1 0

Ideas on a fast and tidy CSV library
by Justin Paston-Cooper 21 Aug '13

21 Aug '13

Dear All, Recently I have been doing a lot of CSV processing. I initially tried to use the Data.Csv (cassava) library provided on Hackage, but I found this to still be too slow for my needs. In the meantime I have reverted to hacking something together in C, but I have been left wondering whether a tidy solution might be possible to implement in Haskell. I would like to build a library that satisfies the following: 1) Run a function <<f :: a_1 -> ... -> a_n -> m (Maybe (b_1, ..., b_n))>>, with <<m>> some monad and the <<a>>s and <<b>>s being input and output. 2) Be able to specify a maximum record string length and output record string length, so that the string buffers used for reading and outputting lines can be reused, preventing the need for allocating new strings for each record. 3) Allocate only once, the memory where the parsed input values, and output values are put. 4) The library's main function should take some kind of data structure describing the types of the function, the function itself and the filenames of input and output (could also be stdin/stdout). I am not sure yet what would be that best value of <<m>>. I would like to most importantly efficiently, and if possible, purely allow changes in state to a number of variables, such as an aggregation over a certain field in the input. I do not currently have knowledge of the FFI, and how it might be used in this case. I would appreciate any suggestions as to where I should look further. Regards, Justin Paston-Cooper

3 6

PPC binaries for GHC 7.x?
by Rogan Creswick 21 Aug '13

21 Aug '13

Does anyone have PPC binaries for GHC 7.x? I've been trying to help a PPC user compile a large haskell application, and it (and it's dependencies) require a newer ghc; the latest ppc binaries we've found are for 6.10, and we have been unable to compile a never ghc from source (6.12 /almost/ worked, but eventually failed with a linker error that appears to be unsolvable in the near-term). Thanks! Rogan

2 2

GHC and backwards compatibility
by Henning Thielemann 21 Aug '13

21 Aug '13

If you use $ cabal install --constraint="array installed" then cabal-install is forced to use the installed version of array. If a package conflicts with this version, then it will report the conflicting packages.

1 0

ANNOUNCE: haskell-src-exts 1.14.0
by Niklas Broberg 21 Aug '13

21 Aug '13

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.14.0! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * git repo: https://github.com/haskell-suite/haskell-src-exts<http://code.haskell.org/haskell-src-exts> There are two primary reasons for this release, and a number of smaller ones. The first primary reason is technical: haskell-src-exts 1.14 revamps the Extension datatype, among other things to allow turning extensions on and off (similar to what Cabal allows). We also introduce the concept of a Language, separate from a set of extensions. This is the only backwards-incompatible change in this release. The second reason is structural: haskell-src-exts is now part of a larger context -- the Haskell Suite. The package has a new home on github (see above), alongside its new cool friends: haskell-names and haskell-packages. There is also a really nice issue tracker there - please help me fill it, or better yet, empty it! What this release does *not* cover is support for the extensions added to GHC in recent time (with the exceptions of CApiFFI and InterruptibleFFI). Work is in progress on many of these, and there will be another major release not far off in the future. This release owes many thanks to Roman Cheplyaka in particular, as well as Erik Hesselink, Simon Meier and David Fox. Thanks a lot! Complete changelog: 1.13.6 --> 1.14.0 =============== * Modernize the Extension datatype in L.H.E.Extension, following the lead of Cabal, to allow negative and positive extension modifiers (turning features on and off). You need to worry about backwards-incompatible changes if any of the following pertains to you: 1) If you use the Extension datatype programmatically - it has changed significantly (see documentation). 2) The ParseMode record now has one more field (baseLanguage :: Language), which might give you a type error. 3) The behavior of the (extensions :: [Extension]) field has changed, which could bite you if you pass custom extensions in the ParseMode. Previously, the ParseMode defaulted to the list of extensions accepted by Haskell2010, and if you set the list explicitly you would override this. Now, the defaults are { baseLanguage = Haskell2010, extensions = [] }, and explicitly setting a list of extensions will be interpreted on top of Haskell2010. See further the documentation for L.H.E.Extension. * Add support for the 'capi' calling convention. It is enabled with the CApiFFI extension. It's been included since GHC 7.4, and advertised since 7.6. * Add support for the 'interruptible' FFI safety annotation, enabled with the InterruptibleFFI extension. * Give better error message when lexing newline fails. In particular, fix the bug when the parser would crash if the file didn't end with a newline. * Support unboxed tuple expressions and patterns. * Fix bug in lexing of primitive integer literals in hex or octal notation. * Disallow negative primitive word literals (such as W# (-0x8000000000000000##)). * Allow phase control for SPECIALIZE pragma. * Derive Foldable and Traversable instances for all annotated AST types. * Fix bug with pretty-printing WARNING and DEPRECATED pragmas. Cheers, Niklas

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ANNOUNCE: posix-paths, for faster file system operations
by Niklas Hambüchen 21 Aug '13

21 Aug '13

John Lato and I would like to announce our posix-paths package. https://github.com/JohnLato/posix-paths It implements a large portion of System.Posix.FilePath using ByteString based RawFilePaths instead of String based FilePaths, and on top of that provides a Traversal module with a fast replacement for `getDirectoryContents` and a recursive `allDirectoryContents`. `getDirectoryContents` is (unsurprisingly?) really slow. Our replacement is 11 times faster in the recursive use case [1], and only 20% slower than `find`. Benchmarks are at [2], code is at [3]. We hope that these improvements will eventually make it into base some day. Until then, we propose our package as a base for discussion and further improvements. Contributions are welcome: Some FilePath operations are not in it yet (especially the Windows / drive related ones), and our traversals might not work on Windows. We would also appreciate some thorough looks at their low level implementations. If you find our benchmarks against getDirectoryContents unfair or would like to add another one, please send a pull request. We have been running this on Linux production machines for a few months now, and are pleased by the speed-up. [1] For the recursive version of the original `getDirectoryContents`, we used the implementation given in Real World Haskell: http://book.realworldhaskell.org/read/io-case-study-a-library-for-searching… [2] Benchmarks: On a real file system: http://johnlato.github.io/posix-paths/usrLocal.html On tmpfs: http://johnlato.github.io/posix-paths/tmpfs.html (note that here find is slow because of process starting overhead) [3] Code: Github: https://github.com/JohnLato/posix-paths RawFilePath operations: https://github.com/JohnLato/posix-paths/blob/master/src/System/Posix/FilePa… Traversals: https://github.com/JohnLato/posix-paths/blob/master/src/System/Posix/Direct… Benchmarks: https://github.com/JohnLato/posix-paths/blob/master/benchmarks/Bench.hs

1 0

Looking for ICFP roommate
by Conal Elliott 21 Aug '13

21 Aug '13

I'm looking for an ICFP roommate. I plan to attend Sunday through Saturday and stay the nights of Saturday the 21st through Saturday the 28th. I missed the discounted price of $225 (yipes) at the Airport Hilton (sold out). Perhaps someone already has a room reserved with two beds or could switch to one with two beds and would like to reduce their cost by room-sharing for part or all of their stay. If so, please let me know. Thanks, - Conal

1 0

wxHaskell mailinglist
by Nathan Hüsken 20 Aug '13

20 Aug '13

Hey, Anyone knows what the proper channel for reporting bugs and asking questions about wxHaskell is? The github page https://github.com/wxHaskell/wxHaskell/wiki seems to have issues disabled, and when I post to the wxHaskell user mailinglist, it tells me that the list is moderated. But my post did not get accepted for the last 2 days. Regards, Nathan

2 3

Re: [Haskell-cafe] Compiling stringable with GHC 7.0.4
by Ketil Malde 20 Aug '13

20 Aug '13

I took the liberty of implementing this fix and uploading stringable-0.1.1.1 to HackageDB. I tested it on GHC 7.0.4 (you know, shipped with the cutting-edge Fedora distribution one year ago, but ancient and no longer to be bothered with by Haskell standards :-) and on 7.6.2. -k Ketil Malde <ketil(a)malde.org> writes: > Hi, > > ------------------------------------------------------- > I don't think FlexibleInstances works with this GHC, I get: > > % cabal install --prefix=$G stringable > Resolving dependencies... > Configuring stringable-0.1.1... > Building stringable-0.1.1... > Preprocessing library stringable-0.1.1... > [1 of 1] Compiling Data.Stringable ( Data/Stringable.hs, dist/build/Data/Stringable.o ) > > Data/Stringable.hs:54:10: > Illegal instance declaration for `Stringable String' > (All instance types must be of the form (T t1 ... tn) > where T is not a synonym. > Use -XTypeSynonymInstances if you want to disable this.) > In the instance declaration for `Stringable String' > ------------------------------------------------------- > > I changed the first line of Data/Stringable.hs to: > > {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} > > and then it compiled. Is there any reason not to retain the TSI pragma? > > -k -- If I haven't seen further, it is by standing in the footprints of giants

1 0

One-element tuple
by AntC 20 Aug '13

20 Aug '13

There's an annoying inconsistency: (CustId 47, CustName "Fred", Gender Male) -- threeple (CustId 47, CustName "Fred) -- twople -- (CustId 47) -- oneple not! () -- nople (That is, it's annoying if you're trying to make typeclass instances for extensible/contractable tuples. Yes, I know I could use HLists.) I'm not happy with either approach I've tried: data Oneple a = Oneple a -- (or newtype) (Oneple $ CustId 47) -- too verbose type Oneple a = [a] [CustId 47] -- at least looks bracket-y What do you do? AntC

8 12

llvm on macos
by Dominic Steinitz 20 Aug '13

20 Aug '13

I am trying to use llvm but am being told I don't have llvm installed. Dominics-MacBook-Pro:Laplace dom$ ghc src-repa/Main.hs -rtsopts -threaded -eventlog -Odph -fllvm -optlo-O3 -fno-liberate-case -isrc-repa [1 of 3] Compiling SolverStencil ( src-repa/SolverStencil.hs, src-repa/SolverStencil.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package array-0.4.0.1 ... linking ... done. Loading package deepseq-1.3.0.1 ... linking ... done. Loading package containers-0.5.0.0 ... linking ... done. Loading package old-locale-1.0.0.5 ... linking ... done. Loading package time-1.4.0.1 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package QuickCheck-2.5.1.1 ... linking ... done. Loading package bytestring-0.10.0.2 ... linking ... done. Loading package primitive-0.5.0.1 ... linking ... done. Loading package vector-0.10.0.1 ... linking ... done. Loading package repa-3.2.3.1 ... linking ... done. <no location info>: Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM ghc: could not execute: opt The ghc documentation (http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/code-generators.html) says that llvm and clang are installed by default for 10.6 and later. I am on 10.8.3. Indeed Dominics-MacBook-Pro:Laplace dom$ llvm- llvm-cpp-4.2 llvm-g++ llvm-g++-4.2 llvm-gcc llvm-gcc-4.2 And Dominics-MacBook-Pro:Laplace dom$ clang clang clang++ But ghc-info tells me Dominics-MacBook-Pro:Laplace dom$ ghc --info [("Project name","The Glorious Glasgow Haskell Compilation System") ,("GCC extra via C opts"," -fwrapv") ,("C compiler command","/usr/bin/gcc") ,("C compiler flags"," -m64 -fno-stack-protector -m64") ,("ar command","/usr/bin/ar") ,("ar flags","clqs") ,("ar supports at file","@ArSupportsAtFile@") ,("touch command","touch") ,("dllwrap command","/bin/false") ,("windres command","/bin/false") ,("perl command","/usr/bin/perl") ,("target os","OSDarwin") ,("target arch","ArchX86_64") ,("target word size","8") ,("target has GNU nonexec stack","False") ,("target has .ident directive","True") ,("target has subsections via symbols","True") ,("LLVM llc command","llc") ,("LLVM opt command","opt") ,("Project version","7.6.2") ,("Booter version","7.4.2") ,("Stage","2") ,("Build platform","x86_64-apple-darwin") ,("Host platform","x86_64-apple-darwin") ,("Target platform","x86_64-apple-darwin") ,("Have interpreter","YES") ,("Object splitting supported","YES") ,("Have native code generator","YES") ,("Support SMP","YES") ,("Unregisterised","NO") ,("Tables next to code","YES") ,("RTS ways","l debug thr thr_debug thr_l thr_p dyn debug_dyn thr_dyn thr_debug_dyn") ,("Leading underscore","YES") ,("Debug on","False") ,("LibDir","/usr/local/lib/ghc-7.6.2") ,("Global Package DB","/usr/local/lib/ghc-7.6.2/package.conf.d") ,("Gcc Linker flags","[\"-m64\"]") ,("Ld Linker flags","[\"-arch\",\"x86_64\"]") ] But I don't have either llc or opt Dominics-MacBook-Pro:Laplace dom$ llc -bash: llc: command not found Dominics-MacBook-Pro:Laplace dom$ opt -bash: opt: command not found Dominics-MacBook-Pro:Laplace dom$ What is the solution? Thanks, Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

5 11

Categories with associated constraints?
by Conal Elliott 20 Aug '13

20 Aug '13

Has anyone given a go at a Category class and friends (including cartesian and closed) with associated constraints (presumably using the ConstraintKinds language extension)? I gave it a try a while back and wasn't able to keep the signatures from getting very complicated. Thanks, -- Conal

2 1

Re: [Haskell-cafe] Errors in non-monadic code
by Donn Cave 20 Aug '13

20 Aug '13

jabolopes(a)google.com, MIME-Version: 1.0 Content-type: text/plain; charset=UTF-8 In-Reply-To: <CAKFCL4VfY-Dz3Xo9ZUZ_SmfRQ2nLGDLbovU=SuF1-sSnqVS8Hw(a)mail.gmail.com> References: <CAKFCL4VfY-Dz3Xo9ZUZ_SmfRQ2nLGDLbovU=SuF1-sSnqVS8Hw(a)mail.gmail.com> quoth Brandon Allbery, > Even in IO, exceptions should be reserved for truly exceptional conditions > (of the "program cannot safely continue" variety), not merely for error > checking when this can be described as a normal flow of evaluation. > Exceptions are not simply alternative flow of control, even in procedural > languages; they are *disruptions* of flow of control. I think that would be debatable, especially as applied to procedural languages in general. It's very common to use exceptions in routine flow of control in Python programs, if I remember right. Meanwhile, the Haskell library in my opinion somewhat weakens your case by making ordinary conditions into exceptions, for example end-of-file. In the end it's a large grey area. Ideally, what the program has to do, or whether it can even continue at all, is determined by the program using information reported by the function that encounters the error. If there's a good reason why exceptions should be used only when the program won't be able to continue, it must be that the language's exception handling facililty isn't very robust. Donn

5 7

GHC and backwards compatibility
by Ketil Malde 20 Aug '13

20 Aug '13

I recently encountered the following problem: -------------------------------------------------------------------------------- $ cabal install Resolving dependencies... Configuring array-0.4.0.1... Building array-0.4.0.1... Preprocessing library array-0.4.0.1... Data/Array/IArray.hs:1:14: Unsupported extension: Trustworthy cabal: Error: some packages failed to install: array-0.4.0.1 failed during the building phase. The exception was: ExitFailure 1 asmeval-0 depends on array-0.4.0.1 which failed to install. binary-0.7.1.0 depends on array-0.4.0.1 which failed to install. biocore-0.3.1 depends on array-0.4.0.1 which failed to install. biofasta-0.0.3 depends on array-0.4.0.1 which failed to install. biopsl-0.4 depends on array-0.4.0.1 which failed to install. biosff-0.3.3 depends on array-0.4.0.1 which failed to install. bytestring-0.10.2.0 depends on array-0.4.0.1 which failed to install. containers-0.5.2.1 depends on array-0.4.0.1 which failed to install. deepseq-1.3.0.1 depends on array-0.4.0.1 which failed to install. hashable-1.2.0.10 depends on array-0.4.0.1 which failed to install. stringable-0.1.1 depends on array-0.4.0.1 which failed to install. system-filepath-0.4.7 depends on array-0.4.0.1 which failed to install. text-0.11.3.1 depends on array-0.4.0.1 which failed to install. unordered-containers-0.2.3.1 depends on array-0.4.0.1 which failed to install. -------------------------------------------------------------------------------- The installed GHC is 7.0.4, and yes, that's many major versions ago, but then again, it's only two years old, and from what I understand, it is shipped by some conservative Linux distributions. The problems here are that: a) the installation tries to install array, but current array requires the "Trustworthy" extension, which appeared in 7.2.1. Ideally, packages should try to be backwards compatible, e.g. by using conditional sections. b) the output isn't very helpful in tracking down the cause of this problem, it claims that all these packages depend on array-0.4.0.1, which is a lie. Somewhere, somehow, somethings depends on this (or at least a newer version), but I have no clue how to figure out which, except examining each of these packages and their dependencies manually. It is also possible that array shouldn't be upgraded like this, but then that needs to be made clear to cabal somehow, so that I don't spend ages trying to. Are there any solutions to this? Advice on how to proceed? -k -- If I haven't seen further, it is by standing in the footprints of giants

4 4

Errors in non-monadic code
by jabolopes＠google.com 19 Aug '13

19 Aug '13

Hi, What is the proper way to implement a non-monadic function that checks whether a given value is correct and gives a proper error message otherwise ? What is the recommended option ? * Either String a check val | valid val = Right val | otherwise = Left errorMsg * Maybe String check val | valid val = Nothing | otherwise = Just errorMsg Cheers, Jose

5 8

inv f g = f . g . f
by Christopher Done 19 Aug '13

19 Aug '13

Anyone ever needed this? Me and John Wiegley were discussing a decent name for it, John suggested inv as in involution. E.g. inv reverse (take 10) inv reverse (dropWhile isDigit) trim = inv reverse (dropWhile isSpace) . dropWhile isSpace That seems to be the only use-case I've ever come across. There's also this one: co f g = f g . g which means you can write trim = co (inv reverse) (dropWhile isSpace) but that's optimizing an ever rarer use-case.

11 11

Database.postgreSQL.Simple - ambigious type
by Hartmut Pfarr 19 Aug '13

19 Aug '13

Hello, I've a problem connecting to my postgresql database. Can You help me fix the ambigious type signature? (The example is identical to the first 5-liner-example in the package documentation) http://hackage.haskell.org/packages/archive/postgresql-simple/0.3.5.0/doc/h… Kind regards Hartmut ------------------------------------------------------------------------ {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" return () ------------------------------------------------------------------------ But this leads to error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) Couldn't match expected type `IO a0' with actual type `q0 -> IO [r0]' In the return type of a call of `query' Probable cause: `query' is applied to too few arguments In a stmt of a 'do' block: query conn "select 2 + 2" In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2"; return () } ------------------------------------------------------------------------ OK, I see, that a parameter q is missing. I change the source code to {-# LANGUAGE OverloadedStrings #-} import Database.PostgreSQL.Simple main = do conn <- connect defaultConnectInfo query conn "select 2 + 2" ( ) {- added ( ) here -} return () ------------------------------------------------------------------------ Now, I run into next error: ------------------------------------------------------------------------ Line 9: 1 error(s), 0 warning(s) No instance for (FromRow r0) arising from a use of `query' The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance (FromField a, FromField b) => FromRow (a, b) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c) => FromRow (a, b, c) -- Defined in `Database.PostgreSQL.Simple.FromRow' instance (FromField a, FromField b, FromField c, FromField d) => FromRow (a, b, c, d) -- Defined in `Database.PostgreSQL.Simple.FromRow' ...plus 10 others In a stmt of a 'do' block: query conn "select 2 + 2" () In the expression: do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () } In an equation for `main': main = do { conn <- connect defaultConnectInfo; query conn "select 2 + 2" (); return () }

3 7

MongoDB single insertions super slow
by Kyle Hanson 19 Aug '13

19 Aug '13

I am trying to write a simple benchmark of testing 1000 single inserts in mongodb, but doing single insertions is super slow. It takes nearly 40 seconds to perform these operations with the haskell mongodb driver, but under a second to perform the same number of operations of single inserts in python. Here are the two files I am working with: https://gist.github.com/hansonkd/6263648 I checked the BSON serialization, and writing it to a socket instead of mongodb driver, makes the script run in under a second so the problem seems to be with the mongodb driver. Any insights would be appreciated. -- Kyle Hanson

2 1

ANN: processing-1.2.0.0. Graphic web applications with processing.js.
by Daniel Díaz Casanueva 18 Aug '13

18 Aug '13

Hello readers of Haskell-Cafe. Yesterday I uploaded a new version of the processing library [1]. == What is this library for? == The library is oriented to create graphic applications that run in a web browser, with no need of running a server. It provides the user with a set of functions and combinators that can be used to create an abstract piece code. This code is rendered to the processing language syntax. Once the code is uploaded, using processing.js [2], the output code is runnable from any browser with JavaScript support. To quickly give you a taste of the library, look at this animation, created with a simple recursive function: http://daniel-diaz.github.io/projects/processing/mill.html The code is here: https://github.com/Daniel-Diaz/processing/blob/master/examples/mill.hs == What's new? == After a couple of weeks working hard to have this library properly working, I think it have reached a point of reasonable usability. As a proof, I have written a simplified version of the Pac-Man game [3]. However, although I have implemented some code optimizations, it seems that the game still does not run smoothly in every web browser. I will definitely keep trying to improve the performance of the code generated by the library. I encourage you to try the library if you are interested, but keep in mind that it is still in an early stage, and may contain bugs, lack some important features, have an incomplete documentation or suffer from API changes. == References == [1] Library at Hackage - http://hackage.haskell.org/package/processing-1.2.0.0 [2] Processing.js - http://processingjs.org [3] Pac-Man game - http://daniel-diaz.github.io/projects/processing/pacman.html

1 0

Alternative name for return
by J. Stutterheim 18 Aug '13

18 Aug '13

Dear Cafe, Suppose we now have the opportunity to change the name of the `return` function in Monad, what would be a "better" name for it? (for some definition of better) N.B. I am _not_ proposing that we actually change the name of `return`. I do currently have the opportunity to pick names for common functions in a non-Haskell related project, so I was wondering if there perhaps is a better name for `return`. - Jurriën

36 73

Applicative is like an Arrow
by Thiago Negri 18 Aug '13

18 Aug '13

I just stumbled upon the Applicative term. Arrows are quite difficult for me to understand at the moment. I guess it needs time to digest. But, as I understand so far, Applicative and Arrows looks like the same thing. Please, enlight me.

9 13

continuations and monads
by Christopher Howard 18 Aug '13

18 Aug '13

Q: Are the "continuations" in Scheme related to the "monads" from Haskell? If so, could someone elaborate on that?

2 1

Haddock multiple definitions
by jabolopes＠google.com 16 Aug '13

16 Aug '13

Hi, I am using GHC: 6.12.1 Haddock: 2.6.0 and the following does not work with Haddock (GHC is fine!): -- Main -- | Blah blah blah (x, y, z) = (1, 2, 3) $ haddock ... /tmp/Main.hs:2:0: parse error on input `(' Is this a bug? Or it's just not part of Haddock? This seems like an interesting feature to document several definitions together, for example, error codes: -- | Syscall error codes for blah... -- -- errA when blah -- ... (errA, errB, errC) = ... Cheers, Jose

3 6

Call for participation IFL 2013
by publicityifl＠gmail.com 16 Aug '13

16 Aug '13

CALL FOR PARTICIPATION 25th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2013 RADBOUD UNIVERSITY NIJMEGEN, THE NETHERLANDS ACM In-Cooperation / ACM SIGPLAN AUGUST 28 - 30 2013 "Landgoed Holthurnsche Hof" http://ifl2013.cs.ru.nl [program available - late registration still open] We are proud to announce that the 25th edition of the IFL series returns to its roots at the Radboud University Nijmegen in the Netherlands. The symposium is held from 28th to 30th of August 2013. Scope ----- The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2013 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2013 will use a post-symposium review process to produce the formal proceedings which will be published in the ACM Digital Library. All participants of IFL 2013 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings do not count as publication for the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. Invited Speaker --------------- Lennart Augustsson, currently employed by the Standard Chartered Bank, well-known for his work on Haskell, parallel Haskell, Cayenne, and Bluespec, is the invited speaker of IFL 2013. The title and abstract of his talk is: "Implementation and Application of Functional Languages - A personal perspective" It is now over 30 years ago since I implemented my first functional language, and over 15 years ago since I wrote my first commercial application. In this talk I will look back to those bygone days and remind you of things that you might have forgotten or never known. The talk will be absolutely free of anything new. Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honoured article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee ------------------- Thomas Arts, Quviq, Gothenburg, Sweden Andrew Butterfield, Trinity College, Dublin, Ireland Edwin Brady, University of St. Andrews, UK Clemens Grelck, University of Amsterdam, Netherlands Adam Granicz, IntelliFactory, Budapest, Hungary Jeremy Gibbons, University of Oxford, UK Fritz Henglein, University of Copenhagen, Denmark Stephan Herhut, Intel Labs, Santa Clara, US Ralf Hinze (co-chair), University of Oxford, UK Zoltán Horváth, Eötvös Loránd University, Budapest, Hungary Zhenjiang Hu, University of Tokyo, Japan Mauro Jaskelioff, Universidad Nacional de Rosario, Argentina Johan Jeuring, University of Utrecht, Netherlands Rita Loogen, University of Marburg, Germany Marco T. Morazán, Seton Hall University, New Jersey, US Dominic Orchard, University of Cambridge, UK Rinus Plasmeijer (chair), Radboud University Nijmegen, Netherlands Tim Sheard, Portland State University, US Sam Tobin-Hochstadt, Northeastern University / Indiana University, US Peter Thiemann, University of Freiburg, Germany Simon Thompson, University of Kent, UK Venue ----- The 25th IFL is organized by the Radboud University Nijmegen, Model Based Software Development Department at the Nijmegen Institute for Computing and Information Sciences. The event is held in the Landgoed Holthurnsche Hof, a rural estate in the woodlands surrounding Nijmegen. It can be reached quickly and easily by public transport. Program ------- Wednesday August 28 ------------------- 8:15 registration 8:50 opening 9:00 Marcos Viera First Class Syntax, Semantics, and Their Composition Doaitse Swierstra 9:25 Olivier Danvy Circularity and Lambda Abstraction Peter Thiemann Ian Zerny 9:50 break 10:20 Loic Denuziere Piglets to the rescue Ernesto Rodriguez Adam Granicz 10:45 Simon Fowler Correct and Secure Web Programming using Dependent Types Edwin Brady and Embedded Domain-Specific Languages 11:10 break 11:40 Jennifer Hackett The Under-Performing Unfold: A new approach to optimising Graham Hutton corecursive programs Mauro Jaskelioff 12:05 Laurence Edward Day Compilation á la Carte Graham Hutton 12:30 Artjoms Sinkarovs Functionally Redundant Declarations for Improved Sven-Bodo Scholz Performance Portability 12:55 lunch 14:00 Xavier Clerc OCaml-Java: Typing Java Accesses from OCaml Programs 14:25 Jonathan Protzenko The implementation of the Mezzo type-checker 14:50 break 15:20 Ralf Lämmel The 101haskell chrestomathy Thomas Schmorleiz Andrei Varanovich 15:45 Chide Groenouwe Instant playful access to serious programming for non-programmers with a visual functional programming language 16:10 break 16:40 Arjan Boeijink Supercompiling Haskell to Hardware Philip Hölzenspies Christiaan Baaij Jan Kuper 17:05 Bas van Gijzel Towards a framework for the implementation and verification Henrik Nilson of translations between argumentation models 17:30 end of talks 18:00 dinner 20:00 25th IFL reunion 23:00 Thursday August 29 ------------------ 9:00 Bas Lijnse Supporting Semi-Structured Work using Higher Order Tasks Jan Martin Jansen 9:25 Viktória Zsók A Prototype of CPS Systems 9:50 break 10:20 Majed Al Saeed A Critical Analysis of Parallel Functional Profilers Patrick Maier Phil Trinder Lilia Georgieva 10:45 Vladimir Janjic Using Erlang Skeletons to Parallelise Realistic Christopher Brown Medium-Scale Parallel Programs Kevin Hammond 11:10 break 11:40 Lennart Augustsson Invited talk: Implementation and Application of Functional Languages A personal perspective 12:40 lunch 13:45 - 23:00 Social event: excursion, symposium dinner, Peter Landin award Friday August 30 ---------------- 9:00 Malak Aljabri The Design and Implementation of GUMSMP: Hans-Wolfgang Loidl a Multilevel Parallel Haskell Implementation Phil Trinder 9:25 Henrique Ferreiro Kindergarten Cop: Laura Castro Dynamic Nursery Resizing for Increasing Parallelism in GHC Vladimir Janjic David Castro Kevin Hammond 9:50 break 10:20 Fangyong Tang User-Defined Shape Constraints in SAC Clemens Grelck 10:45 Leaf Petersen Measuring the Haskell Gap Todd Anderson Hai Liu Neal Glew 11:10 break 11:40 Marco T. Morazán Immediate Dominators in Linear Time 12:05 Merijn Verstraaten On Predicting the Impact of Resource Redistribution Sven-Bodo Scholz in Streaming Applications 12:30 IFL 2014 12:40 lunch 14:00 Melinda Tóth Reduction of regression tests for Erlang based on impact analysis István Bozó Zoltán Horváth 14:25 Macías López A DSL for Web Services Automatic Test Data Generation Henrique Ferreiro Laura M. Castro Thomas Arts 14:50 break 15:20 Kanae Tsushima A Weighted Type Error Slicer Kenichi Asai 15:45 Ben Thorner A Type Inference Debugger for ML in Education Kathryn Gray 16:10 break 16:40 Nicolas Wu Structured Sharing for Dynamic Programming 17:05 Clemens Grelck Towards Persistent and Parallel Asynchronous Adaptive Specialisation Heinz Wiesinger for Data-Parallel Array Processing in SAC 17:30 closing

1 0

Does exist something like "data-files" in Cabal, which works at compile time?
by Alfredo Di Napoli 16 Aug '13

16 Aug '13

Hello guys, I'm pretty sure the answer is "no", but I was hoping to get some extra insight / best practices. The problem can be summarised by this SO question (not the OP, but I have the same problem): http://stackoverflow.com/questions/15731170/cabal-how-to-add-text-file-as-a… As someone states, "data-files" in for run-time, whereas I need to tell cabal "please copy these files in place before trying to compile", so at compile-time. Does something similar exist? I think the best solution, unless someone prove me wrong, is to create a small startup script which copies the files for me (I *think* yesod is using something similar, namely a script called EmbeddedFiles.hs) and then triggers "cabal install" the usual way. Can you come up with a better way? Thanks in advance! Alfredo

3 4

Cabal errors...
by Gregory Guthrie 16 Aug '13

16 Aug '13

I am trying to run ecliseFP to use with Haskell, but it gives an error: SO I tried to rebuild the buildwrapper rogram to get the update, but it fails as below. Any hints or help on what to do next? I think that from some past problems, that the"shadowed" problem is from global and usr-local library clashes, but I have never done anything fancy, I always just use "cabal install xxx" for any packages, so had hoped/assumed that it would keep everything straight. TIA. ------------------------------------------------ Eclipse startup error: Trying to upgrade from exlipse 4.2 to 4.3, the eclipseFP plugin gives me these errors: On startup, The version of helper executable buildwrapper at: ...\users\me\AppData\Roaming\cabal\bin\buildwrapper.exe is 0.71 but at least version version 0.7.2 is required. Choose Install to download and install from hackage, or Cancel to set it up manually in the Haskell preferences page. ------------------------------------------ Then, it tries to do the update, and fails: Resolving dependencies... In order, the following would be installed: List-0.5.1 (new package) base64-bytestring-1.0.0.1 (new package) bktrees-0.3.1 (new package) blaze-builder-0.3.1.1 (new package) blaze-markup-0.5.1.5 (new package) blaze-html-0.5.1.3 (new package) colour-2.3.3 (new package) cpphs-1.16 (new package) data-default-class-0.0.1 (new package) data-default-instances-base-0.0.1 (new package) data-default-instances-containers-0.0.1 (new package) data-default-instances-old-locale-0.0.1 (new package) digest-0.0.1.2 (new package) dlist-0.5 (new package) data-default-instances-dlist-0.0.1 (new package) data-default-0.5.3 (new package) extensible-exceptions-0.1.1.4 (reinstall) changes: base-4.5.1.0 -> 4.6.0.1 haskell-src-exts-1.13.5 (new package) multiset-0.2.2 (new package) polyparse-1.8 (new package) regex-pcre-builtin-0.94.4.7.8.31 (new package) highlighting-kate-0.5.5 (new package) syb-0.3.7 (reinstall) changes: base-4.5.1.0 -> 4.6.0.1 hs-bibutils-5.0 (new package) json-0.7 (new package) pandoc-types-1.10 (new package) tagsoup-0.12.8 (new package) temporary-1.1.2.4 (new package) utf8-string-0.3.7 -bytestring-in-base (new package) hexpat-0.20.3 (new package) citeproc-hs-0.3.8 (new package) wl-pprint-text-1.1.0.0 (new package) graphviz-2999.16.0.0 (new package) xml-1.3.13 (new package) texmath-0.6.3 (new package) zip-archive-0.1.3.4 (new package) pandoc-1.10.1 (new package) Graphalyze-0.14.0.1 (new package) SourceGraph-0.7.0.5 (new package) cabal.exe: The following packages are likely to be broken by the reinstalls: cgi-3001.1.7.4 haskell-platform-2012.4.0.0 haskell-src-1.0.1.5 Use --force-reinstalls if you want to install anyway. ------------------------------------------------ C:\Users\guthrie>cabal install buildwrapper Resolving dependencies... Configuring aeson-0.6.1.0... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package deepseq-1.3.0.0 requires array-0.4.0.0 package text-0.11.3.1 requires array-0.4.0.1 package deepseq-1.3.0.1 requires array-0.4.0.1 package containers-0.5.0.0 requires array-0.4.0.1 package attoparsec-0.10.4.0 requires array-0.4.0.1 package vector-0.10.0.1 requires base-4.5.1.0 package transformers-0.3.0.0 requires base-4.5.1.0 package primitive-0.5.0.1 requires base-4.5.1.0 package mtl-2.1.2 requires base-4.5.1.0 package deepseq-1.3.0.0 requires base-4.5.1.0 package array-0.4.0.0 requires base-4.5.1.0 package unordered-containers-0.2.3.0 requires base-4.6.0.1 package time-1.4.0.1 requires base-4.6.0.1 package text-0.11.3.1 requires base-4.6.0.1 package template-haskell-2.8.0.0 requires base-4.6.0.1 package syb-0.4.0 requires base-4.6.0.1 package pretty-1.1.1.0 requires base-4.6.0.1 package old-locale-1.0.0.5 requires base-4.6.0.1 package hashable-1.1.2.5 requires base-4.6.0.1 package dlist-0.5 requires base-4.6.0.1 package deepseq-1.3.0.1 requires base-4.6.0.1 package containers-0.5.0.0 requires base-4.6.0.1 package bytestring-0.10.0.2 requires base-4.6.0.1 package blaze-builder-0.3.1.1 requires base-4.6.0.1 package attoparsec-0.10.4.0 requires base-4.6.0.1 package array-0.4.0.1 requires base-4.6.0.1 package aeson-0.6.1.0 requires base-4.6.0.1 package Win32-2.3.0.0 requires base-4.6.0.1 package vector-0.10.0.1 requires deepseq-1.3.0.0 package unordered-containers-0.2.3.0 requires deepseq-1.3.0.1 package time-1.4.0.1 requires deepseq-1.3.0.1 package text-0.11.3.1 requires deepseq-1.3.0.1 package containers-0.5.0.0 requires deepseq-1.3.0.1 package bytestring-0.10.0.2 requires deepseq-1.3.0.1 package attoparsec-0.10.4.0 requires deepseq-1.3.0.1 package aeson-0.6.1.0 requires deepseq-1.3.0.1 package vector-0.10.0.1 requires ghc-prim-0.2.0.0 package primitive-0.5.0.1 requires ghc-prim-0.2.0.0 package integer-gmp-0.4.0.0 requires ghc-prim-0.2.0.0 package base-4.5.1.0 requires ghc-prim-0.2.0.0 package text-0.11.3.1 requires ghc-prim-0.3.0.0 package integer-gmp-0.5.0.0 requires ghc-prim-0.3.0.0 package hashable-1.1.2.5 requires ghc-prim-0.3.0.0 package containers-0.5.0.0 requires ghc-prim-0.3.0.0 package bytestring-0.10.0.2 requires ghc-prim-0.3.0.0 package base-4.6.0.1 requires ghc-prim-0.3.0.0 package aeson-0.6.1.0 requires ghc-prim-0.3.0.0 package base-4.5.1.0 requires integer-gmp-0.4.0.0 package text-0.11.3.1 requires integer-gmp-0.5.0.0 package hashable-1.1.2.5 requires integer-gmp-0.5.0.0 package base-4.6.0.1 requires integer-gmp-0.5.0.0 Building aeson-0.6.1.0... Preprocessing library aeson-0.6.1.0... [ 1 of 11] Compiling Data.Aeson.Types.Internal ( Data\Aeson\Types\Internal.hs, d ist\build\Data\Aeson\Types\Internal.o ) Data\Aeson\Types\Internal.hs:35:1: Bad interface file: E:\Plang\Haskell Platform\lib\extralibs\mtl-2.1.2\ghc-7. 4.2\Control\Monad\State\Strict.hi mismatched interface file versions (wanted "7063", got "7042") Failed to install aeson-0.6.1.0 Configuring cmdargs-0.10.5... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package transformers-0.3.0.0 requires base-4.5.1.0 package time-1.4.0.1 requires base-4.6.0.1 package template-haskell-2.8.0.0 requires base-4.6.0.1 package process-1.1.0.2 requires base-4.6.0.1 package pretty-1.1.1.0 requires base-4.6.0.1 package old-locale-1.0.0.5 requires base-4.6.0.1 package filepath-1.3.0.1 requires base-4.6.0.1 package directory-1.2.0.1 requires base-4.6.0.1 package deepseq-1.3.0.1 requires base-4.6.0.1 package containers-0.5.0.0 requires base-4.6.0.1 package cmdargs-0.10.5 requires base-4.6.0.1 package bytestring-0.10.0.2 requires base-4.6.0.1 package array-0.4.0.1 requires base-4.6.0.1 package Win32-2.3.0.0 requires base-4.6.0.1 package integer-gmp-0.4.0.0 requires ghc-prim-0.2.0.0 package base-4.5.1.0 requires ghc-prim-0.2.0.0 package integer-gmp-0.5.0.0 requires ghc-prim-0.3.0.0 package containers-0.5.0.0 requires ghc-prim-0.3.0.0 package bytestring-0.10.0.2 requires ghc-prim-0.3.0.0 package base-4.6.0.1 requires ghc-prim-0.3.0.0 package base-4.5.1.0 requires integer-gmp-0.4.0.0 package base-4.6.0.1 requires integer-gmp-0.5.0.0 Building cmdargs-0.10.5... Preprocessing library cmdargs-0.10.5... [ 1 of 25] Compiling System.Console.CmdArgs.Implicit.Ann ( System\Console\CmdArg s\Implicit\Ann.hs, dist\build\System\Console\CmdArgs\Implicit\Ann.o ) [ 2 of 25] Compiling System.Console.CmdArgs.Implicit.UI ( System\Console\CmdArgs \Implicit\UI.hs, dist\build\System\Console\CmdArgs\Implicit\UI.o ) [ 3 of 25] Compiling System.Console.CmdArgs.Verbosity ( System\Console\CmdArgs\V erbosity.hs, dist\build\System\Console\CmdArgs\Verbosity.o ) [ 4 of 25] Compiling System.Console.CmdArgs.Implicit.Type ( System\Console\CmdAr gs\Implicit\Type.hs, dist\build\System\Console\CmdArgs\Implicit\Type.o ) [ 5 of 25] Compiling System.Console.CmdArgs.Explicit.SplitJoin ( System\Console\ CmdArgs\Explicit\SplitJoin.hs, dist\build\System\Console\CmdArgs\Explicit\SplitJoin.o ) [ 6 of 25] Compiling System.Console.CmdArgs.Explicit.ExpandArgsAt ( System\Conso le\CmdArgs\Explicit\ExpandArgsAt.hs, dist\build\System\Console\CmdArgs\Explicit\ExpandArgsAt.o ) [ 7 of 25] Compiling System.Console.CmdArgs.Explicit.Type ( System\Console\CmdAr gs\Explicit\Type.hs, dist\build\System\Console\CmdArgs\Explicit\Type.o ) [ 8 of 25] Compiling System.Console.CmdArgs.Explicit.Process ( System\Console\Cm dArgs\Explicit\Process.hs, dist\build\System\Console\CmdArgs\Explicit\Process.o) [ 9 of 25] Compiling System.Console.CmdArgs.Explicit.Complete ( System\Console\C mdArgs\Explicit\Complete.hs, dist\build\System\Console\CmdArgs\Explicit\Complete.o ) [10 of 25] Compiling System.Console.CmdArgs.Helper ( System\Console\CmdArgs\Help er.hs, dist\build\System\Console\CmdArgs\Helper.o ) [11 of 25] Compiling System.Console.CmdArgs.Default ( System\Console\CmdArgs\Def ault.hs, dist\build\System\Console\CmdArgs\Default.o ) [12 of 25] Compiling System.Console.CmdArgs.Text ( System\Console\CmdArgs\Text.h s, dist\build\System\Console\CmdArgs\Text.o ) [13 of 25] Compiling System.Console.CmdArgs.Explicit.Help ( System\Console\CmdAr gs\Explicit\Help.hs, dist\build\System\Console\CmdArgs\Explicit\Help.o ) [14 of 25] Compiling System.Console.CmdArgs.Explicit ( System\Console\CmdArgs\Ex plicit.hs, dist\build\System\Console\CmdArgs\Explicit.o ) [15 of 25] Compiling System.Console.CmdArgs.GetOpt ( System\Console\CmdArgs\GetO pt.hs, dist\build\System\Console\CmdArgs\GetOpt.o ) [16 of 25] Compiling Data.Generics.Any ( Data\Generics\Any.hs, dist\build\Data\Generics\Any.o ) Data\Generics\Any.hs:5:1: Bad interface file: E:\Plang\Haskell Platform\lib\extralibs\transformers-0.3 .0.0\ghc-7.4.2\Control\Monad\Trans\State.hi mismatched interface file versions (wanted "7063", got "7042") Failed to install cmdargs-0.10.5 Configuring regex-tdfa-1.1.8... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package transformers-0.3.0.0 requires base-4.5.1.0 package mtl-2.1.2 requires base-4.5.1.0 package transformers-0.3.0.0 requires base-4.6.0.1 package text-0.11.3.1 requires base-4.6.0.1 package regex-tdfa-1.1.8 requires base-4.6.0.1 package regex-base-0.93.2 requires base-4.6.0.1 package parsec-3.1.3 requires base-4.6.0.1 package mtl-2.1.2 requires base-4.6.0.1 package deepseq-1.3.0.1 requires base-4.6.0.1 package containers-0.5.0.0 requires base-4.6.0.1 package bytestring-0.10.0.2 requires base-4.6.0.1 package array-0.4.0.1 requires base-4.6.0.1 package integer-gmp-0.4.0.0 requires ghc-prim-0.2.0.0 package base-4.5.1.0 requires ghc-prim-0.2.0.0 package text-0.11.3.1 requires ghc-prim-0.3.0.0 package regex-tdfa-1.1.8 requires ghc-prim-0.3.0.0 package integer-gmp-0.5.0.0 requires ghc-prim-0.3.0.0 package containers-0.5.0.0 requires ghc-prim-0.3.0.0 package bytestring-0.10.0.2 requires ghc-prim-0.3.0.0 package base-4.6.0.1 requires ghc-prim-0.3.0.0 package base-4.5.1.0 requires integer-gmp-0.4.0.0 package text-0.11.3.1 requires integer-gmp-0.5.0.0 package base-4.6.0.1 requires integer-gmp-0.5.0.0 package regex-tdfa-1.1.8 requires mtl-2.1.2 package regex-base-0.93.2 requires mtl-2.1.2 package parsec-3.1.3 requires mtl-2.1.2 package mtl-2.1.2 requires transformers-0.3.0.0 package mtl-2.1.2 requires transformers-0.3.0.0 Building regex-tdfa-1.1.8... Preprocessing library regex-tdfa-1.1.8... <command line>: cannot satisfy -package-id mtl-2.1.2-5337caef659244e51e2f5fb2e94 4d97f: mtl-2.1.2-5337caef659244e51e2f5fb2e944d97f is shadowed by package mtl-2.1.2- 678980f6077e9f83a86186556aa2a425 (use -v for more information) Failed to install regex-tdfa-1.1.8 cabal: Error: some packages failed to install: aeson-0.6.1.0 failed during the building phase. The exception was: ExitFailure 1 buildwrapper-0.7.4 depends on regex-tdfa-1.1.8 which failed to install. cmdargs-0.10.5 failed during the building phase. The exception was: ExitFailure 1 regex-tdfa-1.1.8 failed during the building phase. The exception was: ExitFailure 1

1 0

ANN: Rlang-QQ-0.0.0.0
by adam vogt 15 Aug '13

15 Aug '13

Hello Cafe, I am pleased to announce the first release of Rlang-QQ. You can find it at <http://hackage.haskell.org/package/Rlang-QQ-0.0.0.0>. The package provides a quasiquoter which makes it easier to call R (<http://www.r-project.org/>) using values calculated with GHC. Variables in quasiquoter prefixed with hs_ lead to the corresponding haskell variable being made available to R (provided there is an instance to marshal values of that type). In other words, `hs_foo' in the quasiquote will start off as whatever `foo' is on the haskell side. For now no attempt is made to translate anything on the R side back to haskell. Regards, Adam Vogt

1 0

Haskell Weekly News: Issue 276
by Daniel Santa Cruz 15 Aug '13

15 Aug '13

Welcome to issue 276 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 4 to 10, 2013. Quotes of the Week * AndrazBajt: co-worker = producer of work tasks * Brend: I have created a revolutionary AI from first principles! Also, can anyone tell me how to draw coloured dots on the screen? * nejucomo: nejucomo can't resist a good wooly yak. Top Reddit Stories * FP Complete is hiring Haskell contract programmers to work from anywhere Domain: self.haskell, Score: 89, Comments: 17 On Reddit: [1] http://goo.gl/MRqlHd Original: [2] http://goo.gl/MRqlHd * What are some killer libraries and frameworks unique to Haskell? Domain: self.haskell, Score: 87, Comments: 149 On Reddit: [3] http://goo.gl/N3aAqW Original: [4] http://goo.gl/N3aAqW * Cartesian Closed Comic #21: A question will be asked Domain: ro-che.info, Score: 64, Comments: 17 On Reddit: [5] http://goo.gl/Kv5Lwh Original: [6] http://goo.gl/pwLne3 * Haskell for all: Composable streaming folds Domain: haskellforall.com, Score: 56, Comments: 38 On Reddit: [7] http://goo.gl/SIFdvq Original: [8] http://goo.gl/OCpBDf * The Monad.Reader issue 22 is out Domain: themonadreader.wordpress.com, Score: 51, Comments: 3 On Reddit: [9] http://goo.gl/yUim7j Original: [10] http://goo.gl/YE6XAV * Typo – A programming language that runs in Haskell's type system Domain: github.com, Score: 49, Comments: 26 On Reddit: [11] http://goo.gl/yceg83 Original: [12] http://goo.gl/g3ae02 * tasty — a new testing framework (successor to test-framework) Domain: documentup.com, Score: 45, Comments: 33 On Reddit: [13] http://goo.gl/08CufZ Original: [14] http://goo.gl/5GN7bO * Get started with GHC development: a starting point for newcomers Domain: ghc.haskell.org, Score: 39, Comments: 4 On Reddit: [15] http://goo.gl/zfGesW Original: [16] http://goo.gl/23IqSe * John Carmack's talk on function programming, Haskell, Lisp, Scheme and concurrency Domain: youtu.be, Score: 38, Comments: 3 On Reddit: [17] http://goo.gl/1yUTKC Original: [18] http://goo.gl/2K6370 * Daily Haskell Exercise Domain: dailyhaskellexercise.tumblr.com, Score: 37, Comments: 13 On Reddit: [19] http://goo.gl/nwmY9b Original: [20] http://goo.gl/Zyy3Sp * "Teach a kid functional programming and you feed her for a lifetime," says Huffington Post CTO Domain: huffingtonpost.com, Score: 27, Comments: 6 On Reddit: [21] http://goo.gl/8YXXgz Original: [22] http://goo.gl/Mm2xa9 Top StackOverflow Questions * Functors and Applicatives for types of kind (* -> *) -> * votes: 15, answers: 1 Read on SO: [23] http://goo.gl/PQ2VmY * How do I correctly use Control.Exception.catch in Haskell? votes: 15, answers: 3 Read on SO: [24] http://goo.gl/G4Eptv * Safer Handles in Haskell? votes: 13, answers: 1 Read on SO: [25] http://goo.gl/b4gWPa * Typeably casting GADTs votes: 8, answers: 1 Read on SO: [26] http://goo.gl/vcuUxt * Repa performance for planetary simulation votes: 7, answers: 0 Read on SO: [27] http://goo.gl/VqggZo * Transforming functions of type `a -> b` into those of type `String -> String` in Haskell votes: 7, answers: 2 Read on SO: [28] http://goo.gl/VzejZ0 * Resolving overlapping instances in external library votes: 6, answers: 1 Read on SO: [29] http://goo.gl/f7QDdu * Cabal - Expose all modules while building library votes: 6, answers: 1 Read on SO: [30] http://goo.gl/C8lKJB * What are Alternative's “some” and “many” useful for? votes: 6, answers: 4 Read on SO: [31] http://goo.gl/o4lLGy Until next time, [32]+Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/1jzatv/fp_complete_is_hiring_haske… 2. http://www.reddit.com/r/haskell/comments/1jzatv/fp_complete_is_hiring_haske… 3. http://www.reddit.com/r/haskell/comments/1k3fq7/what_are_some_killer_librar… 4. http://www.reddit.com/r/haskell/comments/1k3fq7/what_are_some_killer_librar… 5. http://ro-che.info/ccc/21.html 6. http://www.reddit.com/r/haskell/comments/1jp265/cartesian_closed_comic_21_a… 7. http://www.haskellforall.com/2013/08/composable-streaming-folds.html 8. http://www.reddit.com/r/haskell/comments/1jo9gm/haskell_for_all_composable_… 9. http://themonadreader.wordpress.com/2013/08/06/issue-22/ 10. http://www.reddit.com/r/haskell/comments/1jvhj7/the_monadreader_issue_22_is… 11. https://github.com/seliopou/typo 12. http://www.reddit.com/r/haskell/comments/1jwp0s/typo_a_programming_language… 13. http://documentup.com/feuerbach/tasty 14. http://www.reddit.com/r/haskell/comments/1jr8lb/tasty_a_new_testing_framewo… 15. http://ghc.haskell.org/trac/ghc/wiki/Newcomers 16. http://www.reddit.com/r/haskell/comments/1k0uk9/get_started_with_ghc_develo… 17. http://youtu.be/1PhArSujR_A?t=2m5s 18. http://www.reddit.com/r/haskell/comments/1k2w93/john_carmacks_talk_on_funct… 19. http://dailyhaskellexercise.tumblr.com/ 20. http://www.reddit.com/r/haskell/comments/1juqez/daily_haskell_exercise/ 21. http://www.huffingtonpost.com/john-pavley/teach-a-kid-functional-pr_b_36668… 22. http://www.reddit.com/r/haskell/comments/1jr0og/teach_a_kid_functional_prog… 23. http://stackoverflow.com/questions/18052158/functors-and-applicatives-for-t… 24. http://stackoverflow.com/questions/18052619/how-do-i-correctly-use-control-… 25. http://stackoverflow.com/questions/18135648/safer-handles-in-haskell 26. http://stackoverflow.com/questions/18109654/typeably-casting-gadts 27. http://stackoverflow.com/questions/18099878/repa-performance-for-planetary-… 28. http://stackoverflow.com/questions/18106170/transforming-functions-of-type-… 29. http://stackoverflow.com/questions/18071439/resolving-overlapping-instances… 30. http://stackoverflow.com/questions/18084265/cabal-expose-all-modules-while-… 31. http://stackoverflow.com/questions/18108608/what-are-alternatives-some-and-… 32. https://plus.google.com/105107667630152149014/about

1 0

second order types
by Patrick Browne 14 Aug '13

14 Aug '13

1 0

more gtk help
by briand＠aracnet.com 14 Aug '13

14 Aug '13

after getting the entry widget events to work as i wanted, i started looking at some other things to do, and my first attempt failed with this : Couldn't match expected type `Gtk.Signal (Gtk.Signal self0 (Gtk.EventM Gtk.EExpose Bool)) (t0 -> IO ())' with actual type `GtkGL.GLDrawingArea' In the second argument of `Gtk.on', namely `glCanvas' In the expression: Gtk.on Gtk.exposeEvent glCanvas In a stmt of a 'do' block: Gtk.on Gtk.exposeEvent glCanvas $ \ _ -> putStrLn "foo" Gtk.on Gtk.exposeEvent glCanvas $ putStrLn "foo" glCanvas is glCanvas <- GtkGL.glDrawingAreaNew glconfig I looked up the type of Gtk.on and exposeEvent : exposeEvent :: WidgetClass self => Signal self (EventM EExpose Bool) on :: object -> Signal object callback -> callback -> IO (ConnectId object) There are numerous problems starting with the type of 'callback' that I can't seem to trace in the documentation. And I can't figure out why callback is repeated... unfortunately it's not obvious what's going on. The type of the entry widget callback is much simpler, and quite a bit different, so I it's of little help. I'm having a hard time finding appropriate examples because everything seems to use the deprecated convention of on{eventType}, e.g. onExposeEvent or onButtonPress. I was hoping someone could teach me to fish and parse out the type for me and provide a simple example. Just pointing me to up-to-date examples would almost certainly be good enough, but I really need to understand the convention here, since I suspect as I try to attach events to other widget types I will see these sorts of problems again. Thanks, Brian

3 3

Freeing dependent resources
by Alexey Uimanov 14 Aug '13

14 Aug '13

Hello, Haskellers. I am working on HDBI and I faced with the problem. There is an error when I close SQlite database, "unable to close due to unfinalized statements or unfinished backups". This problems occurs when there is some not finalized statements related to this database. So, I must protect the user from this error and garantee the finalisation of all the statements BEFORE the disconnection from the database. There is naive implementation of weak child list inherited from HDBC https://github.com/s9gf4ult/hdbi/blob/master/Database/HDBI/DriverUtils.hs Here you can see that ChildList is just MVar to list of weak pointers. Every time when the statement is created the new weak pointer prepended to this list. Every time when statement becomes not reachable the weak pointer becomes empty and it's finalizer is scheduled to execute in parralel thread. The finalizer finishes the statement, so if finalizer is executed the statement is finished. Before the actual call of disconnection the 'disconnect' method performs 'closeAllChildren'. Call of 'closeAllChildren' performs finishing of just not empty weak pointers, because the finishing of not reachable statements is imposible. You can see the implementation here https://github.com/s9gf4ult/hdbi-sqlite/blob/master/Database/HDBI/SQlite/Im… https://github.com/s9gf4ult/hdbi-sqlite/blob/master/Database/HDBI/SQlite/Im… and here is the simplest code which cause an error {-# LANGUAGE OverloadedStrings #-} module Main where import System.Mem import Database.HDBI.SQlite import Database.HDBI perf c = do runRaw c "create table integers (val)" s <- prepare c "select * from integers" return () main = do c <- connectSqlite3 ":memory:" perf c performGC disconnect c If you remove 'performGC' error not occurs, because the weak pointer in ChildList is still not empty and 'finish' is performed in 'closeAllChildren'. The problem occures when the statement becomes not reachable and weak pointer becomes empty, but finalizer is not performed yet, because the finalizer performed in parralel thread according to https://www.google.ru/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=… The naive solution is to make 'closeAllChildren' to wait until all finalizers is completely performed. But how ? Maybe you have the better solution to garantee that all statements are finished before the database disconnecting ?

1 0

ANN: hspec-smallcheck - SmallCheck support for the Hspec testing framework
by Simon Hengel 14 Aug '13

14 Aug '13

Hi, I'm glad to announce SmallCheck support [1] for the Hspec testing framework. A tiny example on how to use it is here: https://github.com/hspec/hspec-smallcheck/blob/master/example/Spec.hs More documentation for Hspec is here: http://hspec.github.io/ Cheers, Simon [1] http://hackage.haskell.org/package/hspec-smallcheck

1 0

Reminder: today is the deadline for HIW talk proposals (Haskell Implementor's Workshop)
by Ryan Newton 13 Aug '13

13 Aug '13

If you would like to give a talk pertaining to Haskell implementation (including libraries). Please submit a short abstract below. See you in Boston! -Ryan * Call for Talks* ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2013 Boston, USA, September 22th, 2013 The workshop will be held in conjunction with ICFP 2013 http://www.icfpconference.org/icfp2013/ Important dates Proposal Deadline: 13th August 2013 (by midnight, any timezone) Notification: 27th August 2013 Workshop: 22th September 2013

1 0

Diagrams and GTK
by Michael Oswald 13 Aug '13

13 Aug '13

Hello, I am currently writing an application which draws the structure of some packets with help of the diagrams library directly to a GTK GUI. Now the packets can have several hundreds of parameters which have to be drawn so it takes some seconds to calculate the diagram itself. Of course this now blocks the GUI thread, so the basic idea was to put the calculation in a separate thread. Unfortunately this doesn't work as lazyness kicks in and the final diagram is calculated when it is rendered and not evaluated before. Using seq didn't help (because of WHNF) and there seems to be no deepseq instance for the diagrams. Does somebody has an idea on how to speed this up / get the diagram evaluated strictly as especially scrolling the DrawingArea is now really a pain? lg, Michael

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