- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] ~ type operator
by Henning Thielemann 02 Jul '11

02 Jul '11

On 24.06.2011 11:10, Daniel Fischer wrote: > On Friday 24 June 2011, 10:26:42, Guy wrote: >> What does the ~ type operator mean? I've sometimes seen types such as (a >> ~ b) in error messages, but can't understand what GHC is trying to tell >> me. > > Type equality, (a ~ b) means that a and b are the same type (rather, that > the compiler can prove they're the same). This type operator is needed when using type families. With functional dependencies you can assert a bijection between types by writing class C a b | a -> b, b -> a where but using type functions you have to tell the compiler somehow that one type function reverses the mapping of another function. This is where (~) is needed. You can use this type operator, when LANGUAGE TypeFamilies is enabled. I assume that the use of (~) in type error messages reflects that GHC's type checker is now internally based on type families. There are also other circumstances, where GHC uses non-Haskell-98 syntax in its messages, e.g. it uses explicit 'forall' quantifiers in error messages also if they are disabled in the affected module.

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ANNOUNCE: Hs2lib-0.4.8
by Phyx 02 Jul '11

02 Jul '11

What is it? ======== A preprocessor and library which allow you to create dynamic libs from arbitrary annotated Haskell programs with one click. It also allows you to use the generated lib in C, C++ and C# just by including the generated header files. At a minimum it can be considered the inverse of c2hs. Where to get it? ============ You can get it from Hackage (http://hackage.haskell.org/package/Hs2lib) or by using cabal (cabal install Hs2lib). Documentation, Mailing List, Source, etc ======================= Go to http://mistuke.wordpress.com/category/hs2lib/ for information. What's New? ========= - Currently Supported: * Generates Marshaling information for arbitrary data types. Types with kind other then * have to be fully applied. (e.g. Maybe String). A specialized Marshaling instance is then generated for each of these instances. * Generates FFI exports for any function you mark, with FFI compatible type * Properly supports marshaling of lists. [Int] * Supports Ptr, FunPtr and StablePtr. Which introduces the possibility to have caches/sessions. * Supports Callbacks via Higher order functions (everything is auto generated from the function type) * Re-exports existing exports found in source file * Honors existing Storable instances found for types it needs * Avoids unsafePerformIO as much as possible, except for in one specific instance. * Generates Initialization functions for you * Hides unnecessary exports using a DEF file * Allows you to override default type conversions (e.g. String –> CWString) * Provides helper includes for C, C++ and C# (placed into wherever cabal places extra includes. %AppData%\cabal\Hs2lib-0.4.8 for windows) And more Details ===== A more detailed documentation will follow in the next few weeks. Why 0.4.8? -------------- This project has been in development for a very long time. Nearly two years on and off. It was developed mainly to facilitate the creation of Visual Haskell 2010. This is just the first public release of this tool.

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Lambdabot plugin system description
by Richard Wallace 02 Jul '11

02 Jul '11

Hey all, I'm curious if there are any papers or anything else describing the plugin system in Lambdabot. If not I'll dig through the code, but my Haskell isn't yet that strong so a higher level introduction would be very helpful. Thanks, Rich

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compare iteratee with python's yield
by yi huang 02 Jul '11

02 Jul '11

I just read several tutorials on iteratee, i find that iteratee is similar to python's generator, both allow streamlined data processing. For example, i can implement enumFile and printChunks in python like this: EOF = None def enum_file(bufsize, filename): with open(filename) as input: while True: data = input.read(bufsize) if not data: break yield data yield EOF def print_chunks(print_empty, generator): for chunk in generator: if chunk==EOF: print 'EOF' return if len(chunk)==0 and not print_empty: continue print chunk print_chunks(True, enum_file(2, "data")) But i find iteratee far more complicated than python's generator, is that because iteratee can do something python's generator can't, or i simply need to be more familar with functional programming style. -- http://www.yi-programmer.com/blog/

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Re: [Haskell-cafe] Parsing cabal files to calculate average number of dependencies
by Gwern Branwen 01 Jul '11

01 Jul '11

On Fri, Jul 1, 2011 at 4:49 PM, L Corbijn <aspergesoepje(a)gmail.com> wrote: > Is this including or exluding 'or'-ed dependency lists like > http://hackage.haskell.org/package/hugs2yc ? Excluding, it seems. When I run the script on that tarball: $ tar --wildcards "*.cabal" -Oxf `find . -name "*.tar.gz" | g hugs2yc` | runhaskell /home/gwern/deps.hs PackageName "mtl" PackageName "uniplate" PackageName "yhccore" PackageName "ycextra" PackageName "parsec" PackageName "directory" PackageName "filepath" No version of base or containers appears. (mtl appears in both branches and also the general build-depends list.) -- gwern http://www.gwern.net

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Is there Functor instance of Enumerator'
by yi huang 01 Jul '11

01 Jul '11

Say i want to compose Enumerator ByteString m b and Iteratee Builder m b, so I try to transform the enum to Enumerator Builder m b, providing function ByteString -> Builder. It's like implement a Functor instance for Enumerator. But i failed, there are no way to make type system happy. Am I right that there is no Functor instance of Enumerator? -- http://www.yi-programmer.com/blog/

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Twitter API?
by Dmitri O.Kondratiev 01 Jul '11

01 Jul '11

Hi, Does anybody work with any Haskell Twitter API library? In particular supporting OAuth authentication? Thanks!

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Patterns for processing large but finite streams
by Eugene Kirpichov 01 Jul '11

01 Jul '11

Hi, I'm rewriting timeplot to avoid holding the whole input in memory, and naturally a problem arises: How to represent large but finite streams and functions that process them, returning other streams or some kinds of aggregate values? Examples: * Adjacent differences of a stream of numbers * Given a stream of numbers with times, split it into buckets by time of given width and produce a stream of (bucket, 50%,75% and 90% quantiles in this bucket) * Sum a stream of numbers Is this, perhaps, what comonads are for? Or iteratees? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Re: [Haskell-cafe] Patterns for processing large but finite streams
by John Lato 01 Jul '11

01 Jul '11

After the list discussion, I'm surprised nobody mentioned Max Rabkin/Conal Elliott's blog posts on folds and zips. http://squing.blogspot.com/2008/11/beautiful-folding.html http://conal.net/blog/posts/enhancing-a-zip/ They develop a formalism for zipping functions on lists. Iteratee's `zip` set of functions are somewhat similar, but not quite the same. Specifically they still require multiple traversals of the data, but only over a bounded portion of it, so they're much more efficient. Of course you could combine the above patterns with iteratees by creating functions as above, then just running them with a 'fold'. John L. > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > > Thanks but I'm afraid that's still not quite what I'm looking for; > guess I'll have to define my desire by my implementation - so once > it's ready I'll show the result to cafe :) > > 2011/7/1 Alexey Khudyakov <alexey.skladnoy(a)gmail.com>: > > On Fri, Jul 1, 2011 at 12:54 PM, Eugene Kirpichov <ekirpichov(a)gmail.com> > wrote: > >> Alexey, your definition of "mean" does not look like "liftS2 (/) sum > >> length" - you have to manually "fuse" these computations. > >> > > Well it was fused for numerical stability > > > >> I'm asking for a formalism that does this fusion automatically (and > >> guaranteedly). > >> > > Joining accumulators is quite straightforward. So is joining of initial > > state. Just creating a > >> joinAcc :: (acc1 -> x -> acc1) -> (acc2 -> x -> acc2) -> (acc1,acc2) -> > x -> (acc1,acc2) > >> joinAcc f1 f2 (s1,s2) x = (f1 s1 x, f2 s2 x) > > > > Still you have to handle them separately. > >> sum' = foldl (+) 0 > >> len ?= foldl (\n _ -> n+1) 0 > >> sumLen = foldl (joinAcc (+) (\n _ -> n+1)) (0,0) > > > > There is more regular approach but it only works with statistics. > > (function which do not depend on order of elements in the sample) > > For every statistics monoid for its evaluation could be constructed. > > For example sum: > >> newtype Sum a = Sum a > >> instance Num a => Monoid (Sum a) where > >> ? mempty = Sum 0 > >> ? mappend (Sum a) (Sum b) = Sum (a+b) > > > > Composition of these monoids becomes trivial. Just use > > > > > > I pursued this approach in monoid-statistics[1] package. > > It's reasonably well documented > > > > ?[1] http://hackage.haskell.org/package/monoid-statistics > > > >

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Re: [Haskell-cafe] Patterns for processing large but finite streams
by John Lato 01 Jul '11

01 Jul '11

> > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > Subject: [Haskell-cafe] Patterns for processing large but finite > streams > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <BANLkTikdsvQ2Wv4WJR+QMuvksoav0kTqXw(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > Hi, > > I'm rewriting timeplot to avoid holding the whole input in memory, and > naturally a problem arises: > > How to represent large but finite streams and functions that process > them, returning other streams or some kinds of aggregate values? > > Examples: > * Adjacent differences of a stream of numbers > * Given a stream of numbers with times, split it into buckets by time > of given width and produce a stream of (bucket, 50%,75% and 90% > quantiles in this bucket) > * Sum a stream of numbers > > Is this, perhaps, what comonads are for? Or iteratees? > This is exactly what iteratees are for. Specifically, enumeratees are stream transformers, iteratees are stream consumers, and enumerators are stream producers. Consider adjacent differences: Given the stream [a, b, c, d, e ....], you want to produce [b-a, c-b, d-c, e-d, ...] This is a stream transformer, so you need an enumeratee. Using iteratee, there are at least two obvious ways to produce it: 1) High-level, but probably not as good performance. Use Data.Iteratee.ListLike.roll > import Data.Iteratee as I > import Control.Applicative > > diff [x,y] = y-x > diff [x] = 0 > diffs = convStream (map diff <$> roll 2 1) > 2) somewhat explicit, probably better performance > import qualified Data.ListLike as LL > e' iter = do > h <- I.head > unfoldConvStream f h iter > where > f lastEl = do > c <- getChunk > if LL.null c > then return (lastEl, LL.empty) > else do > let h = LL.head c > t = LL.tail c > return (LL.last c, LL.cons (h-lastEl) (LL.zipWith (-) t (LL.init c))) either of these can be run by using an enumerator: *Main> enumPure1Chunk [1..10] (joinI $ e stream2list) >>= run [1,1,1,1,1,1,1,1,1,0] *Main> let e'2 = e' :: Enumeratee [Int] [Int] IO a *Main> enumPure1Chunk [1..10] (joinI $ e'2 stream2list) >>= run [1,1,1,1,1,1,1,1,1] I should optimize 'roll', it wouldn't be hard. Summing is easy; iteratee has "Data.Iteratee.ListLike.sum" built-in, but you could also use a fold. enumPure1Chunk is only useful for small amounts of data, but iteratee packages provide enumerators over files, handles, etc., as well as mechanisms by which you can create your own enumerators. The e' enumeratee is really just a model; I'd probably write one specific to whichever type of stream I wanted to work with. This one assumes a cheap 'cons', for example. For producing a stream of buckets, if the times are ordered it would be simple to do with Data.Iteratee.ListLike.breakE. If the times aren't ordered, I would probably use 'group' instead to collect a set number of samples. In my view, the biggest difference between iteratee and enumerator is the stream abstraction. Iteratee provides > I.Iteratee s m a where 's' is the stream type, e.g. [Int], String, ByteString, Vector Word8, etc. Although the library only processes a chunk at a time (where a chunk is a subsection of the stream), the type is that of the whole stream. Enumerator instead provides > E.Iteratee s m a here, 's' is the type of the chunk. Enumerator treats the stream as having type [s]. The implementations are different too, but ideally that would be hidden from most users. Although the iteratee implementation allows you to use >>= and $, whereas enumerator sometimes requires you to use >>== and $$. I think IterIO mostly follows the same type as Enumerator, although the implementation is again quite different. John

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How to implement the mean function
by Ruohao Li 01 Jul '11

01 Jul '11

Hi guys, I just started learning some Haskell. I want to implement a mean function to compute the mean of a list. The signature of the function is: mean :: (Num a, Fractional b) => [a] -> b But when I implement this simple function, the compiler keep whining at me on type errors. I know this is wrong: mean xs = sum xs / length xs But how to get it right? Thanks.

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Re: [Haskell-cafe] overloading show function
by Holger Siegel 01 Jul '11

01 Jul '11

Am 30.06.2011 um 20:23 schrieb Philipp Schneider: > On 06/30/2011 02:36 PM, Holger Siegel wrote: >> Am 29.06.2011 um 23:50 schrieb Philipp Schneider: >> >>> Hi cafe, >>> >>> in my program i use a monad of the following type >>> >>> newtype M a = M (State -> (a, State)) >>> >>> i use the monad in two different ways. The type variable "a" can be a >>> pair as in >>> >>> interp :: Term -> Environment -> M (Value,Environment) >>> >>> and it can be just a value as in >>> >>> type Environment = [(Name, Either Value (M Value))] >> Simple rule: Never return an environment! >> >> An environment contains local variable bindings, so no subcomputation will ever need to return its environment. I don't know anything about the language your program interprets, but I'm sure that you can rewrite function interp as >> >> interp :: Term -> Environment -> M Value >> >> The structure of the interpreter will become clearer and your problem will vanish. >> > Hello Holger, > > I'm giving two lambda interpreters. The first one is a call by value > interpreter, the second one a call by name interpreter which are > described in Philip Wadler's paper "The essence of functional > programming" page 4 and 12. Now my task is to write a lazy lambda > interpreter. The exercise is more playful than serious since Wadler's > call by value interpreter is, since written in lazy Haskell, already a > lazy lambda interpreter. (To get true call by value one would need to > force evaluations of the arguments with the seq function.) Hello Philipp, that's a nice exercise. > For both of Wadler's interpreters the type of the interpertation > function is: > interp :: Term -> Environment -> M Value > > Now to simulate lazy interpretation i need to do the following: Decide > is the value I need already evaluated or is it still a computation. In > the later case I need to evaluate it and save its value in the > environment. This is the reason I changed the type of the interpretation > function to: > interp :: Term -> Environment -> M (Value,Environment) But that won't work: After you have evaluated an entry of the environment, you store the resulting value but you throw away its updated environment. That means, you lose the results of all subcomputations instead of propagating them to all other copies of the environment. Consider the following expression: let x = big_computation in let y = x in y + x First, big_computation is bound to the name x, resulting in an environment [("x", big_computation)]. Then a closure consisting of this environment together with the right hand side 'x' is bound to the name y. Now y+x is evaluated: The closure is entered, and from its environment the content of x - a call to big_computation - is looked up. Now big_computation is evaluated and the result is bound to x in this environment. After that, this result is also returned as the value of y. But when returning from the evaluation of y, the environment with the updated value of x is lost and you have to re-evaluate it in order to calculate x+y! And that is why I say never return an environment. It is either wrong or unnecessary or the resulting semantics of the interpreter is hard to comprehend. In order to implement lazy evaluation correctly, you have to maintain some global state in which the thunks are updated. For example, your environment could bind IORefs that contain unevaluated thunks to variable names and update them when the thunk is evaluated. But then your interpreter has to run in the IO monad. By the way, do you already know Peter Sestoft's paper "Deriving a lazy abstract machine"? Cheers, Holger

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ANNOUNCING: Hac PDX II - A Portland Haskell Hackathon
by Thomas DuBuisson 30 Jun '11

30 Jun '11

WHAT: A Haskell Hackathon WHEN: July 22-24 (Friday, Saturday, Sunday) 10:00 AM to 5:30 PM WHERE: Forth Avenue Building (FAB, 1900 SW 4th Ave) Room 10 Portland, Oregon 97201 WHERE, take 2: FAB10 is a small auditorium just inside the west most Harrison Street entrance. URL: http://haskell.org/haskellwiki/HacPDX-II WHAT, take 2: HacPDX-II is a Haskell Hackathon where beginners and experts alike can come to work on the Haskell projects of their choice - individually or in groups. If you don't have a project then never fear, come and pick one up or join a group (see the wiki for a list of projects). EQUIPMENT: You should bring a laptop with wireless (802.11). Ethernet is unavailable. Registration: Please RSVP by e-mailing thomas.dubuisson+hacpdx at gmail.com, as well as add your name to the attendees Wiki page (see above URL). Hope to see you there, Thomas

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ANNOUNCE: mime-mail-ses 0.0.0
by Michael Snoyman 30 Jun '11

30 Jun '11

Hi all, I'd like to announce the first release of mime-mail-ses[1]. This provides easy integration between the mime-mail package[2] and Amazon's Simple Email Service. This package was written for the Haskellers website[3], purely because I despise setting up mail servers. There aren't many more features planned, though hopefully in the future it will integrate more closely with Aristid's aws package[4]. Also, a big thank you to Thomas DuBuisson for crypto-api[5] and Vincent Hanquez for cryptohash[6]. Those two packages make it very easy to write applications that require cryptographic authentication. Michael [1] http://hackage.haskell.org/package/mime-mail-ses [2] http://hackage.haskell.org/package/mime-mail [3] http://www.haskellers.com/ [4] http://hackage.haskell.org/package/aws [5] http://hackage.haskell.org/package/crypto-api [6] http://hackage.haskell.org/package/cryptohash

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overloading show function
by Philipp Schneider 30 Jun '11

30 Jun '11

Hi cafe, in my program i use a monad of the following type newtype M a = M (State -> (a, State)) i use the monad in two different ways. The type variable "a" can be a pair as in interp :: Term -> Environment -> M (Value,Environment) and it can be just a value as in type Environment = [(Name, Either Value (M Value))] now in any case when i print the monad, i just want to print the value and never the environment. More specific i want to use somthing like the following instance (Show a,Show b) => Show (M (a,b)) where show (M f) = let ((v,_), s) = f 0 in "Value: " ++ show v ++ " Count: " ++ show s instance Show a => Show (M a) where show (M f) = let (v, s) = f 0 in "Value: " ++ show v ++ " Count: " ++ show s however this gives me the following error message: Overlapping instances for Show (M (Value, Environment)) arising from a use of `print' Matching instances: instance (Show a, Show b) => Show (M (a, b)) -- Defined at /home/phil/code/haskell/vorlesung/ue09/ue09-3c3.hs:78:10-42 instance Show a => Show (M a) -- Defined at /home/phil/code/haskell/vorlesung/ue09/ue09-3c3.hs:82:10-29 In a stmt of an interactive GHCi command: print it Any ideas how to fix it? Thanks! Philipp

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what if two package contains same module?
by 吴兴博 30 Jun '11

30 Jun '11

it seems that cabal install different into different folders. if two package contains same module name, can they all exist? since cabal have no "remvoe" or "uninstall". how can I hide some packages? Cheers! ---------------- 吴兴博 Wu Xingbo

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Haskell Weekly News: Issue 188
by Daniel Santa Cruz 30 Jun '11

30 Jun '11

Welcome to issue 188 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of June 19 to 25, 2011. It was brought up in cafe this week that some would prefer to see full links in the text version of the mailing list as opposed to the shortened links. The reason behind using shortened links is that the newsletter becomes pretty much unreadable with long links in a plain text buffer. In an effort to accommodate both needs, I'm trying a new format for links. One that includes shorter links embedded throughout the newsletter, and the full reference at the bottom of the text. Let me know what you think! If you are reading this on a mobile device, or you would just rather see the HTML version, point your browser to: http://contemplatecode.blogspot.com/2011/06/haskell-weekly-news-issue-188.h… Announcements Alex Mason wrote in to remind us that AusHac2011 (the Australasian Haskell Hackathon) will be held on July 8-10, at UNSW in Sydney. "If you are an Australian haskell hacker, enthusiast, newbie or professional, we would love to see you there." [2] http://goo.gl/d0SXy Florent Becker announced the release of darcs 2.8 beta 1, with many new beta features. Give it a world and let the devs know how things are working! [3] http://goo.gl/rq3DM Quotes of the Week * <[~functor(a)zygohistomorphic-prepromorphism.endofunctor.org] has joined #haskell * kmc: actually, C++'s "typename" keyword is inspired by INTERCAL's "PLEASE" * Toxaris: #haskell delivers a massively multiprogrammer online functional programming (MMOFP) experience * sully: although I'm switching to haskell more and more just on practical grounds * mauke: the greek letter lambda is an imperfect approximation of the symbol \ * Cale: The trouble with sarcasm is that in order for it to really be effective, the listener has to know what the speaker knows. Otherwise, it's indistinguishable from ignorance. Top Reddit Stories * Haskell's evaluation isn't magic. Domain: netsuperbrain.com, Score: 42, Comments: 33 On Reddit: [4] http://goo.gl/jD8jM Original: [5] http://goo.gl/31pyt * Superclass equalities are now supported Domain: hackage.haskell.org, Score: 40, Comments: 16 On Reddit: [6] http://goo.gl/5PuUh Original: [7] http://goo.gl/XhrMR * Free Monads for Less (Part 1 of 3): Codensity Domain: comonad.com, Score: 35, Comments: On Reddit: [8] http://goo.gl/mqkDi Original: [9] http://goo.gl/9eWLr * Renewed LambdaCube 3D engine, Bullet bindings and Stunts example Domain: just-bottom.blogspot.com, Score: 33, Comments: 2 On Reddit: [10] http://goo.gl/UsP59 Original: [11] http://goo.gl/YR3sH * A new design for haskell.org Domain: chrisdone.com, Score: 31, Comments: 47 On Reddit: [12] http://goo.gl/044JZ Original: [13] http://goo.gl/Hvivh * Free Monads for Less (Part 2 of 3): Yoneda Domain: comonad.com, Score: 25, Comments: On Reddit: [14] http://goo.gl/K6fI9 Original: [15] http://goo.gl/bjV5f * Free Monads for Less (Part 3 of 3): Yielding IO Domain: comonad.com, Score: 24, Comments: On Reddit: [16] http://goo.gl/pKXUR Original: [17] http://goo.gl/EDxzo * Snap Framework v0.5.1 released Domain: snapframework.com, Score: 20, Comments: 2 On Reddit: [18] http://goo.gl/sKsqq Original: [19] http://goo.gl/Chk6j * Comparing FRP to RDP (Reactive Demand Programming) Domain: awelonblue.wordpress.com, Score: 19, Comments: 2 On Reddit: [20] http://goo.gl/0wEgx Original: [21] http://goo.gl/B4dYs * Haskell eDSL Tutorial - Shared expenses Domain: lpenz.org, Score: 18, Comments: 11 On Reddit: [22] http://goo.gl/6yf5j Original: [23] http://goo.gl/t14Xs Top StackOverflow Questions * How do you write data structures that are as efficient as possible in GHC? votes: 26, answers: 2 Read on SO: [24] http://goo.gl/cj18P * How does seq force functions? votes: 15, answers: 1 Read on SO: [25] http://goo.gl/f1TX8 * I/O in Haskell is Functional? votes: 14, answers: 5 Read on SO: [26] http://goo.gl/ZT0HQ * Exponentiation in Haskell votes: 12, answers: 4 Read on SO: [27] http://goo.gl/ueo1A * How to compile a resource into a binary in Haskell? votes: 12, answers: 4 Read on SO: [28] http://goo.gl/mxDPV * Haskell Heap Issues with Parameter Passing Style votes: 12, answers: 2 Read on SO: [29] http://goo.gl/b2m10 * Functor is for (a -> b) -> (f a -> f b), what is for (Category c) => c a b -> c (f a) (f b) ? votes: 12, answers: 1 Read on SO: [30] http://goo.gl/Usv7I * Better way to write the following program in Haskell votes: 12, answers: 7 Read on SO: [31] http://goo.gl/Dn7Xj About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18670 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90496 4. http://www.reddit.com/r/haskell/comments/i4k3j/haskells_evaluation_isnt_mag… 5. http://netsuperbrain.com/blog/posts/haskells-evaluation-isnt-magic 6. http://www.reddit.com/r/haskell/comments/i6ftr/superclass_equalities_are_no… 7. http://hackage.haskell.org/trac/ghc/ticket/2715#comment:18 8. http://www.reddit.com/r/haskell/comments/i7sy0/free_monads_for_less_part_1_… 9. http://comonad.com/reader/2011/free-monads-for-less/ 10. http://www.reddit.com/r/haskell/comments/i6qi5/renewed_lambdacube_3d_engine… 11. http://just-bottom.blogspot.com/2011/06/renewed-lambdacube-bullet-bindings-… 12. http://www.reddit.com/r/haskell/comments/i9183/a_new_design_for_haskellorg/ 13. http://chrisdone.com/designs/haskell/ 14. http://www.reddit.com/r/haskell/comments/i7v5r/free_monads_for_less_part_2_… 15. http://comonad.com/reader/2011/free-monads-for-less-2/ 16. http://www.reddit.com/r/haskell/comments/i7vho/free_monads_for_less_part_3_… 17. http://comonad.com/reader/2011/free-monads-for-less-3/ 18. http://www.reddit.com/r/haskell/comments/i3odg/snap_framework_v051_released/ 19. http://snapframework.com/blog/2011/06/19/snap-0.5.1-released 20. http://www.reddit.com/r/haskell/comments/i4i5c/comparing_frp_to_rdp_reactiv… 21. http://awelonblue.wordpress.com/2011/05/21/comparing-frp-to-rdp/ 22. http://www.reddit.com/r/haskell/comments/i8ylw/haskell_edsl_tutorial_shared… 23. http://lpenz.org/articles/hedsl-sharedexpenses 24. http://stackoverflow.com/questions/6432284/how-do-you-write-data-structures… 25. http://stackoverflow.com/questions/6442019/how-does-seq-force-functions 26. http://stackoverflow.com/questions/6399702/i-o-in-haskell-is-functional 27. http://stackoverflow.com/questions/6400568/exponentiation-in-haskell 28. http://stackoverflow.com/questions/6402158/how-to-compile-a-resource-into-a… 29. http://stackoverflow.com/questions/6429085/haskell-heap-issues-with-paramet… 30. http://stackoverflow.com/questions/6445501/functor-is-for-a-b-f-a-f-b-what-… 31. http://stackoverflow.com/questions/6476319/better-way-to-write-the-followin…

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Re: [Haskell-cafe] [Haskell-beginners] Understanding category theory
by Jack Henahan 30 Jun '11

30 Jun '11

Derp, forgot to CC the Cafe. Which ones have you read already? It's difficult to make recommendations without knowing what's already off the table. Additionally, don't be put off if it takes a lot of effort to figure anything out. Math textbooks and so on are unlike any other textbook. It's not unusual to spend a day (or several days) trying to digest a single page of material unless you have a strong mathematical background (or even if you do). This is especially true in the more abstract disciplines (and if there's anything more abstract than category theory, I don't think I've yet encountered it). On Jun 29, 2011, at 8:26 PM, Christopher Howard wrote: > Would anyone recommend a particular book or tutorial to help me understand category theory? I've tried reading a few tutorials i've googled, but even the "beginner" ones are very difficult and seem to assume a lot. > > Sent from my HTC > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners ==== "Computer Science is no more about computers than astronomy is about telescopes." -- Edsger Dijkstra ====

1 0

Exclusive mode in openFile
by Gracjan Polak 30 Jun '11

30 Jun '11

Hi all, It seems I'm not allowed to open same file both for writing and for reading: Prelude System.IO> f_out <- openFile "mylog.log" AppendMode Prelude System.IO> f_in <- openFile "mylog.log" ReadMode *** Exception: mylog.log: openFile: resource busy (file is locked) Usage scenario: I use hslogger to write to logs, but I'd like to read parts of these logs *from inside same Haskell application*. How do I get around exclusive mode in openFile? -- Gracjan

4 7

How to flush with IterIO in echo server
by John Ky 29 Jun '11

29 Jun '11

Hi Haskell Cafe, I've written an echo server using just sockets: import Control.Concurrent import Control.Exception import Control.Monad import Network import Offsync.Data import System.IO import System.IO.Error (isEOFError) main = withSocketsDo $ do sListen <- listenOn (PortNumber 8000) putStrLn "Listening on Port 8000" forkIO $ forever $ do (sSession, hostname, port) <- accept sListen putStrLn ("Connected to " ++ hostname ++ ":" ++ show port) forkIO $ echoLines sSession putStrLn "Press <CTRL-D> to quit." exitOnCtrlD echoLines h = try (hGetLine h) >>= either (\e -> do { hClose h; if isEOFError e then print e else ioError e}) (\l -> do { hPutStrLn h l; hFlush h; echoLines h}) exitOnCtrlD = try getLine >>= either (\e -> unless (isEOFError e) $ ioError e) (const exitOnCtrlD) When I send text to it, it will echo it back immediately after my newline. I then modified it to user IterIO: import Control.Concurrent import Control.Exception import Control.Monad import Control.Monad.Trans import Data.IterIO import Data.IterIO.Inum import Network import System.IO import System.IO.Error (isEOFError) import qualified Data.ByteString.Lazy as L iterHandle' :: (MonadIO m) => Handle -> IO (Iter L.ByteString m (), Onum L.ByteString m a) iterHandle' = iterHandle main = withSocketsDo $ do sListen <- listenOn (PortNumber 8000) putStrLn "Listening on Port 8000" forkIO $ forever $ do (sSession, hostname, port) <- accept sListen putStrLn ("Connected to " ++ hostname ++ ":" ++ show port) forkIO $ do (iter, enum) <- iterHandle' sSession enum |$ iter return () putStrLn "Press <CTRL-D> to quit." exitOnCtrlD exitOnCtrlD = try getLine >>= either (\e -> unless (isEOFError e) $ ioError e) (const exitOnCtrlD) It works, however it doesn't send anything back to me until end of file. I fixed that problem with my sockets version by flushing after each line, but I don't know if IterIO will let me flush on every newline. Any ideas? Cheers, -John

2 1

hmatrix-static with ghc 7.0.2
by Anand Patil 29 Jun '11

29 Jun '11

Hi all, Has anyone managed to get hmatrix-static to compile with ghc 7.0.2, and if so would you be willing to share a patch? With best wishes, Anand

1 0

attoparsec and vectors
by Eric Rasmussen 29 Jun '11

29 Jun '11

Hi everyone, I have a task that involves parsing a flat-file into a vector from the Data.Vector package. In the interest of keeping things simple, I first used Attoparsec.Text's version of "many" and then converted the resulting list to a vector: import qualified Data.Vector as V import Data.Attoparsec.Text as A import Control.Applicative getData = do results <- A.many someParser return (V.fromList results) It runs quickly, but I naively thought I could outperform it by reworking "many" to build a vector directly, instead of having to build a list first and then convert it to a vector: manyVec :: Alternative f => f a -> f (V.Vector a) manyVec v = many_v where many_v = some_v <|> pure V.empty some_v = V.cons <$> v <*> many_v Unfortunately, manyVec either quickly leads to a stack space overflow, or it takes an enormous amount of time to run. Does anyone know of a better way to build up a vector from parsing results? Thanks for any tips or insight, Eric

3 3

Possible bug in GHC 7.0.3
by Alberto G. Corona 29 Jun '11

29 Jun '11

I have an "'impossible' happened" error. The code may look a little bit convoluted but it is part of my real code.: --------- begin of code------ {-# LANGUAGE FlexibleInstances, UndecidableInstances , MultiParamTypeClasses #-} class Serializable a b class IResource a --The rest of the instance definitions does not matter for the error instance Serializable a b => IResource a data DBRef a= DBRef String a instance (IResource a) => Read (DBRef a) data Votation a= Votation{ content :: DBRef a } deriving (Read) ------------------------------- end of code --- gives the following error at compilation time: tests>runghc impossiblelloop.hs ghc: panic! (the 'impossible' happened) (GHC version 7.0.3 for i386-unknown-mingw32): solveDerivEqns: probable loop (impossiblelloop.hs:20:13-16 main:Main.$fReadVotation{v rhI} [a{tv abB} [tv] ] base:GHC.Read.Read{tc 2d} [main:Main.Votation{tc rbo} a{tv abB} [tv]] = [base:GHC.Read.Read{tc 2d} (main:Main.DBRef{tc rbu} a{tv abB} [tv])]) [[main:Main.Serializable{tc rbA} a{tv abB} [tv] b{tv ajE} [tcs]]] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

5 7

Re: [Haskell-cafe] Possible bug in GHC 7.0.3
by Alberto G. Corona 29 Jun '11

29 Jun '11

Ryan' P robably the read instance can not be infered without the functional dependency but the bug is also triggered with it. I simplified ithe coder as much as possible El 29/06/2011 02:26, "Ryan Ingram" <ryani.spam(a)gmail.com> escribió:

1 0

IterIO type restricted functions
by John Ky 29 Jun '11

29 Jun '11

Hi all, >From the IterIO tutorial: enumFile' is like enumFile above, but type restricted to data in the lazy ByteString format, which is more efficient than plain Strings. (enumFile supports multiple types, but in this example there is not enough information for Haskell to choose one of them, so we must use enum*f*ile' or use :: to specify a type explicitly. Which is fine, but shouldn't there also be iterHandle' and iterStream'? Also I'm guessing the *f* is a typo that should be *F*. Cheers, -John

2 2

Problem building with Cabal + TH + Library with FFI to C++ via C Wrapper
by Ozgun Ataman 28 Jun '11

28 Jun '11

Dear Cafe, I have recently run into a very annoying issue that I was not able to get around. I have a local package that makes use of the snappy compression library, which boasts bindings to C++ through a C wrapper. Everything compiles fine with ghc --make, but cabal install hits a wall when TemplateHaskell kicks in and decides to load the entire dependency chain. Here is the important bit of the error message I get after running "cabal install" Loading package snappy-0.2.0.0 ... linking ... ghc: /home/ubuntu/.cabal/lib/snappy-0.2.0.0/ghc-7.0.3/HSsnappy-0.2.0.0.o: unknown symbol `_ZTV8BSSource' ghc: unable to load package `snappy-0.2.0.0' I'm quite certain all the libs are in PATH. "cabal install snappy" installs fine when typed separately by itself and everything works fine (the program works, including compression via snappy) when I compile my local package with ghc --make. The problem only occurs on a Ubuntu 64-bit server, and I wasn't able to replicate the issue on my local OS X box. On my local, cabal install works just fine. I did find a previous note on Cafe with the exact same problem, but no solution was posted at the time: http://www.haskell.org/pipermail/haskell-cafe/2010-December/087748.html Any help is much appreciated. Cheers, Ozgun

1 0

Installation Failed: Haskell Platform 2011.2.0.1-i386 for OS X
by John Velman 28 Jun '11

28 Jun '11

I'm running OS X 10.6.7, XCode 3.2.5. When I try to install "The Haskell Platform 2011.2.0.1 for Mac OS X 10.6 (Snow Leopard)" it goes all the way through to "running package scripts", then says "installation failed" I did two separate downloads, and tried the first installation two time, the second download one time. Same result all three times. Is this a problem with the package, or ? Any suggestions? Thanks, John Velman

3 3

Enumerators, Enumeratees and Iterators
by Sævar Berg 28 Jun '11

28 Jun '11

Hey Café. I've been playing with Enumerators, Iteratees and Enumeratees today after having spent a few days writing a server application using lazy IO, then reading slides from Oleg's DEFUN8 talk notes, and I quote: "Lazy IO in serious, server-side programming is unprofessional", "I can talk a lot how disturbingly, distressingly wrong lazy IO is theoretically, how it breaks all equational reasoning" after which my OCD self has been eating me up to take a proper look at this, so here I am. I'm already beginning to see how Iteratees, Enumerators and Enumeratees can be nifty, but I have a couple of questions about a couple of ideas and whether, and if so how they can be implemented. The first question is, I think, to be solved with enumeratees but I can't really grok how. Let's say I have an iteratee that consumes all input. Is it possible to implement an enumeratee or something else to stick between the enumerator and the iteratee to basically modify the input to the iteratee to only be a part of the input? Something like this: enumFile "someFile" && printFileLines -- prints file with prepended line numbers enumFile "someFile" ?? onlyGet10Lines && printFileLines -- print only 10 lines The second question could actually have an application in the server I'm writing. I was wondering if it was possible to write iteratees/enumerators that would only generate/consume when a certain something was the next chunk to be processed? It's a bit hard to explain, but let me try to give you an example. Let's say my application takes input from the network which contains commands that the server reacts to. Let's say I have a list of command handlers that I want to run this command through, but the handlers will only execute when they are passed the command that they are responsible for handling. Can this list of command handlers be enumerators/iteratees/enumeratees? E.g. is it possible to 'concat' them, making them each peek at the next chunk to see if it's theirs to handle and put it back if it isn't? I know this is possible to do differently and I have an idea for how I would do this, but I'm wondering if this is possible. If I can get answers to either or both of the questions it would help me a bit on my way to completely realize the roles that enumerators vs. iteratees vs. enumeratees have, as the lines seem a bit blurred -- sometimes, anyway. Thanks in advance. :)

4 4

Re: [Haskell-cafe] Possible bug in GHC 7.0.3
by Alberto G. Corona 28 Jun '11

28 Jun '11

Reported: http://hackage.haskell.org/trac/ghc/ticket/5287 2011/6/28 Alberto G. Corona <agocorona(a)gmail.com> > > 2011/6/28 Jason Dagit <dagitj(a)gmail.com> > >> On Tue, Jun 28, 2011 at 3:43 AM, Alberto G. Corona <agocorona(a)gmail.com> >> wrote: >> > I have an "'impossible' happened" error. >> > The code may look a little bit convoluted but it is part of my real >> code.: >> >> I don't know why it's crashing, but did you already report it as a >> bug? If not, you definitely should. >> >> > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug >> >> That URL should have the info you need to report it. >> >> Jason >> > > Not published yet. I wanted to publish it in the list before reporting the > bug. Just in case anyone tell something interesting about it. A sorter > expression that produces the same bug or something similar. >

1 0

What is a "simple pattern binding"?
by dm-list-haskell-cafe＠scs.stanford.edu 28 Jun '11

28 Jun '11

Section 4.4.3.2 of the 2010 Haskell report says: A simple pattern binding has form p = e. The pattern p is matched “lazily” as an irrefutable pattern, as if there were an implicit ~ in front of it. This makes it sound as though p is a pattern, which I assume means what section 3.17 defines as the non-terminal "pat". pat -> lpat qconop pat | lpat This is further suggested by the explicit mention of ~, which would be redundant if p had to be a var, since variables always match (according to section 3.17.2 rule 1). So my reading of section 4.4.3.2 is that the following is considered a simple pattern binding (because it has no guards): (f, g) = (\x -> x, f) However, section 4.5.5 seems to contradict this. It reads: Recall that a variable is bound by either a function binding or a pattern binding, and that a simple pattern binding is a pattern binding in which the pattern consists of only a single variable (Section 4.4.3). Moreover, it goes on to give an example and explanation: [(n,s)] = reads t ... Hence, when non-simple pattern bindings are used This text makes it sound as though a "simple pattern binding" can have only a single variable to the left of the = sign, meaning: f = \x -> x Further confusing things, GHC accepts the following: g1 x y z = if x>y then show x ++ show z else g2 y x g2 :: (Show a, Ord a) => a -> a -> String g2 | False = \p q -> g1 q p () | otherwise = \p q -> g1 q p 'a' where x = True and infers type: g1 :: (Show a, Show a1, Ord a1) => a1 -> a1 -> a -> [Char] According to 4.4.3.2, g2 definitely does not have a simple pattern binding, as its binding is not of the form p = e where p is a pattern. Yet by section 4.5.5, if g2 were not considered a simple pattern binding, the constrained type variables in the binding group containing g1 and g2 (in particular the inferred type (Show a => a) of z in g1) would not be allowed to be generalized. So is section 4.4.3.2 of the Haskell 2010 report just wrong? Or is GHC allowing code prohibited by the standard? Or am I somehow misreading the standard? Anyway, if someone can provide a less ambiguous definition of the term "simple pattern binding", I would appreciated it, particularly if you can point to support for your definition in the Haskell 2010 report... Thanks, David

4 12

Cleaner way to write code and handle errors?
by John Ky 28 Jun '11

28 Jun '11

Hi all, I'm practising my Haskell by writing a simple TCP echo server and finding that getting my program control to be succinct is rather tricky. In particular, I have return () everywhere, my error handling is verbose and I'm not entirely sure my recursion is the cleanest way syntactically to get my loops going and terminating. I must be doing something obviously un-Haskell-like. Any suggestions on how I can improve my code? Code below. Cheers, -John import Control.Concurrent import Control.Exception import Control.Monad import Network import System.IO import System.IO.Error (isEOFError) main = withSocketsDo $ do sListen <- listenOn (PortNumber 8000) putStrLn "Listening on Port 8000" forkIO $ forever $ do (sSession, hostname, port) <- accept sListen putStrLn ("Connected to " ++ hostname ++ ":" ++ (show port)) let processLine = forkIO $ do lineResult <- try (hGetLine sSession) case lineResult of Right line -> do putStrLn line processLine return () Left e -> if isEOFError e then putStrLn (show e) else do ioError e return () return () processLine return() line <- getLine return ()

4 5

Data Flow Programming in FP
by Richard Senington 28 Jun '11

28 Jun '11

Hi all, I have recently become interested in Dataflow programming and how it related to functional languages. I am wondering if the community has any advice on reading matter or other directions to look at. So far I have been looking through the FRP libraries, using Haskell functions with lazy lists for co-routines and the Essence of Dataflow Programming by Uustalu and Vene where they propose using co-monads. It looks as though Iteratees are also relevant but I have not got round to looking at them in detail yet. Have I missed anything? Regards RS

6 8

Fwd: Re: Period of a sequence
by Steffen Schuldenzucker 28 Jun '11

28 Jun '11

Forwarding to -cafe -------- Original Message -------- Subject: Re: [Haskell-cafe] Period of a sequence Date: Mon, 27 Jun 2011 04:46:10 -0700 (PDT) From: michael rice <nowgate(a)yahoo.com> To: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> Hi Steffen, Repeating decimals. 5/7 == 0.714285 714285 7142857 ... Period = 6 It does seem like a difficult problem. This one is eventually repeating, with Period = 3 3227/555 = 5.8144 144 144… Michael --- On *Mon, 6/27/11, Steffen Schuldenzucker /<sschuldenzucker(a)uni-bonn.de>/*wrote: From: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> Subject: Re: [Haskell-cafe] Period of a sequence To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Monday, June 27, 2011, 4:32 AM On 06/26/2011 04:16 PM, michael rice wrote: > MathWorks has the function seqperiod(x) to return the period of sequence > x. Is there an equivalent function in Haskell? Could you specify what exactly the function is supposed to do? I am pretty sure that a function like seqPeriod :: (Eq a) => [a] -> Maybe Integer -- Nothing iff non-periodic cannot be written. If "sequences" are represented by the terms that define them (or this information is at least accessible), chances might be better, but I would still be interested how such a function works. The problem seems undecidable to me in general. On finite lists (which may be produced from infinite ones via 'take'), a naive implementation could be this: > > import Data.List (inits, cycle, isPrefixOf) > import Debug.Trace > > -- Given a finite list, calculate its period. > -- The first parameter controls what is accepted as a generator. See below. > -- Set it to False when looking at chunks from an infinite sequence. > listPeriod :: (Eq a) => Bool -> [a] -> Int > listPeriod precisely xs = case filter (generates precisely xs) (inits xs) of > -- as (last $ init xs) == xs, this will always suffice. > (g:_) -> length g -- length of the *shortest* generator > > -- @generates prec xs g@ iff @g@ generates @xs@ by repitition. If @prec@, the > -- lengths have to match, too. Consider > -- > -- >>> generates True [1,2,3,1,2,1,2] [1,2,3,1,2] > -- False > -- > -- >>> generates False [1,2,3,1,2,1,2] [1,2,3,1,2] > -- True > generates :: (Eq a) => Bool -> [a] -> [a] -> Bool > generates precisely xs g = if null g > then null xs > else (not precisely || length xs `mod` length g == 0) > && xs `isPrefixOf` cycle g > -- Steffen

5 5

Re: [Haskell-cafe] Hoopl: Combining CheckingFuelMonad with State?
by Antoine Latter 27 Jun '11

27 Jun '11

Hi Justin, this message might be better on the haskell-cafe list (or the excellent beginers list!). When you tried to write the get/put implementations, what problems were you running into? Antoine On Sat, Jun 25, 2011 at 7:50 AM, Justin Bailey <jgbailey(a)gmail.com> wrote: > I'd like to carry around some state when rewriting. It seems like > CheckingFuelMonad, etc. are set up to use with other monads but I > can't get the types to agree. > > Using MTL I've managed to come up with these types: > >> newtype RewriteOnce a = R (State Bool a) >> deriving (Monad) >> >> instance MonadState s (CheckingFuelMonad RewriteOnce) where >> get = undefined >> put = undefined > > > But I cannot write the definitions for get and put. Is this possible > or am I misundersanding CheckingFuelMonad? Is there a better approach > altogether? > > Thanks in advance for any help! > > Justin > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users(a)haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >

2 1

toSql and fromSql, for Algebraic Data Type
by Tom Murphy 27 Jun '11

27 Jun '11

Hi *, The title is self-explanatory. I'd like to store information from an algebraic data type in an SQL database, but the type signature of toSql (toSql :: Data.Convertible.Base.Convertible a SqlValue => a -> SqlValue) doesn't make sense to me. How is this done (how do I make an instance of a typeclass like that?) Thanks for your time, Tom

2 1

Help
by Stoyan Peev 27 Jun '11

27 Jun '11

Hello all, I am experiencing some issues to do my course task in university. I have to write a calculator- function in Haskell. The function argument is a list of strings and also form such list, as each string of the argument made definite action: - If the string has the form of an arithmetic _expression_ - calculate this _expression_. The string result becomes part of the list-result. If the _expression_ contains a variable which is not assigned value, the result is displayed "undefined". - If the string has the form- Name = value calculated from the last _expression_ is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type - If there is not a calculated _expression_ to be assigned to form a string "no value". - If the string is non-blank, but there is a species different from the above two case, form the string "error". - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) and parentheses. Where no brackets, the operations are performed from left to right, but * and / precede the + and -. Implementation of any operation gives integer; in the division rejected the fractional part, if any. Variables have names of one letter - from the Latin small letter. In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument ["3 +7 / 2" "2 + x", "= s", "2 * s +4", "", "2 + +4 / 5] function should provide a result-list ["6", "undefined", "s = 6", "16", "error"]. I say another person have the same task, but he didn't do anything. I started doing this task myself but i get stuck in the middle. Then i started searching for something that could help me and find out you :) The code i have written so far uses the library file "Parsing.lhs" but what i have written is taking those actions that i already described, only for a string. I cannot modify it to work for list of string, and complete the whole task. I'll be glad to finish the task myself, but i am going to need some help. Here is the code i have already written: import Parsing expr :: Parser Int expr = do t <- term do symbol "+" e <- expr return (t+e) +++ do symbol "-" e <- expr return (t-e) +++ return t term :: Parser Int term = do f <- factor do symbol "*" t <- term return (f * t) +++ do symbol "/" t <- term return (f-t) +++ return f factor :: Parser Int factor = do symbol "(" e <- expr symbol ")" return e +++ natural eval :: String -> Int eval xs = case (parse expr xs) of [(n,[])] -> n [(_,out)] -> error ("undefined") [] -> error "error" Thanks all in advance :) -- Best Wishes Stoyan Peev

3 9

Re: [Haskell-cafe] Data.Time
by Daniel Schüssler 27 Jun '11

27 Jun '11

On 2011-June-27 Monday 10:15:28 Yitzchak Gale wrote: > The biggest shortcoming, in my opinion, is that the documentation > assumes that the reader is very familiar with the Haskell type > system, and with viewing type signatures and instance lists as an > integral and central part of the documentation. > > In particular, Haskell's standard numeric type classes and the > conversion functions between them play a central role in the API > of Data.Time. Making use of Haddock's recently added(?) support for comments on instance declaration would help a lot here, I think (even if it was just to draw attention to the Num/Integral/Real/Fractional instances). > Before you undertake writing a whole new time library, why not > try writing some improved documentation for Data.Time first? I got the impression that Tony's issue with Data.Time was the lack of some feature, not usability. Seems like the details that were omitted for the sake of constructiveness are quite relevant :) Cheers, Daniel

1 0

Re: [Haskell-cafe] Data.Enumerator.List.concatMap is to Data.Iteratee.?
by John Lato 27 Jun '11

27 Jun '11

> > From: David Place <d(a)vidplace.com> > > Hi: > > I've been studying iteratee IO. Is there a function in the iteratee > package that is analogous to Data.Enumerator.List.concatMap? > Iteratee's 'Data.Iteratee.Iteratee.convStream', or the more general 'Data.Iteratee.Iteratee.unfoldConvStream', would be the rough equivalents. The difference is that DEL.concatMap operates on each chunk, whereas DII.convStream operates on the stream. This makes it possible to specify operations which work on multiple chunks. If you want a function more like DEL.concatMap, it would be > concatMap f = convStream (fmap f getChunk) Note that stream type 's' in enumerator should generally be translated to '[s]' in iteratee. John

1 0

View Thunk Evaluation
by Tom Murphy 27 Jun '11

27 Jun '11

Hi All, Is there a way to determine whether a thunk was evaluated during code's execution? Thanks, Tom

2 2

Trouble with `foldl` in composition chain.
by David Banas 27 Jun '11

27 Jun '11

When profiling this code: consolidateRPR :: [Sample] -> [Sample] smplUnionRecursRev :: [Sample] -> Sample -> [Sample] sortSamps :: [Sample] -> [Sample] smplSetUnion :: [Sample] -> [Sample] smplSetUnion = consolidateRPR . (foldl smplUnionRecursRev []) . sortSamps I'm getting this: smplSetUnion Data.RandProc 1038 32794 0.7 0.2 76.9 72.6 consolidateRPR Data.RandProc 1041 32794 0.0 0.0 8.2 11.8 smplUnionRecursRev Data.RandProc 1040 918112 8.8 5.6 8.8 5.6 sortSamps Data.RandProc 1039 1081514 21.1 55.0 59.2 55.0 I'm confused as to why `sortSamps` is being called so many times. I would expect it to be called 32794 times, just like `consolidateRPR`. It's as if having a `foldl` to the left of `sortSamps` in the composition chain is causing it to execute more than once, per call of `smplSetUnion`, but I wouldn't expect that. This doesn't change anything: smplSetUnion = consolidateRPR . (\ss -> foldl smplUnionRecursRev [] ss) . sortSamps (I thought it might explicitly close the `foldl`.) In fact, even this: smplSetUnion ss = consolidateRPR $ foldl smplUnionRecursRev [] ss' where ss' = sortSamps ss doesn't change anything. `sortSamps` still gets entered 1081514 times! Can anyone help clear up this newbie's fog? Thanks, in advance! -db

1 0

Alex Lexer Performance Issues
by Tristan Ravitch 27 Jun '11

27 Jun '11

Hi Cafe, In one of my projects I have a lexer that seems to be taking an inordinate amount of time and space. The lexer is generated by Alex using the lazy ByteString (with position information) template. I compiled and ran with profiling enabled and I get a report like this: ------------------------------------------------------------------------------- Tue Jun 21 16:56 2011 Time and Allocation Profiling Report (Final) ViewCallGraph +RTS -p -RTS gnutls.bc total time = 51.80 secs (2590 ticks @ 20 ms) total alloc = 9,482,333,244 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc alexScanTokens Data.LLVM.Private.Lexer 24.1 4.5 alex_scan_tkn Data.LLVM.Private.Lexer 21.2 32.7 tokenAs Data.LLVM.Private.Parser.Primitive 6.7 2.9 alexGetChar Data.LLVM.Private.Lexer 6.5 22.7 ------------------------------------------------------------------------------- The entries below these four are marginal. The third entry is from my code and isn't a big deal (yet), but the other three seem to indicate that the lexer is responsible for about 50% of my runtime and memory allocation. For reference, this particular input is about 18M of text, though the ratios are just as bad for smaller inputs. My uneducated suspicion is that Alex is constructing separate ByteStrings that it passes to each of my token constructors, and that this is responsible for a large part of this allocation. Most of my token constructors just ignore this ByteString - assuming there really is an allocation for each token, is there any way to avoid it? I was looking at alexScanTokens, alex_scan_tkn, and alexGetChar but didn't see any obvious ways to improve them. Alternatively, does any one have lexing performance tips that might help? Thanks

4 4

Data.Enumerator.List.concatMap is to Data.Iteratee.?
by David Place 26 Jun '11

26 Jun '11

Hi: I've been studying iteratee IO. Is there a function in the iteratee package that is analogous to Data.Enumerator.List.concatMap? ____________________ David Place Owner, Panpipes Ho! LLC http://panpipesho.com d(a)vidplace.com

1 0

Can't use libraries in haskell
by Александр 26 Jun '11

26 Jun '11

Hello, I'm new in haskell. I want to use xml library (http://hackage.haskell.org/package/xml) in my project. I downloaded it.then try to build and install: runhaskell Setup.hs configure runhaskell Setup.hs build runhaskell Setup.hs install All ok. There are no errors. When i try import modules from this lib to my project, for example: import Text.XML.Light.Cursor I get error: /home/shk/dev/src/XMPP.hs:8:8: Could not find module `Text.XML.Light.Cursor': Use -v to see a list of the files searched for. Failed, modules loaded: none. if i try: > cabal install xml Resolving dependencies... No packages to be installed. All the requested packages are already installed. If you want to reinstall anyway then use the --reinstall flag. What's wrong? How can i install and use library in haskell? Thank you.

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Re: [Haskell-cafe] commutative monoid?
by wren ng thornton 26 Jun '11

26 Jun '11

On 6/25/11 1:34 AM, Evan Laforge wrote: > So there's a range of possible Monoid instances for each type, More for some types than for others. For Maybe there are three: * always take the first/left value; * always take the last/right value; * or, use a semigroup operation defined on the values. The first two options are provided by the First and Last newtypes, and the third option is provided by the instance for Maybe itself (except that it spuriously requires a Monoid instance instead of just a semigroup). For integers there are at least four obvious ones (min, max, addition, multiplication), and numerous less obvious ones. For real numbers we can also add the logarithmic and exponential versions. Etc. > maybe they were chosen by historical happenstance rather than some > kind of "principle monoid" (is there such a thing?). Is there a name > for the thing that's like a monoid, but the operator is commutative > too? "Commutative monoid" :) > That would narrow down the implementation choices, e.g. Data.Map > would have to combine the values. So it seems like if your operation > is commutative, you can put it in "c-monoid" and not rely so much on > the happenstance of the instances, since they are more constrained. They're more constrained, but still not enough to make things unique. All four of the obvious ones I mentioned for integers are commutative. You could keep constraining things further. But how constrained do you want to be? In some sense, there will always be room for multiple interpretations. Occasionally things magic their way into being unique, but usually not. Much of the last century of physics has been spent debating between different interpretations of some extremely constrained systems. So far as I'm aware, there's still no consensus about which interpretation is "right". > And of course you are then free to reorder things, which is a nice bit > of flexibility. Indeed, commutativity is a wonderful property to take advantage of when you can. > So is there a typeclass for that? There might be one hidden in one of the attempts at redesigning the numeric hierarchy (e.g., Numeric Prelude), but there's not a canonical typeclass for them. Unfortunately it's not really a good match for the typeclass system since it doesn't introduce any new operations, it only introduces laws--- which aren't verified nor enforced. Though, if you happen to know the property does hold, then you're free to take advantage of it. (E.g., for the Maybe instance which uses a semigroup operation, if the semigroup op is commutative then so is the monoid op.) -- Live well, ~wren

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Re: [Haskell-cafe] Iteratee IO examples
by wren ng thornton 26 Jun '11

26 Jun '11

On 6/25/11 6:51 AM, John Lato wrote: > Honestly I'm quite dis-satisfied with the current state of code which > depends on iteratee/enumerator. It's nearly all written in a very low-level > style, i.e. directly writing 'liftI step', or 'case x of Yield -> ...'. > This is exactly what I would hope users could avoid, by using the functions > in e.g. Data.Iteratee.ListLike. > > I've recently added more functions to iteratee which greatly reduce the need > for this type of code. I don't know about enumerator, but I expect it isn't > rich enough since most user code I've seen is pretty low-level. I have a rather large suite of list-like functions for the old version of iteratee (used by a project I've been working on for a while). Once I get the time to convert the project to the newer iteratee, I'll send a patch with any you're still missing. (Though, admittedly, I'm not terribly keen on ListLike. The classes still seem too monolithic and ad-hoc. Though I'm not sure there's a way around that without something closer to ML's functor modules.) -- Live well, ~wren

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Re: [Haskell-cafe] Map Monoid instance (was commutative monoid?)
by wren ng thornton 25 Jun '11

25 Jun '11

On 6/25/11 2:15 PM, Erik Hesselink wrote: > On Sat, Jun 25, 2011 at 19:07, Evan Laforge<qdunkan(a)gmail.com> wrote: >> On Sat, Jun 25, 2011 at 9:00 AM, Jens Blanck<jens.blanck(a)gmail.com> wrote: >>>>> So there's a range of possible Monoid instances for each type, >>>> >>>> More for some types than for others. For Maybe there are three: >>>> >>>> * always take the first/left value; >>>> * always take the last/right value; >>>> * or, use a semigroup operation defined on the values. > > Brent Yorgey recently blogged about a fourth instance [1] which also > uses the semigroup operation on the values, but treats Nothing as > failure, returning Nothing. > > Erik > > [1] https://byorgey.wordpress.com/2011/04/18/monoids-for-maybe/ Ah, yes. I forgot about that one. Though technically that one actually requires a monoid, not just a semigroup, since mempty = Just mempty. Which furthers the point that most types have an abundance of monoid instances. -- Live well, ~wren

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Re: [Haskell-cafe] Map Monoid instance (was commutative monoid?)
by wren ng thornton 25 Jun '11

25 Jun '11

On 6/25/11 1:07 PM, Evan Laforge wrote: > In the > case of the overriding version, you have to decide on which side to > merge the new monoid, and on the lifted one the two choices become > four, since you then have to decide whether the unionWith argument > should be flipped or not. > [...] > So I think the Data.Map choice is reasonable for that reason. If it > were lifted, it would have to make a hard-coded decision about the > "side". If you have to write your own, that's under your control. You should pass them through in the same order, to maximize intuitability. Order swapping can be handled by the Dual newtype. -- Live well, ~wren

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Re: [Haskell-cafe] Map Monoid instance (was commutative monoid?)
by wren ng thornton 25 Jun '11

25 Jun '11

On 6/25/11 12:00 PM, Jens Blanck wrote: >>> So there's a range of possible Monoid instances for each type, >> >> More for some types than for others. For Maybe there are three: >> >> * always take the first/left value; >> * always take the last/right value; >> * or, use a semigroup operation defined on the values. >> >> The first two options are provided by the First and Last newtypes, and the >> third option is provided by the instance for Maybe itself (except that it >> spuriously requires a Monoid instance instead of just a semigroup). > > But why does the Map instance of Monoid _not_ mimic the one chosen for > Maybe. I claim this causes unnecessary surprises for users (and lifting the > Monoid instance seems more useful, but that is harder to substantiate). For what it's worth, if people come to a decent enough consensus on which instance should be the default for Data.Map, then we should propose it on the libraries@ list. Of course we'll want to have Data.IntMap behave the same way, and consequently I'd update Data.Trie to follow suit. The various hashable and unordered containers will also probably want to follow suit. For Data.Trie, I only chose the version I did for conformity with IntMap. There's nothing especially interesting about the choice nor difficult about changing it (other than backwards compatibility issues). One issue that will come up, however, is that we'll want to generalize the First and Last newtypes so that they can be used consistently for all of different types which have these three Monoid instances. I'm not sure off hand where those newtypes are living these days, nor how much code it'd break to change them thus (and move them to base, if necessary). -- Live well, ~wren

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commutative monoid?
by Evan Laforge 25 Jun '11

25 Jun '11

Not being especially mathematically inclined, every once and a while I get a little panicked when I notice that, e.g. Data.Map mappend is a plain left-biased union, and doesn't actually mappend the values of the map. Then I remember that monoids are associative, not commutative, and it's totally allowed to do that. Even if you mappend the values, the result is only as commutative as the monoid instance for the values. But clearly you can make that choice. For instance, Maybe's Monoid wants to mappend the elements. This is always a little disconcerting to me because the MonadPlus instance for Maybe doesn't do that. So there's a range of possible Monoid instances for each type, and maybe they were chosen by historical happenstance rather than some kind of "principle monoid" (is there such a thing?). Is there a name for the thing that's like a monoid, but the operator is commutative too? That would narrow down the implementation choices, e.g. Data.Map would have to combine the values. So it seems like if your operation is commutative, you can put it in "c-monoid" and not rely so much on the happenstance of the instances, since they are more constrained. And of course you are then free to reorder things, which is a nice bit of flexibility. So is there a typeclass for that? This article is about the mathematical concept. For the alien creatures in the Doctor Who adventure, see The Ark (Doctor Who). I guess I never saw that episode... but it sounds like they're closer to warm fuzzies than the slimy sleeping bag monster I imagined.

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ANNOUNCE: Win32-junction-point-0.0.1
by Michael Steele 25 Jun '11

25 Jun '11

I'm pleased to announce the initial release of Win32-junction-point * hackage: http://hackage.haskell.org/package/Win32-junction-point * git repository: https://github.com/mikesteele81/Win32-junction-point This package provides the ability to manipulate NTFS junction points as supported by Windows 2000 and above. Junction points, along with NTFS hard links and NTFS symbolic links, are a type of symbolic link that can be made between folders existing on the same filesystem. Please read Microsoft KB205524 [1] for more information on junction points. Junction points have always been left undocumented in the Win32 SDK. The Windows 2000 Resource Kit came with a command-line utility named linkd.exe to work with them. Later, Mark Russinovich of SysInternals distributed a replacement utility named Junction [2] which accomplished the same thing. This source code is based on an article [3] and C++ library [4] written by Mike Nordell at codeproject.com. It is against Microsoft's recommendation to make use of undocumented API features. Use this library at your own risk. [1] http://support.microsoft.com/?kbid=205524 [2] http://technet.microsoft.com/en-us/sysinternals/bb896768 [3] http://www.codeproject.com/script/Articles/ViewDownloads.aspx?aid=194 [4] http://www.codeproject.com/KB/winsdk/junctionpoints.aspx -- Michael Steele

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