- Haskell-Cafe - Haskell.org

same function's type accepted in top level, but rejected in where clause
by Ömer Sinan Ağacan 08 Jul '13

08 Jul '13

Hi all, I came across an interesting type checker error, a function is accepted in top level, but rejected by type checker when moved to `where ...` clause. I moved required code to a new module for demonstration purposes: module Bug where fix :: (a -> a) -> a fix f = let x = f x in x data Fix f = Fix (f (Fix f)) unFix :: Fix f -> f (Fix f) unFix (Fix a) = a data ListF a l = NilF | ConsF a l instance Functor (ListF a) where fmap _ NilF = NilF fmap f (ConsF a l) = ConsF a (f l) fold :: Functor f => (f a -> a) -> Fix f -> a fold f a = f (fmap (fold f) (unFix a {- f (Fix f) -})) unfold :: Functor f => (a -> f a) -> a -> Fix f unfold f a = Fix (fmap (unfold f) (f a)) Now, after this code, type checker accept this function: iterateListF :: (a -> a) -> a -> Fix (ListF a) iterateListF fn e = unfold (foldFn fn) e foldFn :: (a -> a) -> a -> ListF a a foldFn fn a = ConsF a (fn a) But rejects this: iterateListF :: (a -> a) -> a -> Fix (ListF a) iterateListF fn e = unfold f e where f :: a -> ListF a a f a = ConsF a (fn a) With error: bug.hs:27:20: Couldn't match expected type `a1' with actual type `a' `a1' is a rigid type variable bound by the type signature for f :: a1 -> ListF a1 a1 at bug.hs:26:10 `a' is a rigid type variable bound by the type signature for iterateListF :: (a -> a) -> a -> Fix (ListF a) at bug.hs:23:17 In the return type of a call of `fn' In the second argument of `ConsF', namely `(fn a)' In the expression: ConsF a (fn a) Changing type variables in type of `f` to `x` fails with this error: bug.hs:28:20: Couldn't match expected type `x' with actual type `a' `x' is a rigid type variable bound by the type signature for f :: x -> ListF x x at bug.hs:27:10 `a' is a rigid type variable bound by the type signature for iterateListF :: (a -> a) -> a -> Fix (ListF a) at bug.hs:24:17 In the return type of a call of `fn' In the second argument of `ConsF', namely `(fn a)' In the expression: ConsF a (fn a) Failed, modules loaded: none. .. and this is strange because error message describes function as it is before changing `a` to `x`. Any ideas why this definition rejected? Is this a bug in GHC? --- Ömer Sinan Ağacan http://osa1.net

4 4

Catch multiple exceptions using 'Control.Exception'
by Nikita Karetnikov 07 Jul '13

07 Jul '13

I'm trying to update a package that uses 'Control.OldException' (works with GHC 6.10.4). Here is a relevant (and simplified) part: import Control.OldException -- | A predicate matching synchronous exceptions. syncExceptions :: Exception -> Maybe Exception syncExceptions (AsyncException _) = Nothing syncExceptions e = Just e throwAsync :: IO a throwAsync = throwIO $ AsyncException StackOverflow throwArith :: IO a throwArith = throwIO $ ArithException DivideByZero 'syncExceptions' is usually used like this: *Main> tryJust syncExceptions throwArith Left divide by zero *Main> tryJust syncExceptions throwAsync -- pass through *** Exception: stack overflow The above doesn't work with GHC 7.6.3 because 'Control.OldException' [1] was removed. And 'Control.Exception' doesn't have the 'Exception' type. Is there a way to adapt 'syncExceptions' to work with 'Control.Exception'? [1] http://hackage.haskell.org/packages/archive/base/4.1.0.0/doc/html/Control-O…

4 6

Parsec : parse either an interger or a double?
by Fredrik Karlsson 07 Jul '13

07 Jul '13

Dear list, Sorry for asking you another parsec question. I have a text file that is the output of a C/C++ program, involving time references in double format. Now, the program seems to truncate the output if possible, so a file that is one second long will produce this file: File type = "ooTextFile" Object class = "TextGrid" xmin = 0 xmax = 1 .... and a file that is 1.1 seconds long will be this: File type = "ooTextFile" Object class = "TextGrid" xmin = 0 xmax = 1.1 ... Now, how do I handle this case in parsec? I have tried this: start <- try (float haskellDef) <|> fmap ( / 1.0 ) $ integer haskelldef and similar solutions, but I cant get it to accept the 0. /Fredrik -- "Life is like a trumpet - if you don't put anything into it, you don't get anything out of it."

1 1

ANN: HaTeX 3.6. Now with Babel, TikZ and more!
by Daniel Díaz Casanueva 06 Jul '13

06 Jul '13

Hello everyone! It is time for a new release of HaTeX, the Haskell LaTeX library. http://hackage.haskell.org/package/HaTeX-3.6 == What is HaTeX? == HaTeX contains numerous types and functions that will assist you in the process of creating a LaTeX document within Haskell. You can also include your manually created LaTeX code in HaTeX, so you can choose the more convenient tool you want to use for each case. == What's new in this release? == A lot of changes have been made since the last release! The most important changes are explained in the blog post I just wrote: http://deltadiaz.blogspot.com.es/2013/06/hatex-36-texy-class-babel-fontenc-… However, the new feature I am more excited with is the TikZ script generator. Using a very intuitive interface, you will be able to generate high quality graphics for LaTeX documents. It is still far from be complete, but next releases will fill the gaps that you may find right now. Just to give you a taste, consider the following code: myFigure :: Figure myFigure = Scale 2 $ Figures [ RectangleFilled (0,0) 1 1 , Colored Green $ RectangleFilled (-1,1) 1 1 , Colored Red $ RectangleFilled ( 0,2) 1 1 , Colored Blue $ RectangleFilled ( 1,1) 1 1 ] With this simple construction you already have the following picture ready to be inserted in your LaTeX file output. http://daniel-diaz.github.com/projects/hatex/tikzsimple.png In the examples directory contained in the source distribution there is also an example (tikz.hs) that includes the plotting of a Haskell function (sine). However, the interface it uses may be less comfortable than the one shown in the example above. If you are a HaTeX user, don't miss this update, because it also contains some very important bug fixes. Any feedback gladly accepted. Good luck, Daniel Díaz.

8 14

Postdocs and Research Programmer for Compositional Learning via Generalized Automatic Differentiation
by Barak A. Pearlmutter 06 Jul '13

06 Jul '13

We are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination in a production-quality fast system suitable for numeric computing in general, and compositional machine learning methods in particular. To the programming language community, we seek to contribute a way to make numeric software faster, more robust, and easier to write. To the machine learning community, in addition to the above practical benefits, we seek to contribute a system that embodies *compositionality*, in that gradient optimisation can be automatically and efficiently performed on systems themselves consisting of many components, even when such components may internally perform optimisation. (Examples of this include, say, optimisation of the rules of a multi-player game to cause the players actions to satisfy some desiderata, where the players themselves optimise their own strategy with using a simple model of the opponent which they optimise according to their opponent's behaviour; or multi-agent learning where one agent learns an internal model of another agent, where that internal model itself performs learning.) To this end, we are seeking two postdoctoral researchers and one research programmer with interest and experience in a cohert subset of: programming language theory, numerics, automatic differentiation, and machine learning. Inquiries to: Barak A. Pearlmutter <barak(a)cs.nuim.ie> Informal announcment with more details: http://www.bcl.hamilton.ie/~barak/ad-fp-positions.html, which will have a reference to the formal announcement when it becomes available. -- Barak A. Pearlmutter Hamilton Institute & Dept Computer Science NUI Maynooth

1 0

Basic Parsec float & integer parsing question
by Fredrik Karlsson 05 Jul '13

05 Jul '13

Dear list, Sorry for asking a simple parsec question, but both Parsec and Haskell is new to me, so please be gentle :-) I have this code: ------------------------------------------------------------------------------------------------ import Text.ParserCombinators.Parsec import Text.Parsec.Token import Text.ParserCombinators.Parsec.Char data VariableLine = VariableLine String String deriving Show data TierType = IntervalTier | PointTier deriving Show data Tier = Tier TierType String Float Float Integer data Label = Interval Float Float String data LabelFile = LabelFile Float Float symbol :: Parser Char symbol = oneOf "!#$%&|*+-/:<=>?@^_~" testString = "intervals [1]:\n xmin = 0 \n xmax = 0.028199999999999992 \n text = \"\"" headTS1 = "File type = \"ooTextFile\"\nObject class = \"TextGrid\"\n\nxmin =" header :: Parser LabelFile header = do headTS1 start <- float string "\nxmax = " end <- float string "\ntiers? <exists>\nsize = " integer char '\n' return $ LabelFile start end ------------------------------------------------------------------------------------------------ Loading it into ghci I get : Prelude> :l parsectest.hs [1 of 1] Compiling Main ( parsectest.hs, interpreted ) parsectest.hs:21:9: Couldn't match type `[]' with `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity' Expected type: Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity Char Actual type: [Char] In a stmt of a 'do' block: headTS1 In the expression: do { headTS1; start <- float; string "\ \xmax = "; end <- float; .... } In an equation for `header': header = do { headTS1; start <- float; string "\ \xmax = "; .... } parsectest.hs:22:18: Couldn't match expected type `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity Float' with actual type `GenTokenParser s0 u0 m0 -> Text.Parsec.Prim.ParsecT s0 u0 m0 Double' In a stmt of a 'do' block: start <- float In the expression: do { headTS1; start <- float; string "\ \xmax = "; end <- float; .... } In an equation for `header': header = do { headTS1; start <- float; string "\ \xmax = "; .... } parsectest.hs:24:16: Couldn't match expected type `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity Float' with actual type `GenTokenParser s1 u1 m1 -> Text.Parsec.Prim.ParsecT s1 u1 m1 Double' In a stmt of a 'do' block: end <- float In the expression: do { headTS1; start <- float; string "\ \xmax = "; end <- float; .... } In an equation for `header': header = do { headTS1; start <- float; string "\ \xmax = "; .... } parsectest.hs:26:9: Couldn't match expected type `Text.Parsec.Prim.ParsecT String () Data.Functor.Identity.Identity a0' with actual type `GenTokenParser s2 u2 m2 -> Text.Parsec.Prim.ParsecT s2 u2 m2 Integer' In a stmt of a 'do' block: integer In the expression: do { headTS1; start <- float; string "\ \xmax = "; end <- float; .... } In an equation for `header': header = do { headTS1; start <- float; string "\ \xmax = "; .... } Failed, modules loaded: none. I'm sure I'm doing something really stupid here, but I need help to get through this problem. I've used the predefined "letter" parser at other places in the code, so I can't understand why "float" and "integer" does not work. /Fredrik -- "Life is like a trumpet - if you don't put anything into it, you don't get anything out of it."

3 2

Extending Type Classes
by Frantisek Farka 05 Jul '13

05 Jul '13

Hello all, I was looking for my master thesis topic and my supervisor suggested an idea of extending class system so it enables refactoring Type Class hierarchy without affecting client source code which is using refactored classes. One example is Functor - Applicative - Monad problem and corresponding proposal [1]. But this proposal instead of allowing the change through extending Type Classes forces client code to prepare for the new class layout and then switch the classes to the new layout. My goal is rather to allow direct changes in class hierarchy without affecting client source code. I have found different proposals approaching this problem on HaskellWiki, some of them are overlapping, some of them refer each other. The most promising to me seems Default superclass instances proposal [2]. This one is somehow implemented in the Strathclyde Haskell Enhancement (SHE) [3] but I haven't found much reference or user experience really. So the reason why I write this email is to ask you for some tips where above mentioned problem occurs in real source code. I would like to investigate some real examples before designing some ad hoc changes to the Type Classes system. Besides that I'd appreciate anyone who has used default superclass instances in SHE to share his experience. And last but not least I am always grateful for any comments and suggestions. Best wishes Frantisek [1] http://www.haskell.org/haskellwiki/Functor_hierarchy_proposal [2] http://hackage.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances [3] https://personal.cis.strath.ac.uk/conor.mcbride/pub/she/

1 0

newbie question about "Functional dependencies conflict between instance declarations:".....
by Nicholls, Mark 05 Jul '13

05 Jul '13

Hello, I largely don't know what I'm doing or even trying to do, it is a voyage into the unknown....but....if I go... > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FunctionalDependencies #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE UndecidableInstances #-} > class Foo x y | x -> y, y -> x > instance Foo Integer Integer That seems to work....and my head seems to say...your created some sort of binary relation between 2 types...and made <Integer,Integer> a member of it... Something like that anyway.... Then I go.... > data Bar > instance Foo Bar x Error!....but I'm think I understand this....I can't claim that <Bar,x> is a member of Foo and <Integer,Integer> is member of Foo and preserve my functional dependencies, because <Bar,Integer> is now a member of Foo.. Bad programmer....... So how I naively go.... > class NotAnInteger a > instance (NotAnInteger x) => Foo Bar x I haven't declared integer to be "NotAnInteger"....so (in a closed world)....this would seem to exclude the contradiction....but... Functional dependencies conflict between instance declarations: instance Foo Integer Integer -- Defined at liam1.lhs:7:12 instance NotAnInteger x => Foo Bar x -- Defined at liam1.lhs:13:12 So i) I clearly don't understand something about the type system. ii) I don't know how to restrict type variables in instance declarations....i.e. how do I use the notion of "Foo" across different combinations of types, without them colliding. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

3 4

Product-Categories
by Lyndon Maydwell 05 Jul '13

05 Jul '13

Hi Café. I'm wracking my brain trying to figure out a simple, reasonably general, implementation for a category instance for pairs of categories. So far I've looked at [1], which seems great, but doesn't use the built-in category instance, and [2], which I'm just not sure about. Ideally I'd like to be able to express something like - instance (Category a, Category b) => Category (Product a b) where id = Product id id Product o1 o2 . Product i1 i2 = Product (o1 . i1) (o2 . i2) However, it all falls apart when I actually try to define anything. Is this possible? If not, why not? As far as I can tell the issue boils down to not being able to translate "Category i o" to "Product (Fst i) (Fst o) (Snd i) (Snd o)" without breaking the kind expectation of the category instance. Please help me, I'm having a bad brain day :-) [1] - http://twanvl.nl/blog/haskell/categories-over-pairs-of-types [2] - http://hackage.haskell.org/packages/archive/categories/1.0.6/doc/html/Contr…

2 2

Automatically test different versions of single dependency
by Daniel Díaz Casanueva 04 Jul '13

04 Jul '13

Hello everyone. I would like to check what dependencies is one of my packages compatible with. For example, say I have a package called "foo" that depends on package "bar". Most likely, "foo" does not build with each version of "bar". What I want to do is try to build "foo" with each single version of "bar" (not manually). What is the best approach to this?

2 3

getting exported original names using the GHC API
by Pasqualino "Titto" Assini 04 Jul '13

04 Jul '13

Say that I have a module like: module Test(L.find) where import Data.List as L Using the GHC API, I extract the module exports (using GHC.modInfoExports) and I get a list of names. Now, I want to get the name of the export with the namespace stated in the source ("L.find") but I seem to be able to get it only as "Data.List.find". I have tried different variations of functions on Name or OccName but nothing seems to work. Obviously, I could just parse the module and get the info that way, but I wondered if it could be done via the API. Thanks, titto -- Dr. Pasqualino "Titto" Assini http://quid2.org/

2 1

Call for Demo Proposals: Workshop on Functional Art, Music, Modeling and Design
by Brent Yorgey 04 Jul '13

04 Jul '13

Hi all, Below is the call for demonstration proposals for FARM 2013. Please forward to anyone who might be interested -- apologies if you receive multiple copies! Brent Yorgey publicity chair for FARM 2013 ====================================================================== FARM 2013: Call for Demo Proposals ACM SIGPLAN Workshop on Functional Art, Music, Modeling and Design Boston, Massachusetts, USA 28th September, 2013 (directly after ICFP) http://www.cis.upenn.edu/~byorgey/farm13/ ====================================================================== Do you enjoy writing beautiful code to produce beautiful artifacts? Have something cool to show off at the intersection of functional programming and visual art, music, sound, modeling, visualization, or design? FARM 2013 is seeking proposals for 10-20 minute demonstrations to be given during the workshop. For example, a demonstration could consist of a short tutorial, an exhibition of some work, or even a livecoding performance. Slots for demonstrations will be shorter than slots for accepted papers, and will not be published as part of the formal proceedings, but can be a great way to show off interesting work and get feedback from other workshop participants. A demonstration slot could be a particularly good way to get feedback on work-in-progress. Scope ----- The functional programming community is largely interested in writing beautiful programs. This workshop is intended to gather researchers and practitioners interested in writing beautiful programs that generate beautiful artifacts. Such artifacts may include visual art, music, 3D sculptures, animations, GUIs, video games, physical models, architectural models, choreographies for dance, poetry, and even physical objects such as VLSI layouts, GPU configurations, or mechanical engineering designs. The framework used need not be purely functional (“mostly functional” is fine); may be based on abstractions such as higher-order functions, monads, arrows, or streams; and may be manifested as a domain specific language or tool. Theoretical foundations, language design, implementation issues, and applications are all within the scope of the workshop. Important dates --------------- - Demonstration proposals due: Wednesday, 31 July 2013, 18:00 EDT (UTC-4) - Author notification: Monday, 5 August 2013 - Workshop: Saturday, 28 September 2013 (the day after ICFP) A demo proposal should consist of a 1 page abstract, in PDF format, explaining the proposed content of the demonstration and why it would be of interest to the attendees of FARM. Proposals will be judged on interest and relevance to the stated goals and themes of the workshop. Submissions can be made via EasyChair at https://www.easychair.org/conferences/?conf=farm2013. Organizers ---------- Workshop Chair: Paul Hudak Program Chair: Conal Elliott Publicity Chair: Brent Yorgey Program Committee ----------------- Daniel Cukier (University of São Paulo) Conal Elliott (Tabula) (chair) Kathleen Fisher (Tufts University) Richard Gabriel (IBM Research) George Giorgidze (University of Tübingen) Paul Hudak (Yale University) José Pedro Magalhães (University of Oxford) Alex McLean (University of Leeds) John Peterson (Western State Colorado University) Michael Sperber (Active Group) Henning Thielemann Brent Yorgey (University of Pennsylvania)

1 0

How to use Template Haskell based on code that generated by another Template?
by Magicloud Magiclouds 04 Jul '13

04 Jul '13

I have a yesod project, which generated, say, UserPassword in module Model. Then I wrote my template code which generate a piece of code to use UserPassword. I imported Model in my code. Then I got Illegal variable name: `UserPassword' When splicing a TH declaration: -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

2 2

bug in Data.ByteString.Lazy or in me?
by John MacFarlane 04 Jul '13

04 Jul '13

On a 64-bit Windows 8 server EC2 instance, with Haskell Platform freshly installed from the package installer, GHCI> :m + Data.ByteString.Lazy GHCI> Data.ByteString.Lazy.hGetContents stdin gives me an immediate error "hGetBufSome: resource exhausted (Not enough space)", while GHCI> :m + Data.ByteString GHCI> Data.ByteString.hGetContents stdin waits for user input, as expected. On 32-bit Windows, both work as expected. Can anyone explain this? Is this a bug in bytestring? John

1 0

Haskell Weekly News: Issue 272
by Daniel Santa Cruz 04 Jul '13

04 Jul '13

Welcome to issue 272 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 23 to 29, 2013. Quotes of the Week * kmc: johnw: I'm rejoining this channel after months away just to tell you how incredibly wrong you are that monads are the "core concept" of Haskell * dmwit: semigroups are monoids with an identity crisis * olsner: I think doing eta something means to make it more or less pointless. But I don't recall which eta does what * mm_freak: Oleg knows what's at the end of foldr f z [1..]. * copumpkin: it's like grasping desperately for the rope of induction as you fall down the bottomless pit of coinduction Top Reddit Stories * From zero to cooperative threads in 33 lines of Haskell code Domain: haskellforall.com, Score: 78, Comments: 39 On Reddit: [1] http://goo.gl/rtkrO Original: [2] http://goo.gl/wgyAC * RSS reader written in Haskell and Ur/Web Domain: bazqux.com, Score: 66, Comments: 38 On Reddit: [3] http://goo.gl/sUw1U Original: [4] http://goo.gl/gdh5G * FP Haskell Center -- full video demo -- app development, automatic deployment, Git and GitHub integration Domain: youtube.com, Score: 64, Comments: 68 On Reddit: [5] http://goo.gl/gBi5l Original: [6] http://goo.gl/TlGL9 * Building LLVM using Shake Domain: neilmitchell.blogspot.ca, Score: 54, Comments: 6 On Reddit: [7] http://goo.gl/38rWZ Original: [8] http://goo.gl/tA5tM * Hasochism: The Pleasure and Pain of Dependently Typed Haskell Programming [pdf] Domain: personal.cis.strath.ac.uk, Score: 52, Comments: 8 On Reddit: [9] http://goo.gl/6dDsL Original: [10] http://goo.gl/ZZPZL * GHC Core by example, episode 1 - Hello Core! Domain: alpmestan.com, Score: 46, Comments: 3 On Reddit: [11] http://goo.gl/iL5ho Original: [12] http://goo.gl/mUiO5 * HPCwire: Lustre Founder Spots Haskell on HPC Horizon Domain: hpcwire.com, Score: 36, Comments: 2 On Reddit: [13] http://goo.gl/h2dmv Original: [14] http://goo.gl/1WLf8 * Adventures in Three Monads - Logic, Prompt, Failure Domain: web.mit.edu, Score: 33, Comments: 8 On Reddit: [15] http://goo.gl/JEOfw Original: [16] http://goo.gl/AI5wV * Darcs' source code is some of the cleanest Haskell I've read in a while Domain: github.com, Score: 32, Comments: 13 On Reddit: [17] http://goo.gl/TWWMh Original: [18] http://goo.gl/fZGE8 * I accidentally the entire heap: debugging Haskell programs with space leaks Domain: bitba.se, Score: 32, Comments: 6 On Reddit: [19] http://goo.gl/9Vtzy Original: [20] http://goo.gl/HCS5N * GHCJS introduction – Concurrent Haskell in the browser Domain: weblog.luite.com, Score: 30, Comments: 4 On Reddit: [21] http://goo.gl/Fk8Xd Original: [22] http://goo.gl/8CUVT * Functional Reactive Web Interfaces with GHCJS and sodium Domain: weblog.luite.com, Score: 29, Comments: 12 On Reddit: [23] http://goo.gl/5QKn0 Original: [24] http://goo.gl/VpAOv * Who Ya Gonna Call? Getting Dirty with Cmm, the GHC backend, and writing new PrimOps with Carter Schonwald (at NY-Haskell) Domain: vimeo.com, Score: 26, Comments: 14 On Reddit: [25] http://goo.gl/wUVcs Original: [26] http://goo.gl/cZaGd * "Business system" using functional programming? Domain: self.haskell, Score: 24, Comments: 44 On Reddit: [27] http://goo.gl/QcE5P Original: [28] http://goo.gl/QcE5P * The Resource Applicative Domain: haskellforall.com, Score: 23, Comments: 16 On Reddit: [29] http://goo.gl/G7TIs Original: [30] http://goo.gl/4e0Gm Top StackOverflow Questions * What's the conceptual difference between Machines and Conduits (or other similar libraries)? votes: 13, answers: 1 Read on SO: [31] http://goo.gl/hrNMl * “inject” progress logging/tracing in haskell computation? votes: 13, answers: 2 Read on SO: [32] http://goo.gl/RdEKU * Difference between free monads and fixpoints of functors? votes: 12, answers: 0 Read on SO: [33] http://goo.gl/O3oBi * Can Haskell optimize function calls the same way Clang / GCC does? votes: 12, answers: 3 Read on SO: [34] http://goo.gl/Sgf88 * Trying to figure out `random` function in Haskell votes: 10, answers: 3 Read on SO: [35] http://goo.gl/kcHsi * What does !( ) means in data constructor? votes: 9, answers: 2 Read on SO: [36] http://goo.gl/ysnNL * Anything prevents optimizing tail-recursion? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/LaTY2 * Where can I find a good introduction to Haskell optimization passes? votes: 9, answers: 1 Read on SO: [38] http://goo.gl/wMYQH * Passing void * through haskell votes: 9, answers: 1 Read on SO: [39] http://goo.gl/Fiu5Q * Call main function with arguments votes: 9, answers: 2 Read on SO: [40] http://goo.gl/omwDh Until next time, [41]+Daniel Santa Cruz References 1. http://www.haskellforall.com/2013/06/from-zero-to-cooperative-threads-in-33… 2. http://www.reddit.com/r/haskell/comments/1h0wf7/from_zero_to_cooperative_th… 3. https://bazqux.com/ 4. http://www.reddit.com/r/haskell/comments/1ha5dd/rss_reader_written_in_haske… 5. http://www.youtube.com/watch?v=cyyDmQKcHMs 6. http://www.reddit.com/r/haskell/comments/1h7693/fp_haskell_center_full_vide… 7. http://neilmitchell.blogspot.ca/2013/06/building-llvm-using-shake.html 8. http://www.reddit.com/r/haskell/comments/1gxaee/building_llvm_using_shake/ 9. https://personal.cis.strath.ac.uk/conor.mcbride/pub/hasochism.pdf 10. http://www.reddit.com/r/haskell/comments/1gz03w/hasochism_the_pleasure_and_… 11. http://alpmestan.com/2013/06/27/ghc-core-by-example-episode-1/ 12. http://www.reddit.com/r/haskell/comments/1h5ml0/ghc_core_by_example_episode… 13. http://www.hpcwire.com/hpcwire/2013-06-24/lustre_founder_spots_haskell_on_h… 14. http://www.reddit.com/r/haskell/comments/1h24rv/hpcwire_lustre_founder_spot… 15. http://web.mit.edu/~ezyang/Public/threemonads.pdf 16. http://www.reddit.com/r/haskell/comments/1h1etb/adventures_in_three_monads_… 17. https://github.com/asperous/darcs_source_code 18. http://www.reddit.com/r/haskell/comments/1h4rak/darcs_source_code_is_some_o… 19. https://www.bitba.se/i-accidentally-the-entire-heap/ 20. http://www.reddit.com/r/haskell/comments/1h75fa/i_accidentally_the_entire_h… 21. http://weblog.luite.com/wordpress/?p=14&date=2013-06-03 22. http://www.reddit.com/r/haskell/comments/1h4s12/ghcjs_introduction_concurre… 23. http://weblog.luite.com/wordpress/?p=127 24. http://www.reddit.com/r/haskell/comments/1h795v/functional_reactive_web_int… 25. https://vimeo.com/69025829 26. http://www.reddit.com/r/haskell/comments/1gzmw1/who_ya_gonna_call_getting_d… 27. http://www.reddit.com/r/haskell/comments/1hb4ho/business_system_using_funct… 28. http://www.reddit.com/r/haskell/comments/1hb4ho/business_system_using_funct… 29. http://www.haskellforall.com/2013/06/the-resource-applicative.html 30. http://www.reddit.com/r/haskell/comments/1haums/the_resource_applicative/ 31. http://stackoverflow.com/questions/17281556/whats-the-conceptual-difference… 32. http://stackoverflow.com/questions/17290276/inject-progress-logging-tracing… 33. http://stackoverflow.com/questions/17307416/difference-between-free-monads-… 34. http://stackoverflow.com/questions/17344869/can-haskell-optimize-function-c… 35. http://stackoverflow.com/questions/17288614/trying-to-figure-out-random-fun… 36. http://stackoverflow.com/questions/17336774/what-does-means-in-data-constru… 37. http://stackoverflow.com/questions/17346161/anything-prevents-optimizing-ta… 38. http://stackoverflow.com/questions/17354005/where-can-i-find-a-good-introdu… 39. http://stackoverflow.com/questions/17366776/passing-void-through-haskell 40. http://stackoverflow.com/questions/17374356/call-main-function-with-argumen… 41. https://plus.google.com/105107667630152149014/about

1 0

Custom Setup.hs and Paths module
by Daniel Díaz Casanueva 04 Jul '13

04 Jul '13

Hi everyone. I am writing a package where I am using the Paths module that cabal generates automatically. After adding the Paths module to the "other-modules" section in my cabal file everything worked just fine, until I wanted to write a custom Setup.hs. This Setup.hs just writes a couple of files in the system and then calls "defaultMain". The thing is that now "cabal install" does not find the Paths module, so the package is broken. Any thoughts? I need your help, people! Thank you, Daniel Díaz.

2 3

ghci & ghc -> JS (Emscripten)
by B B 03 Jul '13

03 Jul '13

Hi! Does anybody tried, or is there anywhere a project, of online ghc or ghci (compiled to JavaScript with Emscripten)?

6 14

GHC 7.8 (GHCJS)
by B B 03 Jul '13

03 Jul '13

Hi, on the GHCJS website () I found an information: If you are from the future, you have GHC 7.8 or higher installed, and Cabal 1.18, run the following: $ cabal install ghcjs $ ghcjs-boot --auto Is it possible currently to get somehow GHC 7.8 or this information means that AFTER release of 7.8 this functionality will be available?

2 1

Liber Amicorum for Doaitse Swierstra.
by Jurriaan Hage 03 Jul '13

03 Jul '13

Dear all, Prof. Doaitse Swierstra is retiring on August 1 of this year, and is leaving the very functional programming minded Software Technology group that he has led for many decades. We have compiled a Liber Amicorum in his honour that we handed to him after his farewell speech on May 30th. You can find a download of this book at: http://www.cs.uu.nl/people/jur/liberdoaitseswierstra.pdf Not all contributions are ``technical'', and not all of them are in English, but counting up the pages, I guess about 75 percent is. best, Jurriaan Hage

1 0

Re: [Haskell-cafe] some questions about Template Haskell
by oleg＠okmij.org 03 Jul '13

03 Jul '13

TP wrote: > pr :: Name -> ExpQ > pr n = [| putStrLn $ (nameBase n) ++ " = " ++ show $(varE n) |] The example is indeed problematic. Let's consider a simpler one: > foo :: Int -> ExpQ > foo n = [|n + 1|] The function f, when applied to an Int (some bit pattern in a machine register), produces _code_. It helps to think of the code as a text string with the source code. That text string cannot include the binary value that is n. That binary value has to be converted to the numeric text string, and inserted in the code. That conversion is called `lifting' (or quoting). The function foo is accepted because Int is a liftable type, the instance of Lift. And Name isn't. BTW, the value from the heap of the running program inserted into the generated code is called `cross-stage persistent'. The constraint Lift is implicitly generated by TH when it comes across a cross-stage persistent identifier. You can read more about it at http://okmij.org/ftp/ML/MetaOCaml.html#CSP Incidentally, MetaOCaml would've accepted your example, for now. There are good reasons to make the behavior match that of Haskell.

4 6

News about the GSoC Project : Communicating with mobile devices
by Marcos Pividori 03 Jul '13

03 Jul '13

Hi, I am working this summer on the project "Communicating with mobile devices" for the GSoC. I have started a blog to show the progress. To show a simple usage of the API for GCM, I developed a simple test example of a Yesod server, where devices can register to receive GCM messages and users can send messages through the web service. More information: http://gsoc2013cwithmobiledevices.blogspot.com.ar/ Every feedback is welcome!! Thanks, Marcos

1 0

Upgrading GHC 7.6.3 global packages to latest versions
by Rouan van Dalen 02 Jul '13

02 Jul '13

Hi, I am trying to install the latest version of some hackage packages I use often in GHC 7.6.3. However, I am having some problems with packages like [containers] and [time], which are shipped with GHC as global packages. Now when I try to install the latest version of the [time] package, I have 2 time packages, 1 in the global package db (the older version), and 1 in the user package db (the newer version). Now I would like subsequent installed packages to always use the latest version of the [time] package. But if i try to install the [plugins] package, cabal says: #In order, the following would be installed: #ghc-paths-0.1.0.9 (new package) #random-1.0.1.1 (reinstall) changes: time-1.4.1 -> 1.4.0.1 #plugins-1.5.3.0 (new package) So I guess I have 2 options here: 1. unregister all the global packages so that I can install the latest versions of all the global packages. 2. force cabal to only look at latest packages in user package db (not sure if this is possible). Can anyone please point me in the right direction here. I have read about [cabal-dev], but I am not sure if it can be used to solve my problem. Regards --Rouan

3 2

cabal-dev: installing/running with profiling enabled
by Ömer Sinan Ağacan 02 Jul '13

02 Jul '13

Hi all, I want to profile a Haskell program. I have tons of libraries installed without profiling enabled, and some of them are installed with my Linux distribution's package mangaer. So I tried installing the program in a fresh cabal-dev environment with profiling enabled, so that all dependencies would be also installed with profiling enabled. But for some reason even after installing with `cabal-dev install --enable-executable-profiling -prof` I still can't run this program with `caba-dev/bin/program +RTS -p`, it fails with 'program: the flag -p requires the program to be built with -prof'. Can anyone help me on installing/running in cabal-dev with profiling enabled? --- Ömer Sinan Ağacan http://osa1.net

3 4

Spam on list??
by Vlatko Basic 02 Jul '13

02 Jul '13

4 3

Installing Z3 on OS X 10.8.4 ( Off topic )
by mukesh tiwari 02 Jul '13

02 Jul '13

Hello all, Not directly related to Haskell but I am trying to install Z3 to use SBV package[1]. I followed the instructions[2] to install it. *Mukeshs-MacBook-Pro:SMT mukeshtiwari$ git clone https://git01.codeplex.com/z3 -b unstable Cloning into 'z3'... remote: Counting objects: 16070, done. remote: Compressing objects: 100% (3928/3928), done. remote: Total 16070 (delta 12057), reused 16070 (delta 12057) Receiving objects: 100% (16070/16070), 7.62 MiB | 287 KiB/s, done. Resolving deltas: 100% (12057/12057), done.* After installing Z3 *Running python example.py in z3/examples/python gives this result. * *Mukeshs-MacBook-Pro:python mukeshtiwari$ python example.py sat [y = 4, x = 2]* but when I am running sbv package, I am getting this error. *Mukeshs-MacBook-Pro:site-packages mukeshtiwari$ ghci -XScopedTypeVariables GHCi, version 7.6.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m Data.SBV.Bridge.Z3 Prelude Data.SBV.Bridge.Z3> prove $ \(x::SWord8) -> x `shiftL` 2 .== 4*x 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 filepath-1.3.0.1 ... linking ... done. Loading package old-locale-1.0.0.5 ... linking ... done. Loading package time-1.4.0.1 ... linking ... done. Loading package bytestring-0.10.0.0 ... linking ... done. Loading package unix-2.6.0.0 ... linking ... done. Loading package directory-1.2.0.0 ... linking ... done. Loading package process-1.1.0.2 ... linking ... done. Loading package old-time-1.1.0.1 ... linking ... done. Loading package containers-0.5.0.0 ... 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 transformers-0.3.0.0 ... linking ... done. Loading package mtl-2.1.2 ... linking ... done. Loading package syb-0.3.7 ... linking ... done. Loading package sbv-2.10 ... linking ... done. *** An error occurred. *** Unable to locate executable for z3 *** Executable specified: "z3" Prelude Data.SBV.Bridge.Z3> * I have posted every details on installation on pastebin[3]. The documentation file says that[4] it will install *2) Building Z3 using make/g++ and Python Execute: autconf ./configure python scripts/mk_make.py cd build make sudo make install It will install z3 executable at /usr/bin, libraries at /usr/lib, and include files at /usr/include.* Mukeshs-MacBook-Pro:z3 mukeshtiwari$ echo $PATH /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/mukeshtiwari/Programming/GHC/ghc-7.6.1/bin:/Users/mukeshtiwari/.cabal/bin:/Users/mukeshtiwari/Erlang/bin:/Users/mukeshtiwari/Programming/Scala/scala-2.10.0/bin:/usr/local/smlnj-110.75/bin:/Users/mukeshtiwari/.rvm/bin Could some one please tell me how to solve this problem. Regards, Mukesh Tiwari [1] http://hackage.haskell.org/package/sbv [2] http://z3.codeplex.com/wikipage?title=Building%20the%20unstable%20%28workin… [3] http://pastebin.com/K5rq3jCF [4] http://z3.codeplex.com/SourceControl/latest#README

4 6

question about indentation conventions
by Richard Cobbe 02 Jul '13

02 Jul '13

I hope I'm not starting a holy war with this, but I'm curious about an aspect of coding style that's been bugging me for a while, and I'm not finding much discussion of this question on the web or in the mailing list archives. Two questions: 1) Are there wide-spread conventions in the Haskell community for how to indent an application expression that's split across multiple lines? For how to indent an expression that uses infix operators? Or does everyone pretty much do their own thing? 2) If there is such a convention, how do I make Emacs's haskell-mode do it? By default, in most cases Emacs's haskell-mode with turn-on-haskell-indentation does function firstArgument (second argument) thirdArgument Personally, I'd prefer some indentation on the 2nd and 3rd lines to indicate that they're continuing an expression begun on a previous line. I can use parens around the entire application to force haskell-mode to indent subsequent lines (and of course this is necessary in some contexts, like a 'do' expression), but haskell-mode only indents that by a single space: do (function firstArgument (second argument) thirdArgument) nextAction I'd find a larger indent---even just 2 spaces---to be more readable. My inclination to indent the second and following lines of a multi-line application expression is informed by my long experience in Scheme, Racket, and Lisp, whose S-expressions lend themselves to fairly straightforward (and automatable!) indentation conventions. If the Haskell community does things differently, though, I'm happy to adapt. This is the sort of thing that one picks up from the community, as in other languages. I don't, however, have a whole lot of contact with that community outside this list -- thus the post, despite the dangers inherent in discussing subjective stuff like this that people often feel strongly about. Thanks, Richard

10 18

Re: [Haskell-cafe] Subclass or no subclass?
by Patrick Browne 01 Jul '13

01 Jul '13

4 5

some questions about Template Haskell
by TP 30 Jun '13

30 Jun '13

Hi everybody, I am trying to learn Template Haskell, and I have two independent questions. 1/ First, the following code (which is not in its final version, but it is a test) does not compile: ------------------- {-# LANGUAGE TemplateHaskell #-} module Pr where import Language.Haskell.TH pr :: Name -> ExpQ pr n = [| putStrLn $ (nameBase n) ++ " = " ++ show $(varE n) |] ------------------- I obtain: ------------------- No instance for (Lift Name) arising from a use of `n' Possible fix: add an instance declaration for (Lift Name) In the first argument of `nameBase', namely `n' ------------------- Why? Indeed, there is no typeclass constraint on n in the definition of nameBase: ghci> :t nameBase nameBase :: Name -> String Contrary to lift for example: ghci> :t lift lift :: Lift t => t -> Q Exp 2/ If I define in a module: j = 3 and then define in another module: ------------------- h x = $([|j|]) main = do print $ h undefined ------------------- I obtain "3" as expected. However, I do not achieve to make this system work with an infix declaration: infix $([| j |]) + I obtain: parse error on input `$(' What is the problem? Thanks in advance, TP

2 2

Subclass or no subclass?
by Patrick Browne 29 Jun '13

29 Jun '13

1 0

Improving GHC's LLVM Backend
by Alex Ford 29 Jun '13

29 Jun '13

Hello, I'm interested in improving the LLVM backend of GHC by using the existing Haskell LLVM bindings to the C API, as suggested by option 1 in the LLVM FAQ: http://llvm.org/docs/FAQ.html#i-d-like-to-write-a-self-hosting-llvm-compile… At the moment, the backend uses the option 2: emitting LLVM assembly code which is then assembled to LLVM bitcode, optimised, and compiled to machine code by standard LLVM tools. Changing this to use the Haskell FFI bindings to the LLVM C API would allow better tracking of any changes to the LLVM IR, and hopefully a reasonable speed increase by avoiding the overhead of first emitting LLVM Assembly to be assembled. In his bachelor's thesis, David Terei considered this approach when initially writing the LLVM backend, but dismissed it due to the existing Haskell bindings being too large and high level to use with GHC. I have not personally looked at the high level Haskell bindings in much depth, but the low level (llvm-base) bindings appear to be somewhat incomplete with respect to LLVM 3.3. Therefore, I plan to extend the FFI bindings to cover more or all of the C API, and then to modify the GHC LLVM backend to use these bindings to call into the LLVM libraries directly. http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/Backends/LLVMc… a link to David's thesis, as well as documentation of the LLVM backend. I'd really appreciate it if anyone who knows about the Haskell LLVM bindings or about the GHC LLVM backend could give any advice regarding what sort of work needs to be done with the LLVM bindings to make them more appropriate for use within GHC, or how to approach modifying the existing LLVM backend. Many thanks, Alex

2 1

Re: [Haskell-cafe] LLVM Backend status
by JP＠eukor.com 29 Jun '13

29 Jun '13

1 0

LLVM Backend status
by B B 29 Jun '13

29 Jun '13

Hi! I'm new to Haskell and I'm very interested in using it as a base layer for my application. I need to generate the LLVM-IR code from ghc compiler, so I've got 2 questuions to you: 1) Is the LLVM Backend actively developed or should I be afraid it will be discontinued or broken? 2) How can I generate the LLVM IR (or bc) files from the compiler? I see that durign compilation with -fllvm -v3 flags the bc files are created but they are immidietly (after usage) deleted - Is it possible to tell the compiler not to delete them?

3 5

Who uses Travis CI and can help write a cookbook for those guys?
by Ryan Newton 29 Jun '13

29 Jun '13

The Travis folks have decided they want to support Haskell better (multiple compiler versions): https://github.com/travis-ci/travis-ci/issues/882#issuecomment-20165378 (Yay!) They're asking for someone to help them up with setup scripts. They mention their cookbook collection here: https://github.com/travis-ci/travis-cookbooks In that thread above, I pasted our little script that fetches and installs multiple GHC versions, but I have little experiences with cloud technologies & VMs. Can someone jump in and help push this forward? As a community I'm sure it would be great to get a higher percentage of Hackage packages using simple, hosted continuous testing... I'd personally like to replace my Jenkins install if they can get the necessary GHC versions in there. Best, -Ryan

1 0

Re: [Haskell-cafe] LLVM Backend status
by JP＠eukor.com 28 Jun '13

28 Jun '13

1 0

Job opportunities at Galois
by Iavor Diatchki 28 Jun '13

28 Jun '13

Hello, Galois is hiring! We're looking for researchers, principal investigators, and software engineers, including those with expertise in functional programming, formal methods, computer security, control systems, informatics, or networking. For more information, take a look at http://corp.galois.com/careers and please feel free to drop me an e-mail if you have questions. Cheers, -Iavor

2 2

Re: [Haskell-cafe] LLVM Backend status
by JP＠eukor.com 28 Jun '13

28 Jun '13

1 0

Going to speak about Haskell at Flock to Fedora
by Garrett Mitchener 27 Jun '13

27 Jun '13

(Sorry if you get this twice, I had some trouble with google groups and the mailing list...) Hi, Red Hat is holding a conference called Flock to Fedora<http://flocktofedora.com/> in Charleston, SC, and I'm going to be speaking about Haskell: http://sched.co/14twxPj I'm planning to introduce the language, touch a few interesting packages, and show how to use GHC on Fedora. If anyone from the Haskell community is going to Flock and has requests or suggestions for what I should talk about, post a reply! -- Garrett Mitchener, associate professor of mathematics, College of Charleston, SC

1 0

Haskell Weekly News: Issue 271
by Daniel Santa Cruz 27 Jun '13

27 Jun '13

Welcome to issue 271 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 9 to 11, 2013. Quotes of the Week * cmccann: also please note that "malign" remains one of my best function names ever * dolio: Peak file descriptors is a myth. * mami: i think my brain rewired for higher typesystem compatibility in my sleep * elliott: it's impure but only depending on the environment! which is to say it is impure. * johnw: lens makes one into a mutating ninja * applicativ: een my language we say Z for one * DavidJacobs: Code is like poetry; most of it shouldn't have been written. * merijn: If I could program with arbitrary real world side effects I'd use "unsafeStabComputerUser" if someone attempted that * shachaf: Hackage going down regularly is part of the rustic charm of Haskell! * shachaf: JavaScript is the best example of a domain-specific language. Because of the same-origin policy. * chrisdone: [on being invited to help with idris development] dude i've already got 20 yaks lined up waiting to be shaved, don't send me a whole truck full * shachaf: edwardk isn't benevolent or malevolent. edwardk is more of a simple mechanical process tending toward maximal complexity. * AndrejBauer: When have you ever heard a category theorist say: I want to prove this diagram commutesâLet's suppose it doesn't. Top Reddit Stories * Applicative => Monad is coming. Domain: haskell.org, Score: 122, Comments: 62 On Reddit: [1] http://goo.gl/JCvQ4 Original: [2] http://goo.gl/fJxeX * The Homotopy Type Theory Book is now available for free online and in print at Lulu.com Domain: homotopytypetheory.org, Score: 86, Comments: 16 On Reddit: [3] http://goo.gl/x8nLW Original: [4] http://goo.gl/DR9gf * Three useful monads Domain: adit.io, Score: 71, Comments: 25 On Reddit: [5] http://goo.gl/WHqPr Original: [6] http://goo.gl/1r8nC * Beta sign up for FP Complete new IDE Domain: fpcomplete.com, Score: 63, Comments: 11 On Reddit: [7] http://goo.gl/RPV0P Original: [8] http://goo.gl/c03Od * HLearn implements more with 6 trivial lines of Haskell than Weka implements with over 100 complex lines of Java Domain: izbicki.me, Score: 47, Comments: 9 On Reddit: [9] http://goo.gl/EEqwS Original: [10] http://goo.gl/Ukk4E * Top secret School of Haskell articles leaked Domain: self.haskell, Score: 47, Comments: 17 On Reddit: [11] http://goo.gl/zhhYm Original: [12] http://goo.gl/zhhYm * F-algebra tutorial Domain: fpcomplete.com, Score: 41, Comments: 35 On Reddit: [13] http://goo.gl/Ybmbs Original: [14] http://goo.gl/szfGk * John Carmack on haskell today. Domain: self.haskell, Score: 40, Comments: 79 On Reddit: [15] http://goo.gl/8qHkt Original: [16] http://goo.gl/8qHkt * Data Flow Fusion with Series Expressions in Haskell Domain: justtesting.org, Score: 40, Comments: 19 On Reddit: [17] http://goo.gl/ZNeIP Original: [18] http://goo.gl/JAjjy * So you still don't understand Hindley-Milner? Part 1 Domain: akgupta.ca, Score: 38, Comments: 0 On Reddit: [19] http://goo.gl/OOk43 Original: [20] http://goo.gl/SZ1Gb * Ask HN: Any downsides of programming in Haskell? Domain: news.ycombinator.com, Score: 36, Comments: 104 On Reddit: [21] http://goo.gl/vRuHW Original: [22] http://goo.gl/fcleS * FP Complete - Haskell Introduction Domain: youtube.com, Score: 36, Comments: 12 On Reddit: [23] http://goo.gl/GqgC5 Original: [24] http://goo.gl/KoVYs * Fix your posture with haskell Domain: github.com, Score: 35, Comments: 2 On Reddit: [25] http://goo.gl/lznTr Original: [26] http://goo.gl/x21Ax * preprint list for ICFP'13 papers (additional links welcome) Domain: github.com, Score: 34, Comments: 9 On Reddit: [27] http://goo.gl/mZsTn Original: [28] http://goo.gl/bhbC9 * GHC: now with closed type families Domain: github.com, Score: 34, Comments: 15 On Reddit: [29] http://goo.gl/JJSSv Original: [30] http://goo.gl/QlV4s Top StackOverflow Questions * How to compare two functions for equivalence, as in (λx.2*x) == (λx.x+x)? votes: 15, answers: 2 Read on SO: [31] http://goo.gl/A6YDd * Should I use typeclasses or not? votes: 15, answers: 2 Read on SO: [32] http://goo.gl/EOV1M * Is there something like `map2 :: (i -> a) -> (i -> b) -> [i] -> [(a,b)]`? votes: 11, answers: 2 Read on SO: [33] http://goo.gl/WZyEy * Why isn't GeneralizedNewtypeDeriving a Safe Haskell? votes: 11, answers: 1 Read on SO: [34] http://goo.gl/on6Fc * It's not a monad, but what is it? votes: 10, answers: 0 Read on SO: [35] http://goo.gl/sm9gR * Writing efficient iterative loop for ST monad votes: 10, answers: 0 Read on SO: [36] http://goo.gl/WAAwA * Getting multiple results from map with “lens” votes: 10, answers: 2 Read on SO: [37] http://goo.gl/zqX8E * How to run Frege programs on Android? votes: 10, answers: 2 Read on SO: [38] http://goo.gl/XlSCN * Are there problems that can't be solved efficiently without arrays? [duplicate] votes: 9, answers: 0 Read on SO: [39] http://goo.gl/Z5j9a * Haskell: How to organize a group of functions that all take the same arguments votes: 9, answers: 4 Read on SO: [40] http://goo.gl/z64H7 Until next time, [41]+Daniel Santa Cruz References 1. http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal 2. http://www.reddit.com/r/haskell/comments/1gsejt/applicative_monad_is_coming/ 3. http://homotopytypetheory.org/book/ 4. http://www.reddit.com/r/haskell/comments/1gr3uw/the_homotopy_type_theory_bo… 5. http://adit.io/posts/2013-06-10-three-useful-monads.html 6. http://www.reddit.com/r/haskell/comments/1g3a7e/three_useful_monads/ 7. https://www.fpcomplete.com/blog/2013/06/beta-sign-up 8. http://www.reddit.com/r/haskell/comments/1giy7l/beta_sign_up_for_fp_complet… 9. http://izbicki.me/blog/hlearns-code-is-shorter-and-clearer-than-wekas 10. http://www.reddit.com/r/haskell/comments/1g4v7u/hlearn_implements_more_with… 11. http://www.reddit.com/r/haskell/comments/1g5k80/top_secret_school_of_haskel… 12. http://www.reddit.com/r/haskell/comments/1g5k80/top_secret_school_of_haskel… 13. https://www.fpcomplete.com/user/bartosz/understanding-algebras 14. http://www.reddit.com/r/haskell/comments/1g2g3s/falgebra_tutorial/ 15. http://www.reddit.com/r/haskell/comments/1g1mvx/john_carmack_on_haskell_tod… 16. http://www.reddit.com/r/haskell/comments/1g1mvx/john_carmack_on_haskell_tod… 17. http://justtesting.org/post/53175916852/data-flow-fusion-with-series-expres… 18. http://www.reddit.com/r/haskell/comments/1gi5m1/data_flow_fusion_with_serie… 19. http://akgupta.ca/blog/2013/05/14/so-you-still-dont-understand-hindley-miln… 20. http://www.reddit.com/r/haskell/comments/1g0ueo/so_you_still_dont_understan… 21. https://news.ycombinator.com/item?id=5893442 22. http://www.reddit.com/r/haskell/comments/1gknfs/ask_hn_any_downsides_of_pro… 23. https://www.youtube.com/watch?v=Fqi0Xu2Enaw 24. http://www.reddit.com/r/haskell/comments/1glw2o/fp_complete_haskell_introdu… 25. https://github.com/keera-studios/keera-posture 26. http://www.reddit.com/r/haskell/comments/1gr044/fix_your_posture_with_haske… 27. https://github.com/gasche/icfp2013-papers#readme 28. http://www.reddit.com/r/haskell/comments/1gg2xf/preprint_list_for_icfp13_pa… 29. https://github.com/ghc/ghc/commit/569b26526403df4d88fe2a6d64c7dade09d003ad 30. http://www.reddit.com/r/haskell/comments/1gu2yl/ghc_now_with_closed_type_fa… 31. http://stackoverflow.com/questions/17045941/how-to-compare-two-functions-fo… 32. http://stackoverflow.com/questions/17100036/should-i-use-typeclasses-or-not 33. http://stackoverflow.com/questions/17074424/is-there-something-like-map2-i-… 34. http://stackoverflow.com/questions/17137111/why-isnt-generalizednewtypederi… 35. http://stackoverflow.com/questions/17034427/its-not-a-monad-but-what-is-it 36. http://stackoverflow.com/questions/17126923/writing-efficient-iterative-loo… 37. http://stackoverflow.com/questions/17144334/getting-multiple-results-from-m… 38. http://stackoverflow.com/questions/17222383/how-to-run-frege-programs-on-an… 39. http://stackoverflow.com/questions/17128103/are-there-problems-that-cant-be… 40. http://stackoverflow.com/questions/17176829/haskell-how-to-organize-a-group… 41. https://plus.google.com/105107667630152149014/about

1 0

homotopy type theory for amateurs
by Ben 26 Jun '13

26 Jun '13

hello cafe -- by now i'm sure you have heard that the homotopy type theory folks have just written up a free introductory book on their project. http://homotopytypetheory.org/2013/06/20/the-hott-book/ gabriel gonzalez and i are starting up a small reading group for the book. the level of study will be "amateur", though i have high hopes for rigor and thoroughness. personally, i know hardly any type theory or logic, and the last time i thought about homotopy theory in any seriousness was years ago. thankfully the book looks very accessible. being time-constrained we were going to do it mostly over email, maybe starting a google group or other mailing list, but we might meet as well, say once a month in san francisco (where we both are.) if you're interested in joining us, send me an email (midfield at gmail) and i'll try to keep you informed of any developments. best, ben

1 1

Geometric Algebra [Was: writing a function to make a correspondance between type-level integers and value-level integers]
by oleg＠okmij.org 26 Jun '13

26 Jun '13

> It seems very interesting, but I have not currently the time to make a > detailed comparison with vector/tensor algebra. Moreover I have not I would suggest the freely available Oersted Medal Lecture 2002 by David Hestenes http://geocalc.clas.asu.edu/pdf/OerstedMedalLecture.pdf the person who discovered and developed the Geometric Algebra. In particular, see Section V of the above paper. It talks about vectors, geometric products, the coordinate-free representation for `vector product' and the geometric meaning of the imaginary unit i. Section 1 gives a good motivation for the Geometric Algebra. Other sections of the paper develop physical applications, from classical mechanics to electrodynamics to non-relativistic and relativistic quantum mechanics. Computer Scientists might then like http://www.geometricalgebra.net/ see the good and free introduction http://www.geometricalgebra.net/downloads/ga4cs_chapter1.pdf Incidentally, David Hestenes said in the lecture that he has applied for an NSF grant to work on Geometric Algebra TWELVE times in a row, and was rejected every single time.

1 0

writing a function to make a correspondance between type-level integers and value-level integers
by oleg＠okmij.org 26 Jun '13

26 Jun '13

Well, I guess you might be interested in geometric algebra then http://dl.acm.org/citation.cfm?id=1173728 because Geometric Algebra is a quite more principled way of doing component-free calculations. See also the web page of the author http://staff.science.uva.nl/~fontijne/ Geigen seems like a nice DSL that could well be embedded in Haskell. Anyway, the reason I pointed out Vectro is that it answers your question about reifying and reflecting type-level integers (by means of a type class).

2 1

Roman Numeral Problem
by Christopher Howard 25 Jun '13

25 Jun '13

Hi. I am working on some practice programming problems, and one is the Roman numeral problem: write a program that converts Roman numerals into their (arabic) numeral equivalent. I imagine I could hack something together, but I was trying to think about the problem a bit more deeply. I don't know much about parsing, but I was wondering if this problem matched up with some kind of parsing or grammar or other generic approach to thinking about the problem. (I once read something about Context Free Grammars, which was rather interesting.) I can do more of my own research if I can get some initial guidance. -- frigidcode.com

4 3

ANNOUNCE: haskell-names-0.1
by Roman Cheplyaka 25 Jun '13

25 Jun '13

I am pleased to announce the first public release of haskell-names, a name resolution library for haskell-src-exts AST. Namely, it can do the following: * for a module, compute its interface, i.e. the set of entities exported by the module, together with their original names. * for each name in the module, figure out what it refers to — whether it's bound locally (say, by a where clause) or globally (and then give its origin). Thanks to haskell-packages, this library is fully integrated with Cabal, so that you can easily generate name interfaces for any Cabalized package. See more details in the README: http://documentup.com/haskell-suite/haskell-names This library is based on the code written by Lennart Augustsson in 2010. Little of that code survived, but nevertheless it's been a great help. Roman

6 8

tangential request...
by Mark Lentczner 25 Jun '13

25 Jun '13

As you may know, I've been writing a shell in Haskell called Plush<https://code.google.com/p/plush/> . I'm working on the look of the terminal input and output, and I want to do a little data gathering. I want to know what your terminal looks like! You can help me by: 1) Download this file: http://www.ozonehouse.com/mark/plush/TerminalDisplay.txt 2) Open a new terminal window 3) Do not resize the terminal window 4) cat the file 5) Take a screen shot of the whole terminal window 6) E-mail it to me: mark.lentczner+term(a)gmail.com The file contains some sample text, including some screen codes to change colors and bold. This looks like: - in my terminal program <http://www.ozonehouse.com/mark/plush/term.png> - in Plush <http://www.ozonehouse.com/mark/plush/term-plush.png> (bleeding edge, not checked in yet) Thanks!

10 16

[OT]Help with translating Error Monads to C++
by Larry Evans 25 Jun '13

25 Jun '13

Currently in the c++ developers ml, there's a discussion of a proposed Expected<T> or an Either<L,R> template: http://article.gmane.org/gmane.comp.lib.boost.devel/242496 A lot of the discussion appears, at least to me, to not be aware of how haskell handles similar situations. The above link was to my post suggesting they try to emulate the way haskell solves the problem using Monads. However, I've only briefly dabbled in haskell; hence, my responses on that thread are probably missing something. I invite anyone on this list to please provide insights on this particular thread in the boost.devel mailing list. -kind regards, Larry

1 0

Munich Haskell Meeting
by Heinrich Hördegen 24 Jun '13

24 Jun '13

Dear all, if you plan to join our monthly Munich Haskell Meeting on Wed, 26th of June, please go to http://www.haskell-munich.de/dates and hit the button. See you soon, Heinrich -- -- Funktionale Programmierung Dr. Heinrich Hördegen Gutenbergstr. 26 80638 München FON: +49 (89) 12 59 79 49 FAX: +49 (89) 12 59 79 50 hoerdegen(a)funktional.info www.funktional.info --

1 0

A friendly reminder: Ghent-FPG meeting on 26 June, 2013
by Andy Georges 24 Jun '13

24 Jun '13

Hello, This is to remind you that you are kindly invited to attend our next meeting. The original email follows below. The "Functional Programming Group Ghent" (GhentFPG) [1] is a friendly group for all people interested in functional programming, with a tendency towards Haskell. It is organised as part of Zeus WPI [2]. We are pleased to announce that we will hold a next meeting on Wednesday, 26th of June, starting at 19h00! There will be three talks. The main presentation, by Adam Bergmark from Silk [3] is about Fay [4]: Fay is a proper subset of Haskell that compiles to JavaScript. There is a compiler with the same name written in Haskell. Web browsers only speak JavaScript but more and more people find that they want to compile to JavaScript instead. Why do we want to compile Haskell to JavaScript, and what advantages does Fay have compared to other compilers? What are the challenges in compiling Haskell and supporting a language ecosystem, and how do we do it? What can Fay currently do, and what is planned for the future? This will be a broad overview about Fay for prospective users, followed by an in-depth look at interesting parts of the compiler internals. Additionally, there will be two short talks by two students who did an Msc. Thesis about functional programming languages: "Genetic Algorithms in Haskell" by Matthias Delbar "Automatic Detection of Recursion Patterns" by Jasper Van der Jeugt The meeting will take place in the Jozef Plateauzaal at the following address, Faculteit Ingenieurswetenschappen Universiteit Gent Plateaustraat 22 9000 Gent As mentioned above, we aim to start at 19:00. After the meeting we can go for drinks in a nearby pub (this latter part is, of course, completely optional) We hope to see you all there! Regards, On behalf of the GhentFPG organising committee. [1]: http://groups.google.com/group/ghent-fpg [2]: http://zeus.ugent.be/ [3]: http://www.silkapp.com/ [4]: http://www.fay-lang.org/

1 0

New Functional Programming Job Opportunities
by Functional Jobs 24 Jun '13

24 Jun '13

Here are some functional programming job opportunities that were posted recently: Senior software developer/Functional programmer at Vector Fabrics http://functionaljobs.com/jobs/154-senior-software-developer-functional-pro… Cheers, Sean Murphy FunctionalJobs.com

1 0

Fwd: How to make this data type work?
by Miguel Mitrofanov 24 Jun '13

24 Jun '13

Forgot to reply all, as usual. -------- Пересылаемое сообщение -------- 21.06.2013, 12:52, "Miguel Mitrofanov" <miguelimo38(a)yandex.ru>: Actually, this is not the real error you should care about. Try removing FromJSON instance completely, and you'll get a lot more. And these are fundamental: you have to decide what "j" to use when serializing. Haskell won't automagically substitute some suitable type for you. So, that's a classic mismatch: for serializing (ToJSON) you need your "j" type to be known to the AD value (meaning: it should be quantified existentially), but for deserializing you need it to be any type (quantified universally). All in all, AD seems to be the wrong type. 21.06.2013, 12:18, "Magicloud Magiclouds" <magicloud.magiclouds(a)gmail.com>: > data ActionData = AD { oldData :: (FromJSON j, ToJSON j) => j > , newData :: (FromJSON j, ToJSON j) => j} > instance ToJSON ActionData where > toJSON (AD o n) = object [ "oldData" .= o > , "newData" .= n ] > instance FromJSON ActionData where > parseJSON (Object v) = AD > <$> v .: "oldData" > <*> v .: "newData" > parseJSON _ = mzero > > I got when compile: > No instance for (FromJSON (forall j. (FromJSON j, ToJSON j) => j)) > arising from a use of `.:' > Possible fix: > add an instance declaration for > (FromJSON (forall j. (FromJSON j, ToJSON j) => j)) > In the second argument of `(<$>)', namely `v .: "oldData"' > In the first argument of `(<*>)', namely `AD <$> v .: "oldData"' > In the expression: AD <$> v .: "oldData" <*> v .: "newData" > > -- > 竹密岂妨流水过 > 山高哪阻野云飞 > > And for G+, please use magiclouds#gmail.com. > , > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -------- Завершение пересылаемого сообщения --------

3 2

putting the result of a function in `infix` declaration
by TP 24 Jun '13

24 Jun '13

Hi everyone, Consider the following code: -------------- import Prelude hiding ((+)) data Expr = Plus Expr Expr | Minus Expr Expr | Expr String deriving Show f :: Expr -> Int f (Plus _ _) = 1 f (Minus _ _) = 2 -- infix (f (Plus undefined undefined)) + -- (*) infix 6 + (+) :: Expr -> Expr -> Expr e1 + e2 = Plus e1 e2 main = do let a = Expr "a" let b = Expr "b" print $ a + b -------------- I would like to declare the infix precedence of (+) with a function (commented line (*)) instead of directly as above. Is there any means to do that? Do we need some kind of preprocessing machinery? How to do that in Haskell? In a more general way, let us suppose I have a Haskell library able to perform some calculations: how to use it in a pre-processing step, before compilation of the executable? It could be a function to compute symbolically roots of a polynomial of second degree to be used at runtime. We would put some placeholders in the code where the result of the pre- processing calculation would enter. Thanks in advance, TP

3 2