- Haskell-Cafe - Haskell.org

Haskell Weekly News: Issue 226
by Daniel Santa Cruz 09 May '12

09 May '12

Welcome to issue 226 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 29 to May 5, 2012. Quotes of the Week * acowley: I write the most complicated bottoms * Cale: OpenGL is the Rubik's Cube of graphics libraries. It's nearly impossible to change some things without affecting other things Top Reddit Stories * HJ — Haskell-to-JavaScript compiler (WIP) Domain: chrisdone.com, Score: 74, Comments: 23 On Reddit: [1] http://goo.gl/gP51N Original: [2] http://goo.gl/g32rz * Life Without Objects Domain: skipoleschris.blogspot.co.uk, Score: 52, Comments: 17 On Reddit: [3] http://goo.gl/LZmLC Original: [4] http://goo.gl/NLhKH * New major release of the containers package Domain: blog.johantibell.com, Score: 52, Comments: 25 On Reddit: [5] http://goo.gl/yjblJ Original: [6] http://goo.gl/DKlmd * Haskell and the World: Encodings and the Common Misuse of ByteString Domain: a-dimit.blogspot.com, Score: 50, Comments: 27 On Reddit: [7] http://goo.gl/WloCn Original: [8] http://goo.gl/oeIIW * How to write hybrid CPU/GPU programs with Haskell Domain: parfunk.blogspot.com.au, Score: 48, Comments: 3 On Reddit: [9] http://goo.gl/u9A9l Original: [10] http://goo.gl/s917R * phantom tainting with kind error messages Domain: article.gmane.org, Score: 35, Comments: 5 On Reddit: [11] http://goo.gl/V9qRd Original: [12] http://goo.gl/cx4cD * Annotated slides: Comparing Dynamic and Static Language Approaches to Web Frameworks (Rails vs Yesod) Domain: cs.kent.ac.uk, Score: 32, Comments: 11 On Reddit: [13] http://goo.gl/jE2iK Original: [14] http://goo.gl/2dIjQ * SPJ talk: Towards Haskell in the Cloud Domain: skillsmatter.com, Score: 30, Comments: 16 On Reddit: [15] http://goo.gl/fVc0e Original: [16] http://goo.gl/NMtPD * hotswap -- a simple, high level interface to plugins for hotswapping code Domain: hackage.haskell.org, Score: 29, Comments: 8 On Reddit: [17] http://goo.gl/kvhOS Original: [18] http://goo.gl/VMAe0 * Online Haskell typechecker Domain: haskellonline.org, Score: 29, Comments: 12 On Reddit: [19] http://goo.gl/uV3HY Original: [20] http://goo.gl/lSwhb Top StackOverflow Questions * Algebraically interpreting polymorphism votes: 21, answers: 5 Read on SO: [21] http://goo.gl/7krnc * Absolute value of negative zero - bug, or a part of the floating point standard? votes: 16, answers: 3 Read on SO: [22] http://goo.gl/HDpRc * how to implement doubly linked lists votes: 10, answers: 1 Read on SO: [23] http://goo.gl/hXyDM * GHC type inference for higher rank types - assigning to monotypes votes: 10, answers: 1 Read on SO: [24] http://goo.gl/TqYtX * Communication between Java and Haskell votes: 9, answers: 3 Read on SO: [25] http://goo.gl/zMYwx * Default constraint kinds are ignored votes: 9, answers: 2 Read on SO: [26] http://goo.gl/AElL4 * Scrap Your Boilerplate equivalent in Scala? votes: 7, answers: 1 Read on SO: [27] http://goo.gl/Cd7k4 * Does the chain function in underscore.js create a monad? votes: 7, answers: 1 Read on SO: [28] http://goo.gl/DUYkB * Why do Haskell list comprehensions with multiple generators treat the rightmost generator as the tightest loop? votes: 6, answers: 3 Read on SO: [29] http://goo.gl/BnndW * Haskell records, cleaner approach? votes: 6, answers: 1 Read on SO: [30] http://goo.gl/MymyL Until next time, Daniel Santa Cruz References 1. http://chrisdone.com/hj/ 2. http://www.reddit.com/r/haskell/comments/t06ej/hj_haskelltojavascript_compi… 3. http://skipoleschris.blogspot.co.uk/2012/04/life-without-objects.html 4. http://www.reddit.com/r/haskell/comments/t1oen/life_without_objects/ 5. http://blog.johantibell.com/2012/05/new-major-release-of-containers-package… 6. http://www.reddit.com/r/haskell/comments/t688l/new_major_release_of_the_con… 7. http://a-dimit.blogspot.com/2012/04/strings-in-haskell.html 8. http://www.reddit.com/r/haskell/comments/syo8h/haskell_and_the_world_encodi… 9. http://parfunk.blogspot.com.au/2012/05/how-to-write-hybrid-cpugpu-programs.… 10. http://www.reddit.com/r/haskell/comments/t7znh/how_to_write_hybrid_cpugpu_p… 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/98103 12. http://www.reddit.com/r/haskell/comments/t6dn7/phantom_tainting_with_kind_e… 13. http://www.cs.kent.ac.uk/~nccb/rails-yesod-slides.pdf 14. http://www.reddit.com/r/haskell/comments/t1aw8/annotated_slides_comparing_d… 15. http://skillsmatter.com/podcast/home/haskell-cloud/js-4179 16. http://www.reddit.com/r/haskell/comments/t0qg0/spj_talk_towards_haskell_in_… 17. http://hackage.haskell.org/package/hotswap-0.1.9.3 18. http://www.reddit.com/r/haskell/comments/t24lr/hotswap_a_simple_high_level_… 19. http://haskellonline.org/ 20. http://www.reddit.com/r/haskell/comments/t41bk/online_haskell_typechecker/ 21. http://stackoverflow.com/questions/10453558/algebraically-interpreting-poly… 22. http://stackoverflow.com/questions/10395761/absolute-value-of-negative-zero… 23. http://stackoverflow.com/questions/10386616/how-to-implement-doubly-linked-… 24. http://stackoverflow.com/questions/10424375/ghc-type-inference-for-higher-r… 25. http://stackoverflow.com/questions/10370177/communication-between-java-and-… 26. http://stackoverflow.com/questions/10381278/default-constraint-kinds-are-ig… 27. http://stackoverflow.com/questions/10423329/scrap-your-boilerplate-equivale… 28. http://stackoverflow.com/questions/10431999/does-the-chain-function-in-unde… 29. http://stackoverflow.com/questions/10399577/why-do-haskell-list-comprehensi… 30. http://stackoverflow.com/questions/10427217/haskell-records-cleaner-approach

1 0

Fixed point newtype confusion
by Sebastien Zany 09 May '12

09 May '12

Hi, Suppose I have the following types: > data Expr expr = Lit Nat | Add (expr, expr) > newtype Fix f = Fix {unFix :: f (Fix f)} I can construct a sample term: > term :: Expr (Expr (Expr expr)) > term = Add (Lit 1, Add (Lit 2, Lit 3)) But isn't quite what I need. What I really need is: > term' :: Fix Expr > term' = Fix . Add $ (Fix . Lit $ 1, Fix . Add $ (Fix . Lit $ 2, Fix . Lit $ 3)) I feel like there's a stupidly simple way to automatically produce term' from term, but I'm not seeing it. Any ideas? Best, Sebastien

3 9

Handcranked install of ghc / cabal-install and warning from ghc-pkg check
by Graham Berks 09 May '12

09 May '12

Have installed 7.4.1 ghc and 0.14.0 cabal-install and then did alex, happy & haddock. When I do a ghc-pkg check I get $ ghc-pkg check Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/random-1.0.1.1/html/random.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/random-1.0.1.1/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/zlib-0.5.3.3/html/zlib.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/zlib-0.5.3.3/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/HTTP-4000.2.3/html/HTTP.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/HTTP-4000.2.3/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/network-2.3.0.11/html/network.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/network-2.3.0.11/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/parsec-3.1.2/html/parsec.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/parsec-3.1.2/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/text-0.11.2.0/html/text.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/text-0.11.2.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/mtl-2.1/html/mtl.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/mtl-2.1/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/Cellar/cabal-install/0.14.0/share/doc/transformers-0.3.0.0/html/transformers.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/Cellar/cabal-install/0.14.0/share/doc/transformers-0.3.0.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/haddock-2.10.0/html/haddock.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/haddock-2.10.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/xhtml-3000.2.0.5/html/xhtml.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/xhtml-3000.2.0.5/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/ghc-paths-0.1.0.8/html/ghc-paths.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/ghc-paths-0.1.0.8/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/mtl-2.1.1/html/mtl.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/mtl-2.1.1/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/transformers-0.3.0.0/html/transformers.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/transformers-0.3.0.0/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/QuickCheck-2.4.2/html/QuickCheck.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/QuickCheck-2.4.2/html doesn't exist or isn't a directory Warning: haddock-interfaces: /usr/local/share/doc/random-1.0.1.1/html/random.haddock doesn't exist or isn't a file Warning: haddock-html: /usr/local/share/doc/random-1.0.1.1/html doesn't exist or isn't a directory Any suggestions to fix this ? Thanks

1 2

Haskell Hackathon in Munich
by Heinrich Hördegen 09 May '12

09 May '12

Dear all, this is a reminder of our local haskell Hackathon in Munich, Germany, which will be held on Saturday, the 12th of May. If you plan to join, check out the details at: http://www.haskell-munich.de/dates or contact me. We hope to see you on Saturday, Heinrich

1 0

cabal issue
by Graham Berks 09 May '12

09 May '12

Given that I have the dependant packages already installed when I do $ cabal install --dry-run --global buildwrapper Resolving dependencies... In order, the following would be installed: mtl-2.1.1 (new version) aeson-0.6.0.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 parsec-3.1.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 regex-base-0.93.2 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 regex-tdfa-1.1.8 (reinstall) changes: mtl-2.0.1.0 -> 2.1.1 buildwrapper-0.5.2 (new package) And that build wrapper doesn't have a defined dependency version number for MTL can someone tell me why cabal is trying to install a new version of MTL ? Thanks

1 1

Annotaing abstract syntax trees
by José Romildo Malaquias 09 May '12

09 May '12

Hello. I am reading Martijn's MSc Thesis "Generic Selections of Subexpressions", where one can found some discussions about annotating abstract syntax trees (AST). In order to follow the discussion I wrote the attached Haskell program, which is an interpreter for an simple typed expression language. The Annotations package is used. The expression pattern is represented by a single recursive data type. Annotations are used for positions in the input source, and also for the type of expressions and subexpressions. I would like somebody to review the code and comment on it, as I am not sure I am using the concepts right. Also I would like the type checker to produce an expression annotated with both positions and calculated types. Currently it discards the position annotations. Any sugestions on how to modify it is welcome. Next step is adding a new form of expression to introduce local variable bindings. After that I want to start working with ASTs represented by mutually recursive data types. Then I will need multirec... Romildo

2 1

Re: [Haskell-cafe] Un-memoization
by oleg＠okmij.org 08 May '12

08 May '12

Victor Miller wrote: > I was writing a Haskell program which builds a large labeled binary tree > and then does some processing of it, which is fold-like. In the actual > application that I have in mind the tree will be *huge*. If the whole tree > is kept in memory it would probably take up 100's of gigabytes. Because of > the pattern of processing the tree, it occurred to me that it might be > better (cause much less paging) if some large subtrees could be replaced by > thunks which can either recalculate the subtree as needed, or write out the > subtree, get rid of the references to it (so it can be garbage collected) > and then read back in (perhaps in pieces) as needed. This could be fairly > cleanly expressed monadically. So does anyone know if someone has created > something like this? Yes. I had faced essentially the same problem. It is indeed tricky to prevent memoization: GHC is very good at it. The following article explains the solution: ``Preventing memoization in (AI) search problems'' http://okmij.org/ftp/Haskell/index.html#memo-off

1 0

[Q] multiparam class undecidable types - how to get this example to typecheck?
by Matthias Fischmann 08 May '12

08 May '12

Hi everybody, I am torturing the ghc type inference extensions, and I think i managed to break something, either in ghc or (more likely) in my brain: | {-# language FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-} | | module Main | where | | class Table table cell where | toLists :: table -> [[cell]] | fromLists :: [[cell]] -> table | | instance Table [[cell]] cell where | toLists = id | fromLists = id | | -- why i am using a class (not relevant for my question) - | -- data FastTable a = FastTable ... | -- instance Table (FastTable a) a where ... | | instance (Table a c, Show c) => Show a where | showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (toLists t) ghc 7.0.3 sais: | Overlapping instances for Show [[cell0]] | arising from a use of `shows' | Matching instances: | instance Show a => Show [a] -- Defined in GHC.Show | instance (Table a c, Show c) => Show a | -- Defined at /home/mf/tmp/z.hs:18:10-38 | In the second argument of `(.)', namely `shows (toLists t)' I would have thought that there is on overlap: the instance in my code above defines how to show a table if the cell is showable; the (allegedly) overlapping instance defines how to show lists of alpha if alpha is showable. So I was expecting the reduction to go something like: . table ~> [[cell]] (via whichever instance of class Table applies) . [[cell]] ~> [cell] (via instance "Show a => Show [a]") . [cell] ~> cell (via instance "Show a => Show [a]") But it gets worse. I decided to ignore the problem for now and be happy with at least showing the first cell in the upper left corner of the table: | instance (Table a c, Show c) => Show a where | showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (head . head $ toLists t) to that, ghc 7.0.3 sais: | /home/mf/tmp/z.hs:19:66: | Context reduction stack overflow; size = 21 | Use -fcontext-stack=N to increase stack size to N | $dShow :: Show c19 | $dShow :: Show c18 | [...] shouldn't type inference be able to determine that | ((head . head $ toLists t) :: c and use the instance guaranteed by the instance declaration's class constraint (Show c)? I suspect that there should be a type annotation that would make this approach possible, but even then I am curious what this behavior means. Should I upgrade to a more grown-up release of ghc? Any hints or pointers appreciated. (: Thanks, Cheers, Matthias

1 0

LLVM M.D. source code?
by Greg Fitzgerald 08 May '12

08 May '12

A paper, LLVM M.D.: A Denotational Translation Validator, mentions the source code for LLVM M.D is available here: http://llvm-md.seas.harvard.edu/ I can't seem to spot it there or on Hackage. Anyone seen it? Thanks, Greg

1 0

Unit and pair
by MigMit 08 May '12

08 May '12

Hi café, a quick question. Is there a somewhat standard class like this: class Something c where unit :: c () () pair :: c x y -> c u v -> c (x, u) (y, v) ? I'm using it heavily in my current project, but I don't want to repeat somebody else's work, and it seems general enough to be defined somewhere; but my quick search on Hackage didn't reveal anything. I know about arrows; this, however, is something more general, and it's instances aren't always arrows.

4 6