- Haskell-Cafe - Haskell.org

[Template Haskell]
by Dominik Bollmann 25 Jan '16

25 Jan '16

Hi all, I'm just getting my feet wet with template haskell, and I tried to write a tmap function which maps a function over the ith component of an n-tuple (which uses a slightly different approach than the given version on the TH wiki): -- | Selects the ith component of an n-tuple tsel :: Int -> Int -> ExpQ -- n-tuple a -> a tsel i n = [| \t -> $(caseE [| t |] [alt]) |] where alt = match (tupP pats) body [] pats = map varP xs xs = [ mkName ("x" ++ show k) | k <- [1..n] ] body = normalB . varE $ xs !! (i-1) -- | Maps a function over the ith component of an n-tuple tmap :: Int -> Int -> ExpQ -- :: (a -> b) -> n-tuple -> n-tuple tmap i n = do f <- newName "f" t <- newName "t" lamE [varP f, varP t] $ [| let prefix = map extract [1..(i-1)] new = $f ($(tsel i n) $t) suffix = map extract [(i+1)..n] extract k = $(tsel k n) t in tupE $ prefix ++ [new] ++ suffix |] However, this code results in the following error: Sandbox.hs:26:29: Stage error: ‘k’ is bound at stage 2 but used at stage 1 … In the splice: $(tsel k n) In the Template Haskell quotation [| let prefix = map extract [1 .. (i - 1)] new = $f ($(tsel i n) ($t)) suffix = map extract [(i + 1) .. n] extract k = $(tsel k n) $t in tupE $ prefix ++ [new] ++ suffix |] Compilation failed. Could anyone explain to me what stage 2 and stage 1 refer to, and further, what the logical flaw in the above snippet is? What exactly is wrong with line `extract k = $(tsel k n) $t' ? Thanks! Dominik.

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Re: [Haskell-cafe] Local types
by Johannes Waldmann 25 Jan '16

25 Jan '16

> I often wish to be able to define local types and instances ... https://mail.haskell.org/pipermail/haskell-cafe/2014-October/116291.html

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ANN: toysolver 0.4.0 released
by Masahiro Sakai 25 Jan '16

25 Jan '16

I'm announcing the release of the toysolver package, version 0.4.0. The toysolver provides solver implementations of various problems including SAT, SMT, Max-SAT, PBS (Pseudo Boolean Satisfaction), PBO (Pseudo Boolean Optimization), MILP (Mixed Integer Linear Programming) and non-linear real arithmetic. The highlight of this release is the introduction of SMT (Satisfiablity Modulo Theories) solver 'toysmt'. At the moment, toysmt is very experimental and only supports the theory of uninterpreted functions and the theory of linear real arithmetic. http://hackage.haskell.org/package/toysolver https://github.com/msakai/toysolver/releases/tag/v0.4.0 Thanks, -- Masahiro Sakai

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trying to build Hackage hoq-0.3
by Vasili I. Galchin 25 Jan '16

25 Jan '16

1) ghc version 7.6.3 2) cabal version 1.16.0.2 3) when i run "cabal install", I receive on stdout : Resolving dependencies... Configuring readline-1.0.3.0... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for GNUreadline.framework... checking for readline... no checking for tputs in -lncurses... no checking for tputs in -ltermcap... no checking for tputs in -lcurses... no checking for rl_readline_version... no configure: error: readline not found, so this package cannot be built See `config.log' for more details. Failed to install readline-1.0.3.0 cabal: Error: some packages failed to install: hoq-0.3 depends on readline-1.0.3.0 which failed to install. readline-1.0.3.0 failed during the configure step. The exception was: ExitFailure 1 4) I googled Haskell and readline and I found the same error with other Haskell packages like Twitter. How do I resolve my build readline problem? Vasya

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[ANN] nano-erl
by amindfv＠gmail.com 23 Jan '16

23 Jan '16

I've written a tiny library for Erlang-style actor semantics in Haskell: hackage.haskell.org/package/nano-erl It's fast, and meant to be a lightweight abstraction that's easy to integrate with existing code. Enjoy! Tom

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question on the design of protohaskell (in Write You a Haskell)
by Maurizio Vitale 23 Jan '16

23 Jan '16

I know this part has been left incomplete when Stephen got a real job, but I have a question. In http://dev.stephendiehl.com/fun/007_path.html, compiler steps are piped together using the Kleisli (>=>) operator, presumably the one from Control.Monad. they also return some flavour of AST (typically Syn.Module or Core.Module). Assuming that the >=> is the one from Control.Monad, what is the purpose of this returned value (all the steps are in a compilerMonad and presumably would have to update the state w/ the new AST anyhow)? Or is the intention to have a special >=> that also updates the state (but I wouldn't know how to deal w/ Syn.Core vs Core.Module) I would understand some form of phantom type to ensure that compilation steps are in the right order, but as it is I'm puzzled. Anybody has some insight? Thanks, Maurizio

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Haskell Weekly News
by Kim-Ee Yeoh 22 Jan '16

22 Jan '16

Dear Gentle Reader, Many, many beautiful gems in the Haskell Weekly News archives are worth a second look. To give you a taste, I reproduce below excerpts from the quotes section of the Jan 31, 2007 issue -- yes, that's 9 years ago -- under the editorship of Don Stewart. Enjoy. Best, Kim-Ee Yeoh *Top Picks* - Edsko de Vries designs O(1)-amortized and O(1)-worst-case queues <http://www.well-typed.com/blog/2016/01/efficient-queues/> using a technique different from the standard literature by Chris Okasaki. In particular, the O(1)-worst-case queue employs a Progress datatype that could be reused to also optimize data structures other than queues. On the other hand, Lennart Augustsson on /r/haskell was pleased as a plum until he saw the unsafeInterleaveST <https://www.reddit.com/r/haskell/comments/412n6n/new_blog_post_efficient_am…> required to pull off the Progress technique. Elsewhere, Hacker News rates the article highly enough <https://news.ycombinator.com/item?id=10913720> for it to stay on the front page for five hours <http://hnrankings.info/10913720/>. However, the comments there belie that the advanced Haskell goes swoosh over the head of the average HN reader. - Philipp Schuster sketches a FRP implementation based on temporal logic <http://haskellexists.blogspot.com/2016/01/frp-for-free.html>. Neel Krishnaswami explains on /r/haskell why it suffers from space leaks <https://www.reddit.com/r/haskell/comments/40x4wf/frp_for_free/> like most other FRP implementations and ways of fixing it. - Dan Burton reports that the latest version 0.10 of the json parsing package aeson suffers from deal-breaking bugs <https://unknownparallel.wordpress.com/2016/01/14/what-to-do-with-aeson-0-10/>. Aeson author Bryan O'Sullivan, of an older email-centered generation, explains that he has "a life outside of checking github issues" in the /r/haskell discussion <https://www.reddit.com/r/haskell/comments/4101ad/what_to_do_with_aeson010/>. In any case, the next stepping 5 of Stackage LTS rolls it back to version 0.9 <https://www.reddit.com/r/haskell/comments/41gpdk/lts4_with_aeson010_is_bein…> . - A redditor asks, "What's the TypeInType extension planned for the upcoming version 8 of GHC?" The short answer is that it's used for dependent type programming. Detailed answers can be found in the actual /r/haskell Q&A <https://www.reddit.com/r/haskell/comments/4180k3/what_is_typeintype/>. - GHC on ARM used to suffer over 100 failures on the testsuite <https://ghc.haskell.org/trac/ghc/ticket/11206>. Ben Gamari girds his loins and over the last 6 weeks battled against "the villains that plague this poor architecture." Result? Nightly builds now compile clean. Go Ben! *A Blast from the Past (Quotes from #Haskell IRC):* <http://sequence.complete.org/hwn/20070131> - *huschi:* Programing in haskell seems a bit frustrating. i'm missing searching for errors :( - *bakert:* I know all my programs can be reduced to only one tenth the size if only I can learn all these crazy functions *Quote of the Week* - Will Jones: The more I write Haskell, the more it feels like Forth. Where I'm basically just inventing a language for my problem, then writing the program in that instead. (Ed. Dear Will: Remember how Dijkstra once said "Always design your programs as a member of a whole family of programs, including those that are likely to succeed it"? He would have warmly congratulated you on your discovery.) -- Kim-Ee Yeoh

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Munich Haskell Meeting, 2016-01-25 @ 19:30
by Christian Neukirchen 21 Jan '16

21 Jan '16

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 25 at Cafe Puck at 19h30. For details see here: http://chneukirchen.github.io/haskell-munich.de/dates If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jan-2016/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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an automatic refactoring idea
by Jeffrey Brown 21 Jan '16

21 Jan '16

I had a data structure with a redundant field [1]. I refactored to make that field go away. Here is the code [2]. The following is a simplification of it. I was using this type: data X = X1 | X2 Int To make the Int go away, I made a duplicate type: data X' = X1' | X2' Then, every function for which any argument was of type X, I similarly duplicated, replacing every X with X', X1 for X1', and X2 for X2'. Next I had to similarly duplicate every function that used one of those. And so on, and so on ... until eventually I was duplicating the tests. At that point I was able to determine from the duplicate tests that everything was working. There was a little more work involved than that, but the vast, vast majority of the edits that got me there were the simple duplication I just described. I looked once at automatic refactoring in Haskell and was either unimpressed or scared. I may not have understood what I was looking at enough to appreciate its power. Is there something that can do the refactoring described above? [1] The Int in a Tplt could be inferred from context, so keeping a duplicate of that information in the Tplt was dangerous, because it makes invalid state possible (e.g. if one is updated and not the other). So I decided to stop using it. [2] https://github.com/JeffreyBenjaminBrown/digraphs-with-text -- Jeffrey Benjamin Brown

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localized memory management?
by Johannes Waldmann 21 Jan '16

21 Jan '16

Dear cafe, how would you approach this task: find (enumerate) those x from a list xs for which some computation is not successful within some resource bound (it weeds out uninteresting data, leaving just the hard cases, which will be treated later by other means) Input and result could be lazy lists, computations are pure. If the bounded resource is time, then I can use System.Timeout. This puts me into IO, but OK. Perhaps I want to do some logging output anyways. The problem is with bounding space. Assume that "computation on x" (sometimes) allocates a lot. Then the whole program will just die with "heap exhausted", while in fact I want to terminate just the computation on this x, garbage-collect, and continue. I could make the space usage explicit: each step of the computation could additionally compute a number that approximates memory usage. (Assume that this usage varies wildly with each step.) Then I can stop iterating when this reaches some bound. Or, I just compile the computation into a separate executable, and I call it (for each x) via the operating system, because there I can bound space (with ulimit) Is there some way to achieve this in Haskell land? - J.W.

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