October 2012 - Haskell-Cafe

Re: [Haskell-cafe] Explicitly passing an argument to an arrow
by Alfredo Di Napoli 03 Oct '12

03 Oct '12

I see. There exists an equivalent version but more generic? Just out of curiosity, I'm still pretty new to arrows, as you may have read :) Thanks, A. On 3 October 2012 15:59, Ertugrul Söylemez <es(a)ertes.de> wrote: > Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> wrote: > > > Solution was simple, after reading that functions are arrows: > > > > swapA' t = (swapFirst >>> swapSecond) (t,t) > > where > > swapFirst = first snd > > swapSecond = second fst > > No, that's not a solution, unless a = (->) is acceptable. This code is > specific to the function arrow. > > > Greets, > Ertugrul > > -- > Key-ID: E5DD8D11 "Ertugrul Soeylemez <es(a)ertes.de>" > FPrint: BD28 3E3F BE63 BADD 4157 9134 D56A 37FA E5DD 8D11 > Keysrv: hkp://subkeys.pgp.net/ >

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Explicitly passing an argument to an arrow
by Alfredo Di Napoli 03 Oct '12

03 Oct '12

Hello everyone, sorry for the dumb question but I'm wrapping my head around arrow just from this morning. Consider this toy function to swap argument of a tuple: swapA' :: (Arrow a) => a ((b,c), (b,c)) (c,b) swapA' = swapFirst >>> swapSecond where swapFirst = first $ arr snd swapSecond = second $ arr fst It works but requires to pass a tuple of tuple, namely ((b,c), (b,c)). How can I explicitly pass my tuple of tuple to swapFirst so I can simply invoke swapA' (1,2) and get the correct result? ps. I know that swap can be easily and elegantly be written as: swap = snd &&& fst but the point of the exercise was to experiment with first, second and arrow concatenation Thanks in advance, Alfredo

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Data.Text and quasi quoting
by Jake Wheat 03 Oct '12

03 Oct '12

Hello, I have a parser which parses to an ast which contains Text values. I am trying to use this parser with quasiquoting, but the implementation of Data for Text is incomplete. I've attached a smallish test case, when I try to compile Text.hs I get: Text.hs:17:9: Exception when trying to run compile-time code: Data.Text.Text.toConstr Code: Language.Haskell.TH.Quote.quoteExp expr " test " Is there a way to get this working? Thanks, Jake.

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Strange things with type literals of kind Nat
by Troels Henriksen 03 Oct '12

03 Oct '12

Consider the following definition of lists with a statically determined minimum length. data Nat' = Z | S Nat' data NList (n::Nat') a where Rest :: [a] -> NList Z a (:>) :: a -> NList n a -> NList (S n) a uncons :: NList (S n) a -> (a, NList n a) uncons (x :> xs) = (x, xs) This works fine. Now consider an implementation using the new type literals in GHC: data NList (n::Nat) a where Rest :: NList 0 [a] (:>) :: a -> NList n a -> NList (n+1) a uncons :: NList (n+1) a -> NList n a uncons (x :> l) = l This gives a type error: /home/athas/code/nonempty_lists.hs:41:19: Could not deduce (n1 ~ n) from the context ((n + 1) ~ (n1 + 1)) bound by a pattern with constructor :> :: forall a (n :: Nat). a -> NList n a -> NList (n + 1) a, in an equation for `uncons' at /home/athas/code/nonempty_lists.hs:41:9-14 `n1' is a rigid type variable bound by a pattern with constructor :> :: forall a (n :: Nat). a -> NList n a -> NList (n + 1) a, in an equation for `uncons' at /home/athas/code/nonempty_lists.hs:41:9 `n' is a rigid type variable bound by the type signature for uncons :: NList (n + 1) a -> NList n a at /home/athas/code/nonempty_lists.hs:40:11 Expected type: NList n a Actual type: NList n1 a In the expression: l In an equation for `uncons': uncons (x :> l) = l I don't understand why GHC cannot infer that the two types are the same. My guess is that 'n+1' is not "structural" to GHC in the same way that 'S n' is. The page http://hackage.haskell.org/trac/ghc/wiki/TypeNats/MatchingOnNats mentions that "type level numbers of kind Nat have no structure", which seems to support my suspicion. What's the complete story, though? -- \ Troels /\ Henriksen

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ANNOUNCE: cmaes, a generic optimizer
by Takayuki Muranushi 02 Oct '12

02 Oct '12

Hello, I'm pleased to announce "cmaes" http://hackage.haskell.org/package/cmaes , a library for optimization (in mathematical sense). That is, given a function f :: a->Double, to find x of type a so that (f x) is as small as possible. CMA-ES is an optimization algorithm developed by Nikolaus Hansen and his folks http://www.lri.fr/~hansen/cmaesintro.html . It is a stochastic approach, it does not require gradient information. These properties makes it robust against noise and makes it widely applicable. The cmaes packages is a Haskell wrapper to their elaborate Python code. The robustness of CMA-ES and generic programming put together, you can now describe optimization problems for various types of functions just in one line. CMA-ES is not a silver bullet, of course, but it's been working fine for many application including mine, and I hope this library will drastically reduce the initial cost of trying one for you. Type will tell what cmaes can do. Please refer to the running document http://hackage.haskell.org/packages/archive/cmaes/0.2.1/doc/html/Numeric-Op… . You can optimize functions of type ([Double]-> Double) and ([Double]-> IO Double) , that's usual. Next up, you can optimize (Traversable t => t Double-> Double) and (Traversable t => t Double-> IO Double) where you are pretty sure about which parameter you're tuning. Finally, you can do (Data a => a -> Double) and (Data a => a -> IO Double) , where SYB will find the hidden Doubles to be optimized over for you. It was my joy to see optimization of function of type such as (Triangle -> Double) works out of the box. Any comments, suggestions, bug reports are welcome! Thanks, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Copying a syntax tree
by Sergey Mironov 02 Oct '12

02 Oct '12

Hi! I have a syntax tree defined like this: > > data A a = A Int Int (B a) > > data B a = B String String (C a) > > data C a = C Int Int (D a) > and so on, all the data are parametrized with a type variable. This variable is actually used as a field in the very end of a hierarchy: > > data D a = D Int a > Now I have to write a function which would copy (A Int) to (A String). Is it possible to do so using TH/syb without writing copyA (A i1 i2 b) = A i1 i2 (copyB b) copyB = ... copyC = ... .. copyD (D i a) = D i (show a) ? Could you provide me with a hint? Thanks, Sergey

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Possible bug in Criterion or Statistics package
by Till Berger 02 Oct '12

02 Oct '12

Dear all, I may have stumbled upon a bug in the Criterion package. When running the attached Haskell program (Benchmark.hs, a simple test case) on multiple cores (with +RTS -N, +RTS -N2, +RTS -N3 etc.) it sooner or later crashes with the following exception: Benchmark: thread blocked indefinitely in an MVar operation With profiling support enabled and run with the xc flag I get the following output before the crash: *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF --> evaluated by: Main.main, called from Main.CAF *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF I have tested this with GHC versions 7.0.4 and 7.4.2 and Criterion 0.6.0.1. So I am not sure if this is a bug in Criterion itself, the Statistics package or any dependency or if I am doing something obviously wrong. I would be grateful if someone could look into this as it is holding me back from using Criterion for benchmarking my code. Regards, Till Berger

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ANNOUNCE: Sylvia, a lambda calculus visualizer
by Chris Wong 02 Oct '12

02 Oct '12

Hello all Some of you in the audience may have read Dave Keenan's paper, [To Dissect a Mockingbird][]. A subset of that may have wondered if it was possible to generate those pretty pictures programmatically. For that subset, I can answer to you -- yes, yes you can. [To Dissect a Mockingbird]: http://dkeenan.com/Lambda/ Sylvia is a lambda calculus visualizer. It takes in an expression in the untyped lambda calculus and spits out a pretty picture. This is still in very early alpha, but it renders a fair number of combinators correctly. I plan to add animated reduction (once I figure out how to do it), and eventually develop this into a sandbox game of some sort. I'm hoping to get some comments and ideas on how I can take it from here. Obligatory links ---------------- Hackage: http://hackage.haskell.org/package/sylvia Source: https://github.com/lfairy/sylvia Documentation: https://github.com/lfairy/sylvia/wiki ----------------

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Monads
by Vasili I. Galchin 01 Oct '12

01 Oct '12

Hello, I would an examples of monads that are pure, i.e. no side-effects. Thank you, Vasili

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List indexer
by Alex Stangl 01 Oct '12

01 Oct '12

Hi, I have written a small "wrapper" to speed random-access to a list. The usage scenario I have in mind is a "stream" computation yielding an infinite list, and I want to randomly access elements w/o having to traverse the entire list from the beginning for each access. I suspected something similar must already exist, but nothing I looked at seemed to do the trick. IntMap seems to want a finite input list. Ditto for the various array types, except possibly dynamic array. Attached is the list indexer I came up with, and a small test program (I swap the commented-out lines to switch btw. list & list index tests). I am interested to hear any feedback on this -- whether it duplicates something that already exists, or whether there's a better approach, and comments on the code, etc. Also if somebody can suggest a better name (so as not to overlay the word index too much.) I'll publish it on hackage (or at least github) if people think it's useful. It sped up the program I initally wrote it for enormously. Thanks, Alex

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