July 2014 - Haskell-Cafe

OverlappingInstances-extension-like for type families
by Leza Morais Lutonda 23 Jul '14

23 Jul '14

Hello haskell-café, I just started working with type families, and come with some difficulties. Now I have a class: class (...) ⇒ Complexable s where type ComplexSignalType s ... Is there a possibility to have this two instances (in any way)? instance (RealFloat e) ⇒ Complexable [e] where type ComplexSignalType [e] = [ℂ e] instance (RealFloat e) ⇒ Complexable [ℂ e] where type ComplexSignalType [ℂ e] = [ℂ e] Thanks. -- Leza Morais Lutonda, Lemol-C http://lemol.github.io 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu

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How many Haskell programmers are there?
by Johan Larson 23 Jul '14

23 Jul '14

Obviously this is a very fuzzy question. But what's the credible range of answers? Here's one: http://steve-yegge.blogspot.ca/2010/12/haskell-researchers-announce-discove… 38 -- Johan Larson -- Toronto, Canada

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Looking for list comprehensions use cases
by Jan Stolarek 23 Jul '14

23 Jul '14

Haskellers, recently I've been looking into the possibility of creating some new optimisations for GHC. These would be mostly aimed at list comprehensions. Here's where I need your help: 1. Do you have complex list comprehensions usage examples from real code? By complex I mean nested list comprehensions, reading from more than one list ([ ...| x <- xs, y <- ys ... ]) etc. 2. Do you have list comprehensions code that you had to optimize by hand because GHC was unable to make them fast enough? Janek

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Call for participation: Haskell tutorial at CUFP on 4 Sep 2014
by Stefan Wehr 22 Jul '14

22 Jul '14

Dear Haskellers, I'll be giving an advanced Haskell tutorial as part of the CUFP 2014 workshop. * Date/time: 4 Sep 2014, 9:00 AM - 12:30 PM * Place: Gothenburg, Sweden (affiliated with ICFP 2014) * Early registration deadline: 3 Aug 2014! The tutorial teaches important techniques for writing correct, robust, scalable, and fast Haskell programs. The topics covered are: network programming, serialization, persistence, logging, performance, scalability, testing, and concurrency. The tutorial aims at programmers with some Haskell experience. It is not necessary to be an Haskell expert, but you should know how to write functions and data types. For more details, see http://cufp.org/2014/t2-stefan-wehr-haskell-in-the-real-world.html Registration page: https://regmaster4.com/2014conf/ICFP14/register.php Please distribute this call for participation. See you in Gothenburg! Stefan

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Elementary algorithms - (Haskell language is used)
by Xinyu LIU 22 Jul '14

22 Jul '14

Hi, I spent 6 years to write the book ‘Elementary Algorithms’. I finally completed it yesterday. This book is open and free, the PDF version can be downloaded from: https://github.com/liuxinyu95/AlgoXY/blob/algoxy/preview/elementary-algorit… Why There are plenty of books about algorithms, such as ‘Introduction to algorithms’, ‘The art of computer programming’, ‘structure and interpretation of computer programs’… why another book? Is it reinvention of wheel? This book can’t compare with the above classic bibles at all. It has some features like: 1. All algorithms are described in math formulas and pseudo codes. I hope it bring some taste of elegant by using algebraic symbols. 2. All algorithms are realized in both purely functional and imperative approaches. 3. The examples are provided in multiple programming languages, including C, Haskell, Python, C++, Scheme/Lisp. Haskell is the main language for all functional implementations. Content The following content are covered in this book (in the order of appearance) • Binary search tree, insertion sort, red-black tree, AVL tree, Trie, Patricia, suffix tree, B-tree; • Binary heap, Leftist heap, skew heap, splay heap, selection sort, binomial heap, Fibonacci heap, pairing heap; • Queue, Finger tree based sequence, quick sort, merge sort, binary search, saddle-back search, KMP, Boyer-Moore, DFS, BFS, Greedy algorithm, Dynamic programming. There are two books influenced me most, one is ‘Purely functional data structure’ by Chris Okasaki, the other is ‘Introduction to algorithms’. Problems I always don’t have enough time in writing this book during the past 6 years. I am not a native speaker, the biggest problem is about English. It is lack of proof reading by native speakers. I used some result directly, if I had time, I should add more proof for those results. Future work Ideally, I think a serious algorithm book should be written in a stable environment. I planned to write this book in 10 years. But everything changes so fast in modern society. I experienced lay-offs once in 2012 due to Symbian business cut off, and now again in MS. I am not sure if I can continue this work. I think the best way is to open this book to the community. Some future work in my mind: • Proof reading and review • Provide answers to the exercises • Provide proof for the directly referenced result The whole manuscript is available at github: https://github.com/liuxinyu95/AlgoXY Best regards -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/ https://github.com/liuxinyu95/AlgoXY *e*^(*π*i)+1 = 0

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GHCI on ARM (Nvidia Jetson TK1)
by Tomas Korcak 21 Jul '14

21 Jul '14

Hi There, I would like to ask you what is the situation with ghci on arm. I bought Jetson TK1 and I am really sad that ghci does not works there. I have found mentioned somewhere that this should be fixed in 7.4.2 but still it is not. Can somebody clarify what is the situation at the moment and what are further plans? Best regards, T. -- <warning> This e-mail is intended for the named recipient(s). It may contain privileged and/or confidential information. If you are not one of the intended recipients, please notify the sender immediately and destroy this e-mail and attachment(s): you must not copy, distribute, retain or take any action in reliance upon the email or attachment(s). While all reasonable efforts are made to safeguard inbound and outbound e-mails, Tomas Korcak cannot guarantee that attachments are virus-free or are compatible with your systems, and does not accept liability in respect of viruses or computer problems experienced. Thank you. </warning> <notice> Your Skills In Reading Have Improved +1 </notice> <idea> Some days you're the dog; some days you're the hydrant. </idea>

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What is the state of the art in testing code generation?
by oleg＠okmij.org 21 Jul '14

21 Jul '14

Tom Ellis wrote: > The first impediment to using QuickCheck is that my EDSL is typed. I don't > know how to generate random terms in the presence of types. First I should point out Magic Haskeller (which generates typed (Haskell) terms and then accepts those that satisfy the given input-output relationship). http://nautilus.cs.miyazaki-u.ac.jp/~skata/MagicHaskeller.html It is MagicHaskeller on Hackage. The most straightforward method is to generate random terms and filter well-typed ones. This is usually a bad method since many or most randomly generated terms will be ill-typed. One should generate well-typed terms from the beginning, without any rejections. That is actually not difficult: one merely needs to take the type checker (which one has to write anyway) and run it backwards. Perhaps it is not as simple as I made it sound, depending on the type system (for example, the type inference for simply-typed lambda-calculus without any annotations requires guessing. One has to guess correctly all the type, otherwise the process becomes slow). It has been done, in a mainstream functional language: OCaml. The code can be re-written for Haskell using the tricks for non-determinism with sharing (the Share monad). Due to many deadlines I cannot re-write myself, not until the middle of the next week. Also, I don't know your type system. If you are curious about the OCaml code, it can be found at http://okmij.org/ftp/kakuritu/type_inference.ml The main function is let rec typeof : gamma -> term -> tp = fun gamma exp -> match exp () with | I _ -> int | V name -> begin try List.assoc name gamma with Not_found -> fail () end | L (v,body) -> let targ = new_tvar() in let tbody = typeof ((v,targ) :: gamma) body in arr targ tbody | A (e1,e2) -> let tres = new_tvar() in tp_same (typeof gamma e1) (arr (typeof gamma e2) tres); tres It can infer a type of a term (or fail if it is will-typed), it can generate well-typed terms one-by-one, or it can generate all terms of a particular structure (e.g., with two lambdas at the top) and/or of a particular type or type shape (e.g., an arrow).

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Problems with type family in a class
by Leza Morais Lutonda 21 Jul '14

21 Jul '14

Hi, Consider the following type classes: class (Signal sx) => Complexable sx where type ComplexSignalType .... class (Complexable sx) => FourierTransformable sx where fft :: (cx ~ ComplexSignalType sx) => cx -> cx .... and an instance: instance (RealFloat e) => Complexable [e] where type ComplexSignalType = [Complex e] instance (RealFloat e) => FourierTransformable [e] where fft = ... Why this example will give errors: *DSP.Signals.Core Prelude Data.Complex> :t ax ax :: [Complex Double] *DSP.Signals.Core Prelude Data.Complex> fft ax <interactive>:90:1: No instance for (FourierTransformable s0 [Complex Double]) arising from a use of ‘fft’ The type variable ‘s0’ is ambiguous Note: there is a potential instance available: instance [overlap ok] (RealFloat e, Math.FFT.Base.FFTWReal e) => FourierTransformable [e] [Complex e] -- Defined at src/DSP/Signals/Instances/List.hs:40:10 In the expression: fft ax In an equation for ‘it’: it = fft ax Thanks! -- Lemol-C 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu

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Extensible Effects & Multiple Operations
by Justin Bailey 21 Jul '14

21 Jul '14

I am playing around with the extensible-effects framework and cannot figure out how to solve this problem. I am trying to define a set of operations that only work after a file has been opened. For example, reading a character from a file or detecting EOF. My problem is I cannot figure out how to define one "runner" for my Effect that can deal with operations with different result types. For example, I would like my `readFrom` and `atEOF` operations to have the following types: data OpenFile r v = ... -- my "OpenFile" effect -- read a character from the file readFrom :: Eff r Char -- detect EOF atEOF :: Eff r Bool And I would like to use this operations as follows (where I read a byte from the file, check EOF, and return the concatenation of the two values): main = do c <- runOpenFile "Setup.hs" (do a <- readFrom -- `a` has `Char` type eof <- atEOF -- `eof` has `Bool` type return $ [a] ++ show eof) putStrLn c However, I cannot define a `runOpenFile` that compiles. For example, this definition gives me "Overlapping Instance" errors: runOpenFile :: FilePath -> Eff (OpenFile v :> r) result -> IO result The best I've come up with is to define a `Result` type that holds all possible results: data Result = AtEOF Bool | ReadChar Char The `Result` type then lets me put a single type on the `OpenFile` effect in `runOpenFile`: runOpenFile :: FilePath -> Eff (OpenFile Result :> r) result -> IO result But that makes my `main` program pretty ugly, because I have to deconstruct those result types each time: main = do c <- runOpenFile "Setup.hs" (do (ReadChar a) <- readFrom (AtEOF eof) <- atEOF return $ [a] ++ show eof) putStrLn c Does anyone have suggestions? Justin p.s. The complete code (in a sort-of similar form) can be seen at https://github.com/m4dc4p/ext-eff-sample/blob/master/src/OpenFile.hs

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Cabal question
by Jurriaan Hage 21 Jul '14

21 Jul '14

Hi there, I am busy making the Helium compiler available as a Cabal install. What I still need is a way to run some kind of postprocessing (like running a Makefile) after installation (as it happens this is for compiling the libraries that come with Helium). Does anyone know here whether that is supported and how, without having to resort to build-types like Make that for I actually want to avoid? best, Jur

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