February 2014 - Haskell-Cafe

ANN: generic-maybe-0.3.0.2
by Jonathan Fischoff 11 Feb '14

11 Feb '14

generic-maybe utilizes GHC.Generics to generalize the functions of Data.Maybe to arbitrary sum types with two constructors, with one "empty." Here is a quick example: λ> :set -XDeriveGeneric λ> import GHC.Generics λ> data Result a = Success a | Fail deriving (Show, Generic) λ> fromMaybe 'a' Fail 'a' λ> fromMaybe 'a' $ Success 'b' 'b' Additionally, it lets you convert between two representations: λ> convert (Just 'a') :: Result Char Success 'a' Documentation on Hackage: http://hackage.haskell.org/package/generic-maybe-0.3.0.2/docs/Data-Generics… -Jonathan Fischoff

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Lazy lists with with call-by-value reduction strategy.
by flicky frans 11 Feb '14

11 Feb '14

Hello. I am currently writing lists with lazy semantics in the pure lambda-calculus with call-by-value reduction strategy. Here is an example: http://pastebin.com/SvQ5hCSD Here is a simple interpetator: http://pastebin.com/mejCWqpu Am I reinventing the wheel? Are there some sources, from where i can learn more about lazy evaluation in the strict languages?

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Lazy Lambda Calculus implementation
by Chris Warburton 11 Feb '14

11 Feb '14

Hello all, I've written the following implementation of Lambda Calculus, which will evaluate its Terms non-strictly. I've verified this since the Y combinator works without diverging: data Term a = Const a | Var Nat | Lam (Term a) | Term a :@ Term a data Val a = C a | F (Partial (Val a) -> Partial (Val a)) type Env a = [Partial (Val a)] eval' :: Term a -> Env a -> Partial (Val a) eval' (Const c) env = Now (C c) eval' (Var n) env = let Just x = lookUp env n in x eval' (Lam f) env = Now (F (\a -> eval' f (a:env))) eval' (f :@ x) env = do F f' <- eval' f env Later (f' (eval' x env)) eval t = eval' [] t Nat, Partial, lookUp, etc. have pretty obvious implementations. My question is, will this behave lazily? In other words will the contents of the 'env' lists be shared between the different contexts, such that forcing an element the be evaluated twice will only perform the evaluation once? Note that the first "do" line is the only place where evaluation is forced. If anyone could point out an 'obvious' reason why it will or will not be shared, or approaches I can take to check or infer this myself (eg. a Term which would show wildly different RAM usage in each case), I'd be very interested to know. I'd also be interested if someone spots a bug ;) For those who are curious, the code is living at https://gitorious.org/lazy-lambda-calculus and I've written a blog post detailing the iteration's I've been through at http://chriswarbo.net/index.php?page=news&type=view&id=admin-s-blog%2Flazy-… Cheers, Chris

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A new yaml module
by Michael Snoyman 11 Feb '14

11 Feb '14

The yaml package[1] currently provides two modules. Text.Libyaml is a lower level, streaming API which is essentially a raw exposure of the underlying libyaml C library. Data.Yaml uses the aeson Value data type and ToJSON/FromJSON type classes for higher level serialization. For many cases, this approach works well, though there are problems: * There are problems with roundtripping, since YAML allows for ambiguity about the data type of values[2]. For example, in the yaml snippet `foo: 1234`, is 1234 intended to be numeric or a string? Either is valid. * YAML is intended to be human-readable output. But Data.Yaml provides no control over the encoded representation, e.g. should we use single or double quotes for a string (or no quotes at all), or the order of values in a mapping[3]. For other examples, just look at the issue tracker for yaml[4]. I don't want to drop the current aeson-based functionality, since I think it's still valid and useful in many cases. But I do think it's worthwhile to add in an alternative API which handles YAML-specific constructs better. My idea is: * Create a new Data.Yaml.Aeson module, and have it mirror Data.Yaml. * Deprecate Data.Yaml. * Create a new Data.Yaml.? module to contain this YAML-specific API. I'm asking for feedback on that last point. I have some basic ideas on what such an API would look like, but given that there are many people using YAML in ways different than how I'm using it, I don't think an API designed entirely by me will suit all use cases. I've opened up a new issue[5] to track this work. If you're interested in participating in this design, please contact me. I'm happy to have the discussion on this mailing list, but if (as I suspect) there are just a handful of people who are interested in pushing this forward, it likely makes sense to take the discussion offlist. Michael [1] http://hackage.haskell.org/package/yaml [2] https://github.com/snoyberg/yaml/issues/22 [3] https://github.com/snoyberg/yaml/issues/37 [4] https://github.com/snoyberg/yaml/issues [5] https://github.com/snoyberg/yaml/issues/38

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How Plugins works with a Cabal sandbox
by Joey 11 Feb '14

11 Feb '14

I was wondering, could somebody talk me through how the Plugins package works with a Cabal sandbox, or Cabal in general? Basically, I'm writing a program where I want to dynamically load and run user-supplied code. There are two cabal DBs in question: the sandbox which I'm using to build my program, and my "global" sandbox located in /home/user/.cabal. Which database, if either, does a program I load with System.Plugins.Load.load have access to via "import" statements? How can I avoid double-loading of packages? In particular, I'm currently getting this error when I try to load my code. Any thoughts? GHCi runtime linker: fatal error: I found a duplicate definition for symbol transformerszm0zi3zi0zi0_ControlziMonadziTransziError_mapErrorT1_closure whilst processing object file /home/joey/.cabal/lib/transformers-0.3.0.0/ghc-7.6.3/HStransformers-0.3.0.0.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. GHCi cannot safely continue in this situation. Exiting now. Sorry.

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Master's Thesis: Open areas of research in Haskell
by Tyler Huffman 10 Feb '14

10 Feb '14

Hello all! Beginning in September, I'll begin my Master's program in Computer Science, and I'm completely set on doing my thesis in some form of functional programming and/or Haskell. I've seen the Internship opportunities and areas of research for GHC (https://ghc.haskell.org/trac/ghc/wiki/Internships) but they all seem to apply to Ph.D candidates. That said, are there open areas of research that I could look into, or more resources in Haskell that would be useful for a two-year program's research? I'm hoping to have a set thesis by the time I begin my Master's program, which is why I'm giving myself six months in advance. Any help would be highly appreciated! Regards

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Haskell meeting in Regensburg, 2014-02-11, 20:00
by Andres Löh 10 Feb '14

10 Feb '14

Hi. Tomorrow, there will be another Haskell meeting in Regensburg, Germany: Tuesday, 11 February 2014 20:00 Binary Kitchen, Walderdorffstr. 13b, 93053 Regensburg There'll be no official talk or presentation, but the theme of the session is going to be "Performance". If you have Haskell sample code that runs slower than expected, feel free to bring it along. The plan is to discuss a number of small demo problems and consider best practices for improving performance of Haskell programs. Everyone who's interested in Haskell is welcome. No actual knowledge of Haskell is expected or required. We'll speak German and/or English, depending on who's going to be there. If you'd like to get updates about future Regensburg Haskell meetings or vote on when we meet next, please join our mailing list at http://lists.binary-kitchen.de/listinfo/haskell If you have any questions and for some reason don't want to join the mailing list, you can also email me :) Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

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haskell-search.org | research tools for Haskell
by m＠j4.pe 09 Feb '14

09 Feb '14

Hello, I would like to introduce http://haskell-search.org/. The idea is to index the most common Haskell sites to facilitate research on Haskell. It is a version for testing. If you like it, I can leave it running. Regards

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Package for regular expressions
by Daniil Frumin 09 Feb '14

09 Feb '14

Hi, everyone! Due to the recent discussions discussions about regex- packages on libraries@ and cafe, as well as due to the unicode bug in darcs that Erik Kow and I have stumbled upon I wanted to ask the Haskell community the following: What is up with the regular expression packages? Which one is the go-to package? There are really a bunch of them around (regex-compat, regex-tdfa, regex-posicx, regex-compat-tdfa,, ..) and it's not obvious at all what is the difference between them and which one should I use. Any advice or comments are appreciated. Sincerely yours, -- Daniil Frumin

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Installing Ghc 7.8 - ghc-split missing
by Magnus Therning 09 Feb '14

09 Feb '14

I've put together an experimental package of Ghc 7.8 for ArchLinux. It's pulling the sources from git (the branch ghc-7.8 from git.haskell.org) and it basically performs the following steps to build and install: ./sync-all -r git://git.haskell.org get -b ghc-7.8 ./boot ./configure --prefix=/usr make -j 5 make DESTDIR=<the packaging dir> install The build and install completes successfully, and some trivial tests work (compiling hello-world, playing a bit with ghci, that sort of thing). However, compiling mtl fails, with this message: Configuring mtl-2.1.2... Building mtl-2.1.2... Preprocessing library mtl-2.1.2... [ 1 of 21] Compiling Control.Monad.Writer.Class ( Control/Monad/Writer/Class.hs, dist/build/Control/Monad/Writer/Class.o ) ghc: could not execute: /usr/lib/ghc-7.8.0.20140204/ghc-split And indeed, ghc-split is not installed using `make install`. Is this a known bug? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus I invented the term Object-Oriented, and I can tell you I did not have C++ in mind. -- Alan Kay

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