August 2010 - Haskell-Cafe

Partial Signatures with Implicit Parameters
by Alex Rozenshteyn 30 Aug '10

30 Aug '10

I would like to specify that a function takes implicit parameters, without specifying its full return type. My main motivation for this is my xmonad config file and the attempt to remove the need for NoMonomorphismRestriction and some of the code smell associated with global variables that wafts in with implicit parameters. I have read this: http://okmij.org/ftp/Haskell/partial-signatures.lhs and even somewhat understood it; however, I tried to apply it to my config file with an implicit parameter in the signature, and I couldn't get it to compile. Is there something inherently different about using this approach with implicit parameters, or am I probably just doing something wrong? P.S. I decided to ask this here instead of in the xmonad mailing list because I feel like this is a question about haskell that was only slightly inspired by my use of xmonad. -- Alex R

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getting started with HJscript?
by Günther Schmidt 30 Aug '10

30 Aug '10

Hi all, I'll probably have to be doing web stuff soon. And I think I would like to be using JavaScript far more than I have in the past. So I'm thinking about using Haskell (HJscript) to generate it for me. How does on go about learning how to use HJscript? Are there any examples / tutorials? Günther

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Contibution to Haskell world
by Николай Кудасов 30 Aug '10

30 Aug '10

Hi all, I am very interested in haskell and most of related things and as I know there are a lot of things to do for haskell world. I have rather small, as I think, expirience with haskell: I've worked with Language.C, alex, happy, parsec and some other stuff that I haven't looked though attentively. Most of haskell code I've seen was highly readable and most of code I wrote was very easy to write. So I liked to write in haskell and I want to make some contribution to its world. As I am rather new to haskell world I ask you for a problem that can be interesting for me and useful for haskell world. Today I am student at Moscow State University and have at least 2 years for some kind of scientific work. P.S. Some interesting topics for me are transparent parallelization, parallel and distibuted computing and computational models. However, many other topics are also interesting for me =) Thanks in advance, Nick

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Higher-order algorithms
by Eugene Kirpichov 30 Aug '10

30 Aug '10

Most of the well-known algorithms are first-order, in the sense that their input and output are "plain" data. Some are second-order in a trivial way, for example sorting, hashtables or the map and fold functions: they are parameterized by a function, but they don't really do anything interesting with it except invoke it on pieces of other input data. Some are also second-order but somewhat more interesting: * Fingertrees parameterized by monoids * Splitting a fingertree on a monotonous predicate * Prefix sum algorithms, again usually parameterized by a monoid or a predicate etc. Finally, some are "truly" higher-order in the sense that is most interesting to me: * The Y combinator * Difference lists Do there exist other nontrivial higher-order algorithms and datastructures? Is the field of higher-order algorithms indeed as unexplored as it seems? I mean that not only higher-order facilities are used, but the essence of the algorithm is some non-trivial higher-order manipulation. For example, parser combinators are not so interesting: they are a bunch of relatively orthogonal (by their purpose) combinators, each of which is by itself quite trivial, plus not-quite-higher-order backtracking at the core. For example, for the Y combinator and difference lists are interesting: the Y combinator builds a function from a function in a highly non-trivial way; difference lists are a data structure built entirely from functions and manipulated using higher-order mechanisms. -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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On to applicative
by michael rice 30 Aug '10

30 Aug '10

I'm looking at a discussion of Either (as functor) here: http://learnyouahaskell.com/making-our-own-types-and-typeclasses#the-functo… instance Functor (Either a) where fmap f (Right x) = Right (f x) fmap f (Left x) = Left x And this line in Data.Either Functor (Either a) but no fmap defined here. How come? Michael

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Using GHC Api to affect compilation of a module
by Matias Hernandez 29 Aug '10

29 Aug '10

Hello people, I'm evaluating the use of the GHC API (6.12.3) to piggyback into the compilation process (in order, for example, to implement a custom optimization) and I'm having a lot of trouble doing so. In particular, I tried to modify a program being compiled, by working on its Core representation. The idea is that the skeleton of the program would be something like: runGhc (Just libdir) $ do ... c <- compileToCoreModule targetFile compileCoreToObj False (modify c) ... with modify :: CoreModule -> CoreModule being, for now, a rather trivial (identity) function. The first issue I've found is that compileCoreToObj fails (the error is "expectJust mkStubPaths"). However, I've also found that when compileToCoreModule ends, the .hi and .o files have already been produced, which kind of defeats any attempt to change the core code and then proceeding with the rest of the compilation. Am I doing this right? By scanning over the GHC API documentation those functions seemed to be the appropriate ones to start testing with, but I guess there's something trivial (or not) I'm missing. I'd be really thankful for any input. Best regards, - Matias H.

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Looking for a course using Haskell in the Tokyo area
by DekuDekuplex＠Yahoo.com 29 Aug '10

29 Aug '10

Does anybody know where I can find a course (preferably at the graduate level) using Haskell in the Tokyo area? I searched through "Haskell in education - HaskellWiki" (see http://www.haskell.org/haskellwiki/Haskell_in_education) but was unable to find anything. In particular, if anybody is aware of any courses using Haskell at either the Tokyo Institute of Technology (see http://www.titech.ac.jp/english/) or the University of Tsukuba (see http://www.tsukuba.ac.jp/english/) I would be especially interested. The closest I was able to find was a graduate course in Logic in Computer Software at the University of Tsukuba using OCaml, but I am not especially fond of the syntax of OCaml after having become familiar with Haskell. -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Re: [Haskell-cafe] Unicode pretty-printing
by Peter Gromov 29 Aug '10

29 Aug '10

Thanks for getting back to me. I was imprecise, by UTF8 characters I mean Unicode. My source files are UTF8-encoded, and Haskell reads them fine, it only has problems outputting them in a readable way. At this point I'm not talking of any I/O besides plain console output. Not using Show is not that of a choice, since I'm using HUnit which uses Show and prints the test results via the standard output functions. I've tried to wrap my strings and redefine Show so that it doesn't escape anything, but the standard output functions don't accept that, and HUnit doesn't know anything about System.IO.UTF8: ---- import System.IO.UTF8 import qualified System.IO import Test.HUnit newtype UString = UString String instance Show UString where show (UString s) = s instance Eq UString where (==) (UString s1) (UString s2) = s1 == s2 test1 = TestCase (assertEqual "fail" (UString "абв") (UString "где")) main = System.IO.hSetBinaryMode System.IO.stdout True >> System.IO.UTF8.putStrLn "это тест" --------- Prelude> :load utest.hs [1 of 1] Compiling Main ( utest.hs, interpreted ) Ok, modules loaded: Main. *Main> main это тест *Main> runTestTT test1 ### Failure: fail expected: *** Exception: <stderr>: hPutChar: invalid argument (Illegal byte sequence) --------- I've tried replacing UString X in the test with Data.Text.pack X and even desperately with Data.Text.Encoding.encodeUtf8 (Data.Text.pack X), but no dice. Though this time instead of crashes I get the good old escapes. On 29 August 2010 00:09, Yitzchak Gale <gale(a)sefer.org> wrote: > Peter Gromov wrote: >> Unfortunately, Haskell escapes UTF8 characters. > > What do you mean by "UTF8 characters"? > > Each element of the Char type represents a single Unicode > character, not encoded in UTF-8 or any other encoding. > > When you read a text file using the traditional IO functions, > recent versions of GHC will use the encoding of the > "current locale" (whatever that means on your system) > to decode the input into Unicode, unless you specify > otherwise. The same is true for writing to the console or > to a file. > > As Don pointed out, you may be interested in > using the newer Data.Text instead, especially when > encodings matter to you. It will usually be faster than > traditional IO, and it is designed to be the new standard > for representing text in Haskell. > > A third option would be to read the data as raw binary > bytes, without any decoding, using Data.ByteString. > Then it is totally up to you to do any decoding or > encoding. > > In any case, the standard Show instances will not be > able to do a very good job of displaying non-ASCII > characters; Show cannot make very many assumptions > about your data or your environment. As Don suggested, > you may want to define your own type class similar to > Show that does what you want. > > Regards, > Yitz >

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Unicode pretty-printing
by Peter Gromov 28 Aug '10

28 Aug '10

Hi all, I've been considering using Haskell for my natural language processing project. Due to its nature, it has much to do with Unicode. Unfortunately, Haskell escapes UTF8 characters. I've been able to output these strings via System.IO.UTF8.putStrLn (though I wish it was less painful), but still there are two problems. First, I want to pretty-print not only strings, but any structures made of lists and algebraic data types. Their 'show' calls string's 'show' which escapes all UTF8 symbols. The second problem is that I'm not the only one who wants to output the structures. For example, I want to have tests which check that certain operations produce certain natural language strings. So, I considered using HUnit for that. When its assertEqual fails, it displays the expected versus actual data, which is fine. But, of course, it doesn't know anything about Unicode, and I can't read escaped strings very well to understand what exactly has failed. I really don't want to write yet another unit test framework only for the sake of UTF8. Is there any way to make Haskell display values containing Unicode strings in a readable way? Peter

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(automatic) type classes context inference
by João Paulo 28 Aug '10

28 Aug '10

Hello everyone, I am developing a toolset in which I have several (multiparameter) type classes; It is often the case that I can only define a data-type X as an instance of one such class (say A), if X is an instance of another class (say B); The thing is that, while it is hard for me, because of all the type parameters that I have to deal with, to add 'X is an instance of B' to the context cxt_A in 'instance cxt_A => A X' ghc is always able to correctly infer all type parameters; In fact, I always get: 'Could not deduce (B X t1 ... tn) from the context cxt_A arising from ... Probable fix: add (B X t1 ... tn) to the context cxt_A ...' In my case, this is the fix that I always need: most of the times, I am just copy-pasting (B X t1 ... tn) to cxt_A! Is there a way, say a compilation option, to avoid this? can anyone please help me here? :) thank you very much -- João Paulo Fernandes Universidade do Minho www.di.uminho.pt/~jpaulo

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