March 2008 - Haskell-Cafe

ANN: haskell-src-exts 0.3.2
by Niklas Broberg 18 Mar '08

18 Mar '08

Hi all, I'm pleased to announce a new release for the haskell-src-exts package. haskell-src-exts 0.3.2 =========================== haskell-src-exts is a package for handling and manipulating Haskell source code. It is based on the haskell-src package that is part of the standard libraries, but extends this to support a number of syntactic extensions, e.g. MPTCs, fundeps, GADTs, TH etc. It is intended as a drop-in replacement for the standard haskell-src package, and exports the same functions and data types, plus some more. Apart from the more standard extensions supported by e.g. GHC, haskell-src-exts also provides support for HaRP (Haskell Regular Patterns) and HSX (Haskell Source with XML) syntax. Note that as of 0.3, haskell-src-exts /= HSX. * cabal sdist: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts… * darcs repo: darcs get http://code.haskell.org/HSP/haskell-src-exts === Changes from 0.2: === * Added support for - Indexed type families (including associated types/datatypes) - Explicit kind signatures - Standalone deriving * haskell-src-exts is now decoupled from hsx/trhsx and harp. - Modules renamed to Language.Haskell.Exts.* - Module Transform is removed from the package (now in package hsx) * New repository (i.e. darcs pull in an old repo won't work, use darcs get), containing only the haskell-src-exts package (no hsx or harp). * Builds with 6.8.2 (thanks Duncan Coutts) === Complete list of supported extensions === * Multi-parameter type classes (MPTCs) * Functional dependencies * Associated types, indexed type families * Liberal class and instance heads * Implicit parameters * Explicit kind signatures * Pattern guards * Generalized algebraic data types (GADTs) * Template Haskell (TH) * Universal and existential quantification (forall) * Empty data type declarations * Unboxed tuples (# #) * Standalone deriving * Regular patterns * Haskell XML, HSX style === Build Requirements === * happy >= 1.17 - It might work with 1.16 though I haven't tested. In that case change the cabal file. - It would work with older versions as well, though they insert a dependency on Array (haskell98) instead of Data.Array. If that's all you have to work with, update the cabal file with a dependency on haskell98. * Cabal >= 1.2 Patches are more than welcome. :-) Cheers, /Niklas

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Haskell board game, anyone?
by Benjamin L. Russell 18 Mar '08

18 Mar '08

Maybe off topic, but when I saw this, I thought, "Why isn't there a Haskell version of this game?" (Or is there?) C-jump is a neat idea, mapping something fun (downhill skiing) with programming. I look at this game and wonder what it would look like in the wonderful world of higher-order functions. Begin forwarded message from "Geoff Knauth" (with subject "[plt-scheme] Scheme board game, anyone?" on "PLT-list Mailing List" <plt-scheme(a)list.cs.brown.edu>, dated "Mon, 17 Mar 2008 12:38:32 -0400"): > Maybe off topic, but when I saw this, I thought, "Why > isn't there a > Scheme version of this game?" (Or is there?) > > C-jump is a neat idea, mapping something fun (downhill > skiing) with programming. I look at this game and > wonder what it would look like > in the wonderful world of continuations. > > Begin forwarded message from Chase Turner: > > > Subject: Is there a Scheme version of this game? > > > > http://www.c-jump.com Benjamin L. Russell

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problem with type equality constraints
by Ganesh Sittampalam 18 Mar '08

18 Mar '08

Hi, When I try to compile this code with ghc-6.9.20080310: module Test2 where type family Id a type instance Id Int = Int type instance Id (a, b) = (Id a, Id b) class Id a ~ ida => Foo a ida instance Foo Int Int instance (Foo a ida, Foo b idb) => Foo (a, b) (ida, idb) I get these errors: Test2.hs:12:0: Couldn't match expected type `ida' against inferred type `Id a' `ida' is a rigid type variable bound by the instance declaration at Test2.hs:12:16 When checking the super-classes of an instance declaration In the instance declaration for `Foo (a, b) (ida, idb)' Test2.hs:12:0: Couldn't match expected type `idb' against inferred type `Id b' `idb' is a rigid type variable bound by the instance declaration at Test2.hs:12:27 When checking the super-classes of an instance declaration In the instance declaration for `Foo (a, b) (ida, idb)' It seems to me that since Foo a ida and Foo b idb are superclassess, Id a ~ ida and Id b ~ idb should be known and so this should have worked - am I missing something? Cheers, Ganesh

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Network.Browser and getCookies
by Adam Smyczek 18 Mar '08

18 Mar '08

Somehow I cannot get cookies from the Response using Network.Browser module (HTTP 3001.0.4). The cookie header part seams to be empty and getCookies returns empty list as well. Any idea how to debug this or does someone have a working example? Thanks, Adam

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2-level type analysis of introducing naming into data types
by Greg Meredith 17 Mar '08

17 Mar '08

All, The following Haskell code gives a 2-level type analysis of a functorial approach to introducing naming and name management into a given (recursive) data type. The analysis is performed by means of an example (the type of Conway games). The type is naturally motivated because through the application of one functor we arrive at a lambda calculus with an embedded type for Conway games (giving access to field operations for arithmetic on virtually every type of number ever considered) -- while another functor provides a process calculi with a similarly embedded data type. Moreover, the technique extends to a wide variety of algebra(ic data type)s. The recipe is simple. 1. Provide a recursive specification of the algebra. Example. data Game = Game [Game] [Game] 2. Abstract the specification using parametric polymorphism. Example. data PolyGame g = PolyGame [g] [g] 2.a. Identify the 2-Level knot-tying version of the recursive type Example. data RGame = RGame (PolyGame RGame) 3. Insert name management type into the knot-tying step of the 2-Level version Example. data DefRefGame var = DefRefGame (PolyDefRef var (PolyGame (DefRefGame var))) given the basic form data PolyDefRef var val = Def var (PolyDefRef var val) (PolyDefRef var val) | Ref var | Val val is the name management technology We illustrate the idea with two other forms of name management technology: the lambda calculus and the pi-calculus. Then we show that names themselves can be provided as the codes of the terms of the new type. At the root of this work is the proposal that communication begins from the concrete -- a "closed" recursive type -- representing some observation or proposal of how something works in the universe (of discourse). As said proposal develops or is explored, calculations need to rely more and more on naming (i.e. "let x stand for ... in ..."). Thus, this capability is "injected" into the data type -- moving from the concrete to the general. The analysis above provides a concrete, disciplined procedure for achieving this in both a sequential and concurrent computational setting. Best wishes, --greg -- This code summarizes the previous post. It shows that for any -- "closed" recursive type we can devise a closed extension of this -- type embedding the type as a value in the lambda calculus module Grho( Game, PolyGame, PolyDefRef, DefRefGame, QDefRefGame ,Term, GTerm, QGTerm ,Sum, Agent, Process, GProcess, QGProcess ) where -- Conway's original data type data Game = Game [Game] [Game] deriving (Eq,Show) -- Abstracting Conway's data type data PolyGame g = PolyGame [g] [g] deriving (Eq,Show) -- Recovering Conway's data type in terms of the abstraction newtype RGame = RGame (PolyGame RGame) deriving (Eq,Show) -- RGame ~ Game -- Expressions with references and values data PolyDefRef var val = Def var (PolyDefRef var val) (PolyDefRef var val) | Ref var | Val val deriving (Eq,Show) -- Games with references and values data DefRefGame var = DefRefGame (PolyDefRef var (PolyGame (DefRefGame var))) deriving (Eq,Show) -- Games with references and values in which variables are quoted games newtype QDefRefGame = QDefRefGame (DefRefGame QDefRefGame) deriving (Eq,Show) -- Lambda terms with values data Term var val = Var var | Abs [var] (Term var val) | App (Term var val) [Term var val] | Value val deriving (Eq,Show) -- Lambda terms with games as values data GTerm var = Term var (PolyGame (GTerm var)) deriving (Eq,Show) -- Lambda terms with games as values and variables that are quoted lambda terms data QGTerm = GTerm QGTerm -- And the following defn's/eqns take it one step further by providing a notion of process with -- Conway games as embedded values -- Process terms with values -- Sums data Sum var val agent = PValue val | Locate var agent | Sum [Sum var val agent] deriving (Eq,Show) -- One of the recent observations i've had about the process calculi -- is that '0' is Ground, and as such is another place to introduce -- new Ground. Thus, we can remove the production for '0' and replace -- it with the types of values we would like processes to 'produce'; -- hence the PValue arm of the type, Sum, above. Note that this -- design choice means that we can have expressions of the form -- v1 + v2 + ... + vk + x1A1 + ... xnAn -- i am inclined to treat this as structurally equivalent to -- x1A1 + ... xnAn -- but there is another alternative: to allow -- sums of values to represent superpositions -- Agents data Agent var proc = Input [var] proc | Output [proc] deriving (Eq,Show) -- Processes data Process var sum = Case sum | Deref var | Compose [Process var sum] deriving (Eq,Show) -- Processes over games data GProcess var = Process var (Sum var (PolyGame (GProcess var)) (Agent var (GProcess var))) deriving (Eq,Show) -- Processes over games with quoted process for variables newtype QGProcess = QGProcess (GProcess QGProcess) deriving (Eq,Show) -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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floating point operations and representation
by Jacob Schwartz 17 Mar '08

17 Mar '08

I have two questions about using the Double data type and the operations in the Floating typeclass on a computer that uses IEEE floating point numbers. I notice that the Floating class only provides "log" (presumably log base 'e') and "logBase" (which, in the latest source that I see for GHC is defined as "log y / log x"). However, in C, the "math.h" library provides specific "log2" and "log10" functions, for extra precision. A test on IEEE computers (x86 and x86-64), shows that for a range of 64-bit "double" values, the answers in C do differ (in the last bit) if you use "log2(x)" and "log10(x)" versus "log (x) / log(2)" and "log(x) / log(10)". I am under the restriction that I need to write Haskell programs using Double which mimic existing C/C++ programs or generated data sets, and get the same answers. (It's silly, but take it as a given requirement.) If the C programs are using "log2", then I need "log2" in the Haskell, or else I run the risk of not producing the same answers. My first thought is to import "log2" and "log10" through the FFI. I was wondering if anyone on Haskell-Cafe has already done this and/or has a better suggestion about how to get specialized "log2" and "log10" (among the many specialized functions that the "math.h" library provides, for better precision -- for now, I'm just concerned with "log2" and "log10"). My second question is how to get at the IEEE bit representation for a Double. I am already checking "isIEEE n" in my source code (and "floatRadix n == 2"). So I know that I am operating on hardware that implements floating point numbers by the IEEE standard. I would like to get at the 64 bits of a Double. Again, I can convert to a CDouble and use the FFI to wrap a C function which casts the "double" to a 64-bit number and returns it. But I'm wondering if there's not a better way to do this natively in Haskell/GHC (perhaps some crazy use of the Storable typeclass?). Or if someone has already tackled this problem with FFI, that would be interesting to know. Thanks. Jacob

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STM example code
by Galchin Vasili 17 Mar '08

17 Mar '08

Hello, I am playing around with the STM API. I would like to see examples of STM other than the Santa.hs as I am having problems with STM vs IO. Thanks, Vasili

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Re: [Haskell-cafe] QuickCheck
by rodrigo.bonifacio 17 Mar '08

17 Mar '08

Hi all, Is it possible to define a limit for the size of children list bellow? I've tried: children <- resize (10 (listGen featureGenNormal)) But it didn't work. Thanks a lot, Rodrigo. > Sebastian Sylvan: > >> featureGenNormal = do > >> id <- stringGen > >> name <- stringGen > >> featuretype <- arbitrary > >> grouptype <- arbitrary > >> children <- arbitrary > >> properties <- listGen stringGen > >> return (Feature id name featuretype grouptype children properties) > > Ryan Ingram wrote: > > Also, you can shorten this significantly with liftM or ap (from Control.Monad): > > True, but in this case I like being able to see meaningful > names for each parameter of the constructor. > ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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Re: Haskell-Cafe Digest, Vol 55, Issue 20
by Maverick 17 Mar '08

17 Mar '08

Thanks Sterl, you're right, this was because I have don´t compiled with the -threaded flag, this works fine now. > This answer may be way off base, but if differences appear between > ghci and compiled versions, I've often found its as simple as > remembering to compile with the -threaded flag. The ghci runtime is > threaded by default, as I understand it, while compiled binaries are > not, and IO operations will block in very different fashions (i.e. in > their own thread, or stalling the entire app) depending on the runtime. > Regards, > sterl. Alvaro ______________________________________________ ¿Con Mascota por primera vez? Sé un mejor Amigo. Entra en Yahoo! Respuestas http://es.answers.yahoo.com/info/welcome

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QuickCheck
by rodrigo.bonifacio 17 Mar '08

17 Mar '08

Hi all, I'm trying to use the quick-check library for checking some properties of a user defined data type. Bellow the target data type: data Feature = Feature Id Name FeatureType GroupType Children Properties | FeatureError where: Id = String Name = String FeatureType = int GroupType = int Children = [Feature] Propertyes = [String] I've written the following quick-check property: prop_AlternativeFeature :: Feature -> Feature -> QuickCheck.Property prop_AlternativeFeature fm fc = length (children fc) == 0 ==> length (checkAlternativeFeature fm fc) > 0 When I try to check such property, the result is: ERROR "./EshopModelChecking.hs":11 - Type error in instance member binding *** Term : arbitrary *** Type : Feature *** Does not match : Gen Feature I think that I need to write some arbitrary or generator functions, but I didn't realize how to do that with the availalble quick-checking documentation. Any help will be welcome. Thanks in advance. Rodrigo.

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