- Haskell-Cafe - Haskell.org

Merging SMT solving and programming languages: two EDSLs for SMT in Haskell
by Don Stewart 19 Jan '11

19 Jan '11

Hey all, For a while now, Galois has been interested in connecting automated solvers to our programming language of choice, Haskell, to make it possible to prove automatically some properties of our functions (rather than just testing, e.g. with QuickCheck). We've pushed two efforts out this week, as previews of what we're thinking in this space: * SBV; and http://hackage.haskell.org/package/sbv * yices-painless. http://hackage.haskell.org/package/yices-painless Both are embedded DSLs for representing propositions to an SMT solver via Haskell functions and values. They take different approaches (a compiler from Haskell to the SMT-LIB format, versus an interpreter for the Yices SMT solver). SBV is the more mature package, while yices-painless emphasizes a type-preserving translation from a minimal core language. SBV was built by Levent Erkok, yices-painless by Don Stewart. Documentation for the design of yices-painless is available, as is documentation on SBV. Both are ready for experimentation and feedback, and we welcome your comments. More information is on the blog: http://corp.galois.com/blog/2011/1/18/merging-smt-solvers-and-programming-l… -- Don

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H98, OOHaskell - getting started with objects in Haskell
by gutti 19 Jan '11

19 Jan '11

Hi, I wanted to check whether Haskell offers reasonably easy object oriented programming -- I already had a look into "Haskell's overlooked object system" und "A Gentle Introduction to Haskell 98" [H98] There are a few things where I could use advice with: 1. OOHaskell doesn't seem to be available in the HackageDB (cabal) -- so how do I install it especially on Debian ? 2. is OOHaskell the right way to go - is it standardised (H98) or is it extension and is it widely used and recommended ? 3. Haskell 98 offers datatypes and some sort of monadic classes -- is it possible to build simple objects without OOHaskell ? 4. When I want object properties to change, when should I use IORef when STRef ? Thanks in advance, Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/H98-OOHaskell-getting-started-with-obj… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANN: extcore 1.0
by Tim Chevalier 19 Jan '11

19 Jan '11

Hello, I've recently released version 1.0 of extcore, a library for processing code in GHC's text-based External Core format. extcore includes a parser, prettyprinter, typechecker, and interpreter for External Core, as well as modules for computing module dependencies and combining multiple Core modules into a single module. The library was originally written by Andrew Tolmach and was part of the GHC distribution. After becoming its maintainer, about a year ago I moved the library to Hackage. Since in the past year, the library has acquired some users but has received no fatal bug reports, I thought it was time for a 1.0 release. You might be interested in using this library if you're a compiler or tool writer who wants to write a tool that processes the output of GHC's front-end and simplifier -- that is, code in the Core intermediate language, a typed functional language in which many of Haskell's features have been desugared into a polymorphically typed lambda-calculus (extended with type coercions, as are necessary to support type system features such as GADTs). Such tools might include alternative back-ends or new program transformations. Using the extcore library allows tool writers to experiment with implementing new compiler features without modifying GHC itself. While the GHC API is also a route to adding new stages to the GHC pipeline, using extcore has the advantage that External Core is well-specified (read on) and less subject to change than GHC's internal Core format. The library has been tested on External Core produced by GHC 6.10.* and GHC 6.12.*. It has not yet been tested on External Core produced by GHC 7.*. Documentation for the External Core format itself lives at http://www.haskell.org/ghc/docs/latest/html/ext-core/core.pdf - but documentation for the library is limited, and I would welcome documentation or other patches. Currently the best source of documentation for the library is the README file, found in the distribution. To get the distribution, please visit http://hackage.haskell.org/package/extcore or cabal install extcore. Please direct replies to glasgow-haskell-users(a)haskell.org, with me CCed. Please make sure not to reply to haskell(a)haskell.org. Cheers, Tim Chevalier -- Tim Chevalier * http://cs.pdx.edu/~tjc/ * Often in error, never in doubt "an intelligent person fights for lost causes,realizing that others are merely effects" -- E.E. Cummings

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Re: [Haskell-cafe] HDBC, postgresql, bytestrings and embedded NULLs
by John Lato 19 Jan '11

19 Jan '11

> > From: Michael Snoyman <michael(a)snoyman.com> > > > I took a quick look at the enumerators library, but it doesn't seem to > have > > the necessary support for handling data that comes from arbitrary C API > > function calls rather than handles or sockets. > > It does support this, for prior art see yaml[1] or yajl-enumerator[2]. > I'd be happy to help you with this, as having some enumerator > experience is a big help here. > This is one of the things that iteratee supports rather better than enumerator, IMHO. See "enumFromCallback"[1]. But if you want this style of code with database access, why not try Takusen? John L. [1] http://hackage.haskell.org/packages/archive/iteratee/0.6.0.1/doc/html/Data-…

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Re: [Haskell-cafe] Mailing lists on projects.haskell.org
by John Lato 18 Jan '11

18 Jan '11

> > Message: 10 > Date: Tue, 18 Jan 2011 09:53:42 +1100 > From: Erik de Castro Lopo <mle+hs(a)mega-nerd.com <mle%2Bhs(a)mega-nerd.com>> > Subject: Re: [Haskell-cafe] Mailing lists on projects.haskell.org? > To: haskell-cafe(a)haskell.org > Cc: haskell(a)haskell.org > Message-ID: <20110118095342.89b92b21.mle+hs(a)mega-nerd.com<20110118095342.89b92b21.mle%2Bhs(a)mega-nerd.com> > > > Content-Type: text/plain; charset=US-ASCII > > Erik de Castro Lopo wrote: > > > Hi all, > > > > I tried sending mail to the haskell-llvm mailing list (xxxx AT > > projects.haskell.org) several days ago and today I received a bounce > > message. > > > > Looking into the issue a little further, I find that DNS has no MX > > record for projects.haskell.org and projects.haskell.org doesn't > > seem to be listening on port 25. > > > > I would email the administrator, but that is also an @projects.h.o > > address and I doubt it would get through. > > > > Posting here in the hope that someone who sees this can fix it. > > > Anyone? > > There is still no mx record and still nothing listening on port 25. > In case anyone was replying off-list, could you please cc me as well? I believe this is the cause of problems with the iteratee list (which Maciej just mentioned) as well. Although the problem does seem intermittent. John

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Template Haskell a Permanent solution?
by Jonathan Geddes 18 Jan '11

18 Jan '11

Cafe, First let me say that Template Haskell is very powerful and a lot of great work has been done in this area. It fills in a number of holes in Haskell's feature set. But TH gives me the same feeling as other language features that have been described as "bolted on." Also, TH is both library and built-in syntax (via an extension) which feels strange to me. Finally, It's very complicated to do some simple things. I see TH used most for the following tasks: #1 Parse a string at compile-time so that a custom syntax for representing data can be used. At the extreme, this "data" might even be an EDSL. #2 Provide instances automatically. I would propose that more specialized features be implemented to accomplish these tasks. To start, I'll throw out some ideas that provide these capabilities. For TH use #1, compile-time parsing of arbitrary strings, I think it would be nice for quasiquote semantics to be modified so that code like > json :: String -> JsonObject > json = ... > > data = [ json | > { "name" : "Jonathan" > , "favorite language": "Haskell" > } >|] causes the function json to be called at compile time with a string argument of " {\"name\" : \"Jonathan\"\n , \"favorite language\": \"Haskell\"\n }". The whole expression being then replaced with the result of the function application. What I like about this is that defining "quasiquoters" is trivial. They're just functions of the form String -> a. Many such quasiquoters already exist and would be ready for use! I imagine certain rules would apply, ie a quasiquoter must be defined prior to use and in a separate module, etc. For TH use #2, automatic instances, I would propose a way of declaring that a class can be automatically derived, and therefore added to the set [Eq, Ord, Show, Read, ... , etc]. This is the set of classes that can be put in the "deriving" clause of a type declaration. I don't know exactly what the syntax for this would look like, but I imagine it would look a bit like the various current implementations of automatic instances in TH. Again, TH is very powerful, and fills in a number of holes in Haskell's feature set. But it leaves me wondering if these holes should not be filled in by other, more specialized features, leaving TH to continue to find other holes to fill. I'm wondering if others see TH as a permanent solution, or if you agree with me that some of TH's most common usages should have more specialized features dedicated to them. It may very well be that I am simply not experienced enough with TH to fully appreciate and embrace it. By the way, did I miss any uses of TH as common as the ones I mentioned? --Jonathan

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multiple instances or contexts?
by John Lato 18 Jan '11

18 Jan '11

Hello, This post is (hopefully) literate Haskell. I recently noticed that there are two ways to specify instances in a common situation. Suppose I have something like this: > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, NoMonomorphismRestriction, OverlappingInstances #-} > > data A = A > data B = B > data C = C > > newtype Repr a = Repr { unRepr :: State MyState a } > > class SomeClass a b where > If I want to make instances of SomeClass for Repr, A and Repr, B, I have two choices: > instance SomeClass Repr A where > instance SomeClass Repr B where > or I can introduce a new class and make an instance with a context, > class RClass c where > > instance RClass A where > instance RClass B where > -- no C instance for RClass > > instance RClass x => SomeClass Repr x is there any reason to prefer one form over the other? Of course the first requires more instance declarations, but they're auto-generated so that doesn't bother me. Thanks, John L.

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HDBC, postgresql, bytestrings and embedded NULLs
by Iustin Pop 18 Jan '11

18 Jan '11

Hi all, It seems that (at least) the postgresql bindings do not allow pure binary data. I have a simple table: debug=# create table test (name bytea); byteas seems to be the backing type on the DB side for bytestrings. and then I run this: import Database.HDBC.PostgreSQL import Database.HDBC import Data.ByteString main = do db <- connectPostgreSQL "dbname=debug" stmt <- prepare db "INSERT INTO test (name) VALUES($1)" execute stmt [toSql $ pack [0]] execute stmt [toSql $ pack [65, 0, 66]] commit db What happens is that the inserted string is cut-off at the first NULL value: the first row is empty, and the second row contains just "A". http://www.postgresql.org/docs/8.4/static/datatype-binary.html says: “When entering bytea values, octets of certain values must be escaped (but all octet values can be escaped) when used as part of a string literal in an SQL statement. In general, to escape an octet, convert it into its three-digit octal value and precede it by two backslashes”, and continues to list that NULL should be quoted as E'\\000'. However, I find no such quoting in the HDCB.Postgresql sources. Anyone else stumbled on this? thanks, iustin

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Denotational semantics for the lay man.
by David Sankel 18 Jan '11

18 Jan '11

Hello All, I've recently had the opportunity to explain in prose what denotational semantics are to a person unfamiliar with it. I was trying to get across the concept of distilling the essence out of some problem domain. I wasn't able to get the idea across so I'm looking for some simple ways to explain it. Does anyone know of a way to explain what's the meaning and objective of "distilling the essence" without introducing more jargon. One thing that comes to mind is how Newton's equations for gravity were a distillation of the essence of the way things fall. Thanks in advance, David -- David Sankel Sankel Software www.sankelsoftware.com

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Mailing lists on projects.haskell.org?
by Erik de Castro Lopo 18 Jan '11

18 Jan '11

Hi all, I tried sending mail to the haskell-llvm mailing list (xxxx AT projects.haskell.org) several days ago and today I received a bounce message. Looking into the issue a little further, I find that DNS has no MX record for projects.haskell.org and projects.haskell.org doesn't seem to be listening on port 25. I would email the administrator, but that is also an @projects.h.o address and I doubt it would get through. Posting here in the hope that someone who sees this can fix it. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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is datetime actively maintained?
by Carter Schonwald 18 Jan '11

18 Jan '11

Hey All, I can't tell if datetime is actively maintained, and its version constraints need to be adjusted to allow base4 (which would then allow a whole mess of other packages including gitit build under ghc7). I've been sucessful building gitit with ghc7 64 on OS X and using it if that one constraint on datetime is modified. any thoughts? cheers, -Carter

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Zipper with two focii for concurrent fun
by Yaakov M. Nemoy 17 Jan '11

17 Jan '11

Hey List, So i've been pondering zippers in a concurrent situation, where more than one thread has access to the same data structure, but with different focal points. I've gotten a bit stuck though trying to figure out how to do this using a zipper. The use case i'm thinking of is slightly more complex, but using a basic example below should suffice for an explanation. Say we have a list of characters in a text file which we're using in the form of a zipper to show the current user's cursor position in the text file. This means a data type such as: data ZipperList a = ZL [a] a [a] where the second element is the current position of the cursor. Now as long as one user is editing the text file, we're fine. Now let's say we're working on a newfangled multipointer desktop where two users have a keyboard and mouse each and want to work on the same open file. We could have a list like this: data ZipperList a = ZL [a] a [a] a [a] which leaves us with a number of limitations. We're now limited to two cursors rather than one, but once we start assuming multipointer, two seems like a bigger limitation than just one cursor. Also, we can't tell which user is at which focal point of the zipper easily. We could replace the user with: data User a = User String a data ZipperList a = ZL [a] (User a) [a] (User a) [a] but that gets into some ugly pattern matching. It still leaves us stuck with a two user limit. Say we create a zipper that can contain other zippers. Such as: data ZipperList a = ZL (ZipperList a) a (ZipperList a) | L [a] which lets us include as many users as we want. Now this creates a very different problem. Each cursor in our fictional text editor has a totally different data structure. This is how any calculation at any cursor knows where in the text file it is. If one user edits the character at the cursor, every other zipper-like data structure has to be updated for each other users. Thus a huge concurrency issue. This is where i hit a block. Am i totally crazy in thinking i can solve this problem using a zipper? Or has someone thought about this problem already and perhaps there's even a paper on it? Please advise. -Yaakov

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ANNOUNCE: Haskell binding to the ANTLR parser generator C runtime library
by Mark Wright 17 Jan '11

17 Jan '11

Hi, I'm pleased to announce the release of a Haskell binding to the ANTLR LL(*) parser generator C runtime library. The book on ANTLR is "The Definitive ANTLR Reference: Building Domain-Specific Languages" by Terence Parr: http://www.pragprog.com/titles/tpantlr/the-definitive-antlr-reference ANTLR is a very powerful parser generator. It has support for predicated parsing, where dis-ambiguating semantic predicates can be used to look up information in the symbol table to handle context sensitive programming languages. The lexing and parsing handled in C. The C parser actions call Haskell code to do things such as: - enter data into the symbol table - look up data in the symbol table to implement dis-ambiguating semantic predicates for parsing context sensitive programming languages. - create Abstract Symbol Table entries. The ANTLR parser generator site is at: http://antlr.org/ The Haskell binding to the ANTLR parser generator C runtime library is at: http://hackage.haskell.org/package/antlrc An example based on the implementation of arithmetic expressions example from "Types and Programming Languages" by Benjamin Pierce, chapter 4, is on github: https://github.com/markwright/antlrc-examples/tree/master/src/tapl/arith Building the example is somewhat challenging. I provide a Makefile, however it is specific to my system and will need to be customised. Documentation for the ANTLR C runtime library is at: http://www.antlr.org/wiki/display/ANTLR3/ANTLR3+Code+Generation+-+C Documentation for the ANTLR parser generator is at: http://www.antlr.org/wiki/display/ANTLR3/ANTLR+v3+documentation Thanks, Mark

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Haskell, C and Matrix Multiplication
by Blake Rain 17 Jan '11

17 Jan '11

Dear Haskellers, I thought I'd take some time to share something of my weekend with you all. Not because of anything new, but because it is a feel-good example of Haskell being win. A good read for Monday morning, perhaps. What feels like decades ago, I used to work in computer graphics. Mostly my work was on software for the amateur/semi-professional film industry - developing various filters and effects for the likes of Adobe AfterEffects and so on. I have not touched computer graphics from a programming perspective for quite a long time now, and I thought it was about time that I flexed those old muscles. So, I have taken it upon myself to write an SVG renderer for Blender. To cut what is going to be a very long and humdrum story short, I needed to write some matrix arithmetic code. Having not done this in a long time, I thought I'd see if I can still actually remember what matrix multiplication is by writing a function to multiply two 4x4 matrices. I thought I'd write one in C and then one in Haskell, and see how long they took and how easy they were to write. Here is the one in C: void multiplyM4 (float m1[4][4], float m2[4][4], float m3[4][4]) { m1[0][0] = m2[0][0] * m3[0][0] + m2[0][1] * m3[1][0] + m2[0][2] * m3[2][0] + m2[0][3] * m3[3][0]; m1[0][1] = m2[0][0] * m3[0][1] + m2[0][1] * m3[1][1] + m2[0][2] * m3[2][1] + m2[0][3] * m3[3][1]; m1[0][2] = m2[0][0] * m3[0][2] + m2[0][1] * m3[1][2] + m2[0][2] * m3[2][2] + m2[0][3] * m3[3][2]; m1[0][3] = m2[0][0] * m3[0][3] + m2[0][1] * m3[1][3] + m2[0][2] * m3[2][3] + m2[0][3] * m3[3][3]; m1[1][0] = m2[1][0] * m3[0][0] + m2[1][1] * m3[1][0] + m2[1][2] * m3[2][0] + m2[1][3] * m3[3][0]; m1[1][1] = m2[1][0] * m3[0][1] + m2[1][1] * m3[1][1] + m2[1][2] * m3[2][1] + m2[1][3] * m3[3][1]; m1[1][2] = m2[1][0] * m3[0][2] + m2[1][1] * m3[1][2] + m2[1][2] * m3[2][2] + m2[1][3] * m3[3][2]; m1[1][3] = m2[1][0] * m3[0][3] + m2[1][1] * m3[1][3] + m2[1][2] * m3[2][3] + m2[1][3] * m3[3][3]; m1[2][0] = m2[2][0] * m3[0][0] + m2[2][1] * m3[1][0] + m2[2][2] * m3[2][0] + m2[2][3] * m3[3][0]; m1[2][1] = m2[2][0] * m3[0][1] + m2[2][1] * m3[1][1] + m2[2][2] * m3[2][1] + m2[2][3] * m3[3][1]; m1[2][2] = m2[2][0] * m3[0][2] + m2[2][1] * m3[1][2] + m2[2][2] * m3[2][2] + m2[2][3] * m3[3][2]; m1[2][3] = m2[2][0] * m3[0][3] + m2[2][1] * m3[1][3] + m2[2][2] * m3[2][3] + m2[2][3] * m3[3][3]; m1[3][0] = m2[3][0] * m3[0][0] + m2[3][1] * m3[1][0] + m2[3][2] * m3[2][0] + m2[3][3] * m3[3][0]; m1[3][1] = m2[3][0] * m3[0][1] + m2[3][1] * m3[1][1] + m2[3][2] * m3[2][1] + m2[3][3] * m3[3][1]; m1[3][2] = m2[3][0] * m3[0][2] + m2[3][1] * m3[1][2] + m2[3][2] * m3[2][2] + m2[3][3] * m3[3][2]; m1[3][3] = m2[3][0] * m3[0][3] + m2[3][1] * m3[1][3] + m2[3][2] * m3[2][3] + m2[3][3] * m3[3][3]; } Urgh! This took me over 20 minutes to write out, and a further 34 minutes of debugging. That's 54 minutes to write a simple function! This included 23 invocations of the compiler and 18 invocations of the test program. I was disgusted with myself! I should have automated the writing of it with Haskell! The reason for this was mostly that I kept pressing the wrong buttons when writing those many, many array indices. I became blind to the numbers in the end, and it became one of those nightmare debugging sessions where you check your printMatrix function and other stupid things. I'd like to say that I was a bit tired, but that's just an excuse. Some points about the C version: 1. It's only 21 lines of code. 2. It took me 54 minutes to write and debug. Therefore I can write eight of these in a standard working day. Hmm. 3. It's written in C, so everybody and their dog understands it completely. Honest. 4. It's written in C, so it can just be pasted into nearly any other language (because they're all just augmented C). Therefore no one needs to understand it. 5. It does not consume much space (in terms of memory). 6. It's pretty fast (being written in C): Performing 100,000,000 matrix multiples takes on average 6.117s (with gcc -O2). 7. Passing a NULL pointer anywhere will cause it to explode. 8. If the pointers are not to arrays of 16 floats, it will trash memory (being as it destructively updates one of it's arguments). 9. It will only operate on 4x4 float matrices. 10. How am I storing matrices again...? Column major? So, after drinking some coffee and having a smoke, I started the Haskell version: [foldl (+) 0 $ zipWith (*) x y | x <- m1, y <- transpose m2] Ah. So... anyway, some points about the Haskell version: 1 It's only one line of code - that's 20 less than the C version. 2. It took me just over five minutes to think about and to code. That's a saving of 49 minutes (1180% or something). It compiled straight away and worked first time. I can write too many of these in a day. 3. It's laughably clear and concise. Anyone can easily tell what I'm doing. 4. It does *not* operate in constant space. I think. I can't tell. I don't know how to tell. Maybe it does. 5. It's very fast (being written in Haskell): Performing 100,000,000 matrix multiples takes on average 1.435s (with ghc -O2). 6. Passing in empty lists will not crash it. 7. It will operate on square matrices of any size (3x3, 4x4...). 8. It will operate on a matrix of any type that is of class Num. The Haskell version seems to win on all counts. Interestingly enough (to some at least), other matrix operations also become trivial when expressed in Haskell: Determinant is a bit fiddly, but quite trivial. However it is woefully slow at O(n!). I need to make use of LU decomposition (in which the determinant is the sum of the diagonal values in U). Adjoint is even more of a fiddle, but actually a one-liner after reusing bits of the determinant function (including the determinant function). Matrix inversion is easy in one line, but dependent on the determinant and the adjoint, therefore also woefully slow. Just another example of Haskell being awesome :) Now over to you guys! What experiences have you had which make you glad you use Haskell? Anything make you smile recently? - Blake

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Announcement: SiteBridge version 1.0
by Sukit Tretriluxana 17 Jan '11

17 Jan '11

Dear all, I am proudly announcing SiteBridge version 1.0. Briefly about it, SiteBridge is a site bridging system that allows developers to connect his/her locally developed app to an externally accessible site. It's aimed to get rid of the pain of the fact that you are still internally developing an app but want to test how it looks and feels when externally accessible. An obvious example of this is a web app for mobile devices. Usually these devices are on a different network than that used in development. So you won't be able to test it with real devices unless you have a test site, or you have control over everything including providing WIFI network that you mobile devices can get on. However, so many people do not have this luxury especially those working in enterprise environment. This is a pain and even you have a test site it's often still very awkward to incorporate it into the rapid development cycle. Sometimes you just want a quick test to see how things go such as troubleshooting. See this short introduction video. http://www.youtube.com/watch?v=UxhLthKSr_E For more information, check out http://code.google.com/p/sitebridgeserver/ Enjoy! Ed

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Please explain the Lazy type differences
by Roderick Ford 17 Jan '11

17 Jan '11

I wrote this little clip, and haven't been able to figure out why the L.ByteString is different type than Data.ByteString.Internal.ByteString, or if I am just doing everything wrong here. import System.FilePath import System.Environment import qualified Data.ByteString.Lazy as L import qualified Crypto.Hash.SHA1 as SH import Data.Hex (hex) hashFile :: FilePath -> IO L.ByteString hashFile f = do content <- L.readFile f let hasher = SH.init bhsh = SH.update hasher content return (hex (SH.finalize bhsh)) I really just want to get the digest of a file as a [Char], but then of course, SH.update is not very happy about the type I am passing to it either. Can't wait till this becomes easy! Roderick

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Needed: A repeatable process for installing GHC on Windows
by Johan Tibell 17 Jan '11

17 Jan '11

Hi, Earlier today I was trying to set up a Windows build bot for the 'network' package. That turned out to be quite difficult. Too much playing with PATHs, different gcc versions, etc. Does anyone have a repeatable, step-by-step process to install GHC and get a build environment (where I could build network) going? Johan

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Guy Steele's Praise For Haskell @ Strange Loop Keynote
by aditya siram 17 Jan '11

17 Jan '11

Guy Steele did the keynote on parallelism [1] at the Strange Loop [2] conference in which he said that he could do it over Fortress [3] would have been modeled on Haskell rather than Fortran. The relevant portions are between 49:36 - 49:50. Thought it might interest readers of this list. -deech [1] http://www.infoq.com/presentations/Thinking-Parallel-Programming [2] http://strangeloop2010.com/ [3] http://en.wikipedia.org/wiki/Fortress_(programming_language)

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Misleading MVar documentation
by Edward Z. Yang 16 Jan '11

16 Jan '11

Merry Christmas all! Is it just me, or does the Control.Concurrent.MVar documentation seem a bit misleading? In particular, we should explicitly note the race conditions for not just swapMVar but also readMVar, withMVar, modifyMVar_ and modifyMVar, and clarify that the safety guarantees of the latter three pertain to their handling of asynchronous exceptions. It might also be good to tell people that if they need race-free operations of this style, STM is a good alternative to look at, even if only one variable is being synchronized over. Cheers, Edward

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hoogle command line program on Linux
by Erik de Castro Lopo 15 Jan '11

15 Jan '11

Hi all, I'm testing out hoogle 4.1.2 on Debian Linux and getting the following when trying to update the local hoogle databases: erik > sudo hoogle data Starting default Starting keyword hoogle: keyword.txt: commitAndReleaseBuffer: invalid argument (Invalid or incomplete multibyte or wide character) My LANG related envorinment variables are set as follows: erik > env | grep LANG LANG=en_AU.UTF-8 GDM_LANG=en_AU.UTF-8 LANGUAGE=en_AU.UTF-8 LC_LANG=en_AU.UTF-8 When faced with a similar problem while working on Ben Lippmeier's compiler, I fixed this by replacing all instances of readFile with readUtf8File which is defined as: readUtf8File :: FilePath -> IO String readUtf8File filePath = do h <- openFile filePath ReadMode hSetEncoding h utf8 hSetEncoding stdout utf8 hGetContents h Is it possible to fix this without hacking the hoogles sources? I'd like to package hoogle for Debian, but first I need to get it working. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Question concerning Network.Curl.Opts
by Michael Litchard 15 Jan '11

15 Jan '11

I'm having trouble passing header strings properly, and I'd like some advice on how to proceed. Below is a capture of what is being sent, versus what I am trying to send. I won't include all code, only what I think is necessary. If I have omitted something important, please let me know. How could I discover what the cause of the discrepancy is? Thanks again for any feedback. Here's a snippet from the header, what is being sent. > GET /resourceList.do?form=webForwardsForm&readOnly=false&policyLaunching=true&resourcePrefix=webForwards&path=%2FshowWebForwards.do&messageResourcesKey=webForwards&actionPath=%2FresourceList.do HTTP/1.1 User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0) Host: 172.16.1.18 Accept: */* Accept-Encoding: gzip,deflate Referer: https://172.16.1.18/showWebForwards.do Cookie: domainLogonTicket=SLXa10225c6e8389b3eb181e3df5dcf08de; logonTicket=SLXa10225c6e8389b3eb181e3df5dcf08de; lbTrack=OAIAGHMWQDOLYYTJEXQHXBYPXVALXNREKIHAYYRZSOGYJLUYNNCJ--------; SSLX_SSESHID=bvgx4mggmy6v ^ compare this to CurlHttpHeaders Here's the part of the source I think is relevant > launch :: String -> String -> IO (Either String String) > launch user pass = do > -- Initialize Curl > curl <- initCurl > -- Sequence of steps > let steps = do > curlResp curl urlInitial method_GET > curlResp curl urlLogin $ loginOpts user pass > curlResp curl urlFlash1 method_GET > curlResp curl urlFlash2 method_GET > curlResp curl urlGetResource resourceOpts <---- here's where the problem is revealed > runErrorT steps > main :: IO () > main = do > -- username and password > user:pass:_ <- getArgs > -- Launch webpage > resp <- launch user pass > -- Response comes as Either String String > -- You have to handle each case > case resp of > Left err -> print err > Right body -> putStrLn body > resourceOpts :: [CurlOption] > resourceOpts = > [ CurlHttpHeaders > [ "Accept text/javascript, text/html, application/xml, text/xml, */*" > , "Accept-Language en-us,en;q=0.5" > , "Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7" > , "Keep-Alive 115" > , "Connection keep-alive" > , "X-Requested-With XMLHttpRequest" > , "X-Prototype-Version 1.6.0.3" > ] > , CurlEncoding "gzip,deflate" > , CurlReferer "https://172.16.1.18/showWebForwards.do" > ]

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Monad transformer: apply StateT to List monad
by michael rice 14 Jan '11

14 Jan '11

{- From: http://en.wikibooks.org/wiki/Haskell/Monad_transformers if for instance we apply StateT to the List monad, a function that returns a list (i.e., a computation in the List monad) can be lifted into StateT s [], where it becomes a function that returns a StateT (s -> [(a,s)]). That is, the lifted computation produces multiple (value,state) pairs from its input state. -} import Control.Monad.Trans.State.Lazy type GeneratorState = StateT Int -- a function in the list monad f :: Int -> [Int] f n = [0..n] Will someone please demonstrate the above comment from the wiki page. Michael

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questions about network.curl/ web client programming
by Michael Litchard 14 Jan '11

14 Jan '11

I'm learning both haskell and web programming as I go here, this question entails both. I'm writing a screen scraping program, and I'm at the point where I need to send certain data in the header. My question is, is that what method_HEADER is for? If so, could I see an example? If not, how does one go about shaping header data?

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Re: [Haskell-cafe] parsec2 vs. parsec3... again
by Permjacov Evgeniy 14 Jan '11

14 Jan '11

On 12/23/2010 06:01 AM, Evan Laforge wrote: > This is not very encouraging! Especially strange is how Text > generates *more* allocation... I'd expect less since it doesn't unpack > all the Texts. Errgh. To check against predicate, library HAS to unpack checked character. There is no way around it. > There's an obvious problem where I get the digits as a String and then > parse that with list functions, but I can't see any way to get parsec > to return a chunk of Text. This is roughly how parsec itself parses > numbers, in Text.Parsec.Token. > > Any ideas or experience? > If you wish performance so desperatley, you can try hand-coded parsing. What I mean is, that if library has to unpack characters to check them against isDigit predicate, why not to use it in building numeral value immidiatley? This will eliminate intermidiate list. However, every back-tracking parser is slow by definition. If you wish maximum possible speed, consider hand-written lexer (this is not too hard) and possibly Happy to generate parser. BTW, how much utf16 text is around? I never found any in wild web.

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Writer monad corresponds to synthesized attribute?
by Sean Leather 13 Jan '11

13 Jan '11

I have been attempting to translate something I did using UUAG into monadic code. It involved inherited, synthesized, and chained attributes. It has been said that such attributes correspond to the Reader, Writer, and State monads, respectively [1]. The former and latter are straightforward, but I'm somehow missing the correlation between the Writer monad and synthesized attributes. I couldn't figure out the appropriate combination of tell/listen/pass/censor to do what I was doing with a synthesized Map. I eventually gave up and passed the synthesized values along with the monadic result. Has anybody done this before? Regards, Sean [1] http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue4/Why_Attribute_Gr…

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Haskell for Gingerbread
by Chris Kuklewicz 13 Jan '11

13 Jan '11

Hi folks, I have been looking at developing for my Android phone which is running Gingerbread (Android version 2.3). The important thing about the official development kit is this: The new native development kit (C/C++ cross compiler for ARM) is that the you can create android applications from pure C without using the Dalvik/Java virtual machine at all. The thinking behind this was probably for game developers to be able to avoid their VM. So all that might be needed is a Haskell compiler with a C-backend that emits ARM-compatible code and an initially minimal android runtime. Implementing to the new "native_activity.h" allow for the usual application life-cycle: onStart, onPause, onResume, onStop... Some options I have not had a chance to look into: 1) GHC arm port and -via-C 2) jhc (and lhc) generated C 3) port nhc98 4) port yhc bytecode runtime Does anyone know who else is thinking along any of these lines? Are there 5th or 6th routes to take? Cheers, Chris Kuklewicz

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ghci seg faulting, have no idea why.
by Michael Litchard 13 Jan '11

13 Jan '11

this just started happening, don't know why. Could anyone offer suggestions, troubleshooting methods? ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Segmentation fault

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The "type & data" constructor confused me !
by z_axis 13 Jan '11

13 Jan '11

data Layout a = forall l. (LayoutClass l a, Read (l a)) => Layout (l a) readsLayout :: Layout a -> String -> [(Layout a, String)] readsLayout (Layout l) s = [(Layout (asTypeOf x l), rs) | (x, rs) <- reads s] Why is the first parameter of readsLayout "(Layout l)" not "Layout (l a)" ? Sincerely! ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/The-type-data-constructor-confused-me-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Weekly News: Issue 164 - January 12, 2011
by Daniel Santa Cruz 13 Jan '11

13 Jan '11

Welcome to issue 164 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 2 to 8, 2011. Announcements Ben Millwood [2]announced the release of haskell-src-meta 0.3. He tells us that this release adds support for GHC 7, and for TH 2.5, along with other improvements. Mitar announced [3]a new version ef Etage, a general data-flow framework. Malcolm Wallace had [4]some news from the Haskell Language Committee. One of the points mentioned is that there will _not_ be a language standard called 2011, with the committee intending to issue a new language standard in 2012. Simon Peyton-Jones [5]invited members from the community to submit papers for the Workshop on Intermediate Representations 2011. The deadline is January 21. Clemens Grelck wanted to alert everyone that the [6]new deadline for papers for the Eighth International Workshop on Practical Aspects of High-Level Parallel Programming (PAPP 2011) to be held June 1-3, 2011, in Tsukuba, Japan. Emanuel Kitzelmann extended a [7]call for papers for the 4th International Workshop on Approaches and Applications of Inductive Programming (AAIP 2011) to be held July 19, 2011, in Odense, Denmark. Maciej Piechotka announced the [8]first release of nanoparsec. "Nanoparsec is currently simply a port of attoparsec on the ListLike--the abstraction of lists used by iteratee. Top Reddit Stories * Why Lisp is a Big Hack (And Haskell is Doomed to Succeed) Domain: axisofeval.blogspot.com, Score: 69, Comments: 15 On Reddit: http://www.reddit.com/r/haskell/comments/evbz5/why_lisp_is_a_big_hack_and_h… Original: http://axisofeval.blogspot.com/2011/01/why-lisp-is-big-hack-and-haskell-is.… * I ported Clojure's Persistent Vectors and Hash Maps to Haskell Domain: blog.wuwon.id.au, Score: 54, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/evzhz/i_ported_clojures_persistent… Original: http://blog.wuwon.id.au/2011/01/persistent-vectors-and-hash-maps-for.html * 3 Big Haskell Infrastructure Projects That Would Be Cool Domain: cdsmith.wordpress.com, Score: 31, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/evt56/3_big_haskell_infrastructure… Original: http://cdsmith.wordpress.com/2011/01/03/big-ideas-for-haskell/ * Merging binary and cereal Domain: blog.johantibell.com, Score: 31, Comments: On Reddit: http://www.reddit.com/r/haskell/comments/ewvpv/merging_binary_and_cereal/ Original: http://blog.johantibell.com/2011/01/merging-binary-and-cereal.html * Haskell library improvements I'd like to see Domain: blog.johantibell.com, Score: 28, Comments: On Reddit: http://www.reddit.com/r/haskell/comments/evm0g/haskell_library_improvements… Original: http://blog.johantibell.com/2011/01/haskell-library-improvements-id-like-to… * You can use GHC for the mac app store Domain: github.com, Score: 20, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/exxal/you_can_use_ghc_for_the_mac_… Original: https://github.com/nfjinjing/haskell-cocoa-currency-converter-example * Sudoku Solver in Haskell Domain: antimatroid.wordpress.com, Score: 18, Comments: 7 On Reddit: http://www.reddit.com/r/haskell/comments/ev1i0/sudoku_solver_in_haskell/ Original: http://antimatroid.wordpress.com/2009/06/01/sudoku-solver-in-haskell/ * Job Board for Functional Programmers Domain: functionaljobs.com, Score: 15, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/evi5d/job_board_for_functional_pro… Original: http://functionaljobs.com * Haskell library for analysis and synthesis of Go code. Domain: hackage.haskell.org, Score: 15, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/ex43z/haskell_library_for_analysis… Original: http://hackage.haskell.org/package/language-go * Hello Reddit! I implemented Levenshtein automata in Haskell after reading that 'Damn Cool Algorithm' blog post -- what do you think? Domain: self.haskell, Score: 14, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/ewl7o/hello_reddit_i_implemented_l… Original: /r/haskell/comments/ewl7o/hello_reddit_i_implemented_levenshtein_automata/ Top StackOverflow Questions * Why do we need 'seq' or 'pseq' with 'par' in Haskell? votes: 9, answers: 2 Read on SO: http://stackoverflow.com/questions/4576734/why-do-we-need-seq-or-pseq-with-… * Complex data structures in Haskell - how do they work? votes: 8, answers: 2 Read on SO: http://stackoverflow.com/questions/4597262/complex-data-structures-in-haske… * Mutability in functional programming votes: 8, answers: 3 Read on SO: http://stackoverflow.com/questions/4619500/mutability-in-functional-program… * Where can I learn advanced Haskell? votes: 8, answers: 6 Read on SO: http://stackoverflow.com/questions/4634962/where-can-i-learn-advanced-haske… * in haskell, why do I need to specify type constraints, why can't the compiler figure them out? votes: 7, answers: 4 Read on SO: http://stackoverflow.com/questions/4590975/in-haskell-why-do-i-need-to-spec… * Can someone explain to me the following Haskell expression votes: 6, answers: 2 Read on SO: http://stackoverflow.com/questions/4589957/can-someone-explain-to-me-the-fo… * Parse XML in Haskell votes: 6, answers: 5 Read on SO: http://stackoverflow.com/questions/4619206/parse-xml-in-haskell * Haskell: Record Update for Existential Types votes: 5, answers: 1 Read on SO: http://stackoverflow.com/questions/4580211/haskell-record-update-for-existe… * Are there any more original, more functional Haskell web-frameworks? votes: 5, answers: 2 Read on SO: http://stackoverflow.com/questions/4598091/are-there-any-more-original-more… * Defining a data type that doesn't want to be defined.. votes: 5, answers: 1 Read on SO: http://stackoverflow.com/questions/4612830/defining-a-data-type-that-doesnt… About the Haskell Weekly News You can find an html version of this newsletter at http://contemplatecode.blogspot.com/2011/01/haskell-weekly-news-issue-164.h… To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Hope your 2011 is off to a good start! Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18414 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85062 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18418 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18415 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18416 7. http://article.gmane.org/gmane.comp.lang.haskell.general/18417 8. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85177

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Fwd: [TYPES/announce] Oregon Programming Languages Summer School]
by Don Stewart 13 Jan '11

13 Jan '11

This will be of interest to many Haskellers. The line up is always outstanding, Oregon is lovely in the summer, and you can drop by Galois while you're here :-) -- Don ----- Forwarded message from Robert Harper <rwh(a)cs.cmu.edu> ----- Date: Wed, 12 Jan 2011 16:49:46 -0500 From: Robert Harper <rwh(a)cs.cmu.edu> To: types-announce(a)lists.seas.upenn.edu Cc: pierre-louis curien <eu.sunt.picasso(a)gmail.com> Subject: [TYPES/announce] Oregon Programming Languages Summer School [ The Types Forum (announcements only), http://lists.seas.upenn.edu/mailman/listinfo/types-announce ] We are pleased to announce the preliminary program for the 10th annual Oregon Programming Languages Summer School (OPLSS) to be held 15 June to 1 July 2011 at the University of Oregon in Eugene. This year's program is titled Types, Semantics, and Verification, and features the following speakers: Amal Ahmed Logical relations Indiana University Andrew Appel Software verification Princeton University Nick Benton Monadic effects Microsoft Research Robert Constable Cornell University Pierre-Louis Curien Polarization and Focalization pi.r2 team, PPS, CNRS-Paris 7 University-INRIA Robert Harper Type theory foundations Carnegie Mellon University Hugo Herbelin Foundation of Coq pi.r2 team, PPS, CNRS-Paris 7 University-INRIA Xavier Leroy Compiler verification INRIA Paul-Andre' Mellies Category theory pi.r2 team, PPS, CNRS-Paris 7 University-INRIA Greg Morrisett Ynot programming Harvard University Frank Pfenning Proof theory foundations Carnegie Mellon University Benjamin Pierce Software foundation in Coq University of Pennsylvania Dana Scott Carnegie Mellon University Full information on registration will be available shortly at http://www.cs.uoregon.edu/Activities/summerschool/summer11. Robert Harper Zena Ariola Pierre-Louis Curien ----- End forwarded message -----

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MonadRandom-computation that does not terminate
by Tim Baumgartner 13 Jan '11

13 Jan '11

Hi, I'm having difficulties with this function I wrote: iterateR :: (MonadRandom m) => (a -> m a) -> a -> m [a] iterateR g s = do s' <- g s return (s:) `ap` iterateR g s' I'm running the computation with evalRandIO and surprisingly the first call of main in ghci succeeds, but the second does not terminate. Reproducible. Any clues what I'm doing wrong here? Thanks in advance, Tim

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HXT is slow?
by Patrick Hurst 12 Jan '11

12 Jan '11

Is it just me, or is HXT slow? I noticed that both reading a document from a file, as well as running computations, are exceedingly slow, with simple stuff like 'get the contents of everything with a given class' taking .3 seconds for a 400KB HTML file in Python using lxml and 2 seconds using HXT with tagSoup and compiled with -O2.

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Set monad
by Peter Padawitz 12 Jan '11

12 Jan '11

Hi, is there any way to instantiate m in Monad m with a set datatype in order to implement the usual powerset monad? My straightforward attempt failed because the bind operator of this instance requires the Eq constraint on the argument types of m. Peter

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Re: [Haskell-cafe] Type System vs Test Driven Development
by Serge Le Huitouze 12 Jan '11

12 Jan '11

Evan Laforge <qdunkan(a)gmail.com> wrote: > QuickCheck seems to fit well when you have small input and output > spaces, but complicated stuff in the middle, but still simple > relations between the input and output. I think that's why data > structures are so easy to QuickCheck. I suppose I should look around > for more use of QuickCheck for non-data structures... the examples > I've seen have been trivial stuff like 'reverse . reverse = id'. I second this feeling... For example, I've never seen (I've not looked hard, though) Quickcheck's testing applied on graphs. Generating "interesting" (whatever that means for your particular problem) graphs doesn't seem to be a trivial test, even if it's a mere data structure... Does anyone know of such examples? Maybe genetic programming producing graphs is of relevance here (where generating candidate graphs have exactly the same problem). Also, I wonder how you guys do when you're trying to tests code using a lot of numbers (be them floating point or even integer). E.g., integers: A code doing addition and substraction of some sort. A property such as "X = (X add Y) sub Y" is easily falsifiable when the number of bits of your integer is too small for your numbers. E.g. floating point numbers: A code doing coordinate transformations of some sort. Properties akin to "roundtripping" can be easily formulated (e.g. "X = trfY_toX(trfX_toY(X))"), but they'll be falsified often due to lost bits in the mantissa. Replacing strict equality (almost always meaningless for floats) with approximate equality will work. However, definitions of such an approximate equality might vary (the immediate idea is to compare the difference of the two numbers with a small "epsilon", say 10^-9, but that will obviously not work if you numbers are around 10^-12...). And there are also, of course, overflows and underflows, and singularities (e.g. division by zero), non invertibility... So, in addition to defining the approximation (not always easy as I tried to "demonstrate" above) to be used in comparisons, one probably needs ad'hoc generators whose complexity might very well exceed that of the code one wants to test... So, do you have any "methodology" for such use cases? --Serge

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ANNOUNCE: haskell-src-exts 1.10.1
by Niklas Broberg 12 Jan '11

12 Jan '11

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.10.1! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts Unless you are using the XmlSyntax extension, this major version bump should behave just like a minor version bump. Changes: 1.10.0 --> 1.10.1 =============== * Enable the unicode version of DoubleColon (x2237). Thanks to Andrés Sicard-Ramírez for the patch! 1.9.6 --> 1.10.0 =============== * Ensure that implied extensions are always picked up, not only when using the parseFile* family of functions as previously. * Add the newly devised <%>...</%> syntax to the XmlSyntax support. This causes changes to pretty much everything, including adding a case to the AST which prompts the major version bump. Cheers, /Niklas

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Improving HList programming/debugging (longish)
by jeff p 12 Jan '11

12 Jan '11

This message shows how to slightly reformulate HLists (and other type-level things) to get better type-checking and more informative error messages. The technique is interesting in that it uses GADTs and functional dependencies and seems to not be implementable with associated type synonyms. It also makes higher-order HList programming/debugging much more tractable. Programming with HLists, at least in our experience, encourages GHC to produce long and inscrutable type errors. This confusing behaviour is caused by the open nature of type classes-- GHC (as per the Haskell specification) will meekly report a conditional type (i.e. expression has type a if a constraint holds) rather than decisively commit to reporting a type error. By using GADTs and equality constraints, we can force GHC to be a lot more decisive. > {-# LANGUAGE > EmptyDataDecls, > FlexibleInstances, > FunctionalDependencies, > GADTs, > MultiParamTypeClasses, > NoMonomorphismRestriction, > ScopedTypeVariables, > TypeFamilies, > TypeOperators, > UndecidableInstances > #-} We'll start with a standard type level list. > data Nil > data x :* xs > infixr 5 :* Rather than have direct terms for constructing these lists, we'll make a GADT for HLists, and use the constructors from the GADT. > data HList a where > Nil :: HList Nil > (:*) :: x -> HList xs -> HList (x :* xs) Notice that every construction of an HList has a type of the form HList a, e.g.: *Main> :t 'a' :* () :* Nil 'a' :* () :* Nil :: HList (Char :* (() :* Nil)) This definition of HLists allows the type checker to rule out malformed term level representations of HLists: *Main> :t 1 :* () <interactive>:1:5: Couldn't match expected type `HList xs' against inferred type `()' In the second argument of `(:*)', namely `()' In the expression: 1 :* () We can now write hHead and hTail: > hHead :: HList (x :* xs) -> x > hHead (x :* _) = x > hTail :: HList (x :* xs) -> HList xs > hTail (_ :* xs) = xs In order to have higher-order functions on HLists (e.g. fold), we need a way to apply and evaluate type-level representations of functions to arguments. > class HEval f x res | f x -> res where > hEval :: f -> x -> res By assuming the function argument to HEval is in weak head normal form, we will avoid having to write a general eval function at the expense of having to explicitly sequence calls to hEval ourselves. We can now write hMap. > class HMAP f xs res | f xs -> res where > hMap :: f -> HList xs -> HList res > instance HMAP f Nil Nil where > hMap _ Nil = Nil > instance (HEval f x v, HMAP f xs vs) => HMAP f (x :* xs) (v :* vs) where > hMap f (x :* xs) = hEval f x :* hMap f xs We can turn hHead and hTail into type-level functions which can be mapped over HLists. > data HHead = HHead > instance (arg ~ HList (x :* xs)) => HEval HHead arg x > where hEval _ = hHead > data HTail = HTail > instance (arg ~ HList (x :* xs)) => HEval HTail arg (HList xs) > where hEval _ = hTail Notice that the result of HTail is a HList. Here is an example: *Main> :t hMap HHead (('a' :* "b" :* Nil) :* (() :* Nil) :* Nil) hMap HHead (('a' :* "b" :* Nil) :* (() :* Nil) :* Nil) :: HList (Char :* (() :* Nil)) By putting an equality constraint in the context of the HEval instances and a variable in the head, rather than putting the actual type in the head, we force GHC to use the proper HEval clause for each type-level function, and to eagerly type-check the evaluation of the function. *Main> :t hMap HHead (('a' :* "b" :* Nil) :* "c" :* Nil) Top level: Couldn't match expected type `HList (v :* xs)' against inferred type `[Char]' If we use a more naive version of the HEval clause for HHead > data HHead0 = HHead0 > instance HEval HHead0 (HList (x :* xs)) x where > hEval _ = hHead then GHC won't notice the type-level type error *Main> :t hMap HHead0 (('a' :* "b" :* Nil) :* "c" :* Nil) hMap HHead0 (('a' :* "b" :* Nil) :* "c" :* Nil) :: (HEval HHead0 [Char] v) => HList (Char :* (v :* Nil)) until it is actually forced to look at the result type of the hMap. *Main> :t hMap HHead0 (('a' :* "b" :* Nil) :* "c" :* Nil) :: HList (Char :* Char :* Nil) <interactive>:1:0: No instance for (HEval HHead0 [Char] Char) arising from a use of `hMap' at <interactive>:1:0-46 Possible fix: add an instance declaration for (HEval HHead0 [Char] Char) In the expression: hMap HHead0 (('a' :* "b" :* Nil) :* "c" :* Nil) :: HList (Char :* (Char :* Nil)) And the resulting error message is a missing instance, rather than a type clash. Incidentally, an associated type synonym version of this technique seems to break down at this point since type indices on an associated type must match the class instance head; i.e. we can't seem to write HEval with an associated type synonym instead of a functional dependency. The remainder of this message is just an exercise meant to show that this approach really does make HList (and type-level) programming/debugging easier. We will build the machinery needed to write a hFilter function directly (as a type class) as well as in terms of a hFoldr. These two forms of hFilter will behave identically in terms of type checking and error messages. In order to write a filter for HLists, we need type-level booleans > data HTrue > data HFalse > data HBool a where > HTrue :: HBool HTrue > HFalse :: HBool HFalse and an if-then-else construct. > class HIF yes no bool res | yes no bool -> res where > hIf :: yes -> no -> HBool bool -> res > instance HIF yes no HTrue yes where > hIf yes _ _ = yes > instance HIF yes no HFalse no where > hIf _ no _ = no > data HIf yes no = HIf yes no > instance (arg ~ HBool b, HIF yes no b res) => HEval (HIf yes no) arg res where > hEval (HIf yes no) = hIf yes no Note that HIF is the type class for hIf, a term level function, while HIf is a type-level version of hIf. We can now write hFilter > class HFILTER p xs res | p xs -> res where > hFilter :: p -> HList xs -> HList res > instance HFILTER p Nil Nil where > hFilter _ Nil = Nil > instance (HFILTER p xs vs, > HEval p x (HBool b), > HIF (HList (x :* vs)) (HList vs) b (HList res) > ) => HFILTER p (x :* xs) res > where > hFilter p (x :* xs) = hIf (x :* vs) vs (hEval p x) where vs = hFilter p xs and observe its type-level computation in ghci: *Main> :t hFilter HHead ((HTrue :* "a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* (HTrue :* Nil) :* Nil) hFilter HHead ((HTrue :* "a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* (HTrue :* Nil) :* Nil) :: HList (HList (HBool HTrue :* ([Char] :* (() :* Nil))) :* (HList (HBool HTrue :* Nil) :* Nil)) We can also see that type checking works as expected: *Main> :t hFilter HHead ((HTrue :* "a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* HTrue :* Nil) Top level: Couldn't match expected type `HList (HBool b :* xs)' against inferred type `HBool HTrue' *Main> :t hFilter HHead (("a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* (HTrue :* Nil) :* Nil) Top level: Couldn't match expected type `HBool b' against inferred type `[Char]' Expected type: HList (HBool b :* xs) Inferred type: HList ([Char] :* (() :* Nil)) We now move on to hFoldr. > class HFOLDR f acc xs res | f acc xs -> res where > hFoldr :: f -> acc -> HList xs -> res > instance HFOLDR f acc Nil acc where > hFoldr _ acc _ = acc > instance (HFOLDR f acc xs v, > HEval f x f1, > HEval f1 v res > ) => HFOLDR f acc (x :* xs) res > where > hFoldr f acc (x :* xs) = hEval (hEval f x) (hFoldr f acc xs) Note that since we assume the function argument to hEval is in weak head normal form, we must evaluate each piece of the resulting application of f to its two arguments. In order to have curried functions and partial applications, we will have type-level syntactic application which is used to build up type-level applications. > data f :$ x = f :$ x > infixl 3 :$ We can now write a type-level cons (:*) function, which also serves as an example for how :$ is meant to be used. > data Cons = Cons > instance HEval Cons x (Cons :$ x) where > hEval = (:$) > instance (arg ~ HList xs) => HEval (Cons :$ x) arg (HList (x :* xs)) where > hEval (Cons :$ x) = (x :*) Note that :$ must be used with care since hEval is not a general evaluation function. *Main> hEval HHead (Cons :$ "a" :$ Nil) <interactive>:1:0: Couldn't match expected type `HList (res :* xs)' against inferred type `(Cons :$ [Char]) :$ HList Nil' When generalising the type(s) for `it' *Main> hEval HHead (hEval (Cons :$ "a") Nil) "a" The lack of type-level lambdas forces higher-order type-level programming to be point-free. In order to facilitate point-free programming we need to be able to uncurry type-level functions. > data Uncurry = Uncurry > instance (arg ~ (x, y), HEval f x f1, HEval f1 y res) => HEval (Uncurry :$ f) arg res where > hEval (Uncurry :$ f) (x, y) = hEval (hEval f x) y Here is some machinery for manipulating pairs. > data Fst = Fst > instance (arg ~ (x, y)) => HEval Fst arg x where > hEval _ = fst > data Snd = Snd > instance (arg ~ (x, y)) => HEval Snd arg y where > hEval _ = snd > (f &&& g) x = (f x, g x) > data f :&&& g = f :&&& g > instance (HEval f x fv, HEval g x gv) => HEval (f :&&& g) x (fv, gv) where > hEval (f :&&& g) = hEval f &&& hEval g Note that we have not made :&&& fully partially applicable (nor HIf from above either). This limitation is for convenience (we won't be needing to partially apply either combinator) as nothing prevents writing the fully partially applicable combinators. (*) Since our goal is to write hFilter in terms of hFoldr, we should start by writing filter in terms of a point-free foldr: > fltr p = foldr (uncurry (myIf fst snd) . (p &&& ((:) &&& flip const))) [] > where myIf yes no b = if b then yes else no Now we just have to provide type-level versions of composition, flip and const. > data f :. g = f :. g > instance (HEval g x v0, HEval f v0 v) => HEval (f :. g) x v where > hEval (f :. g) = hEval f . hEval g > data Flip = Flip > instance HEval Flip f (Flip :$ f) where > hEval = (:$) > instance HEval (Flip :$ f) x (Flip :$ f :$ x) where > hEval = (:$) > instance (HEval (f :$ x) x0 res) => HEval (Flip :$ f :$ x0) x res where > hEval (Flip :$ f :$ x0) x = hEval (f :$ x) x0 > data Const = Const > instance HEval Const x (Const :$ x) where > hEval = (:$) > instance HEval (Const :$ x) y x where > hEval (Const :$ x) = const x Finally we can simply write our type-level filter in terms of hFoldr > hFltr p = hFoldr ((Uncurry :$ (HIf Fst Snd)) :. (p :&&& (Cons :&&& (Flip :$ Const)))) Nil and observe how it works in ghci: *Main> :t hFltr HHead ((HTrue :* "a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* (HTrue :* Nil) :* Nil) hFltr HHead ((HTrue :* "a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* (HTrue :* Nil) :* Nil) :: HList (HList (HBool HTrue :* ([Char] :* (() :* Nil))) :* (HList (HBool HTrue :* Nil) :* Nil)) *Main> :t hFltr HHead ((HTrue :* "a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* HTrue :* Nil) Top level: Couldn't match expected type `HList (HBool b :* xs)' against inferred type `HBool HTrue' *Main> :t hFltr HHead (("a" :* () :* Nil) :* (HFalse :* 'a' :* Nil) :* (HTrue :* Nil) :* Nil) Top level: Couldn't match expected type `HBool b' against inferred type `[Char]' Expected type: HList (HBool b :* xs) Inferred type: HList ([Char] :* (() :* Nil)) Any comments/feedback will be appreciated. Jeff (*) Here is a partially applicable fanout (:&&&): > data Fanout = Fanout > instance HEval Fanout f (Fanout :$ f) where > hEval = (:$) > instance HEval (Fanout :$ f) g (Fanout :$ f :$ g) where > hEval = (:$) > instance (HEval f x fx, HEval g x gx) => HEval (Fanout :$ f :$ g) x (fx, gx) where > hEval (Fanout :$ f :$ g) = hEval f &&& hEval g

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Manatee-0.1.8 release!
by Andy Stewart 12 Jan '11

12 Jan '11

Hi all, Manatee-0.1.8 release! New features in manatee-0.1.8: * Manatee now can works in XMonad * Use Ping/Pong solution fix immortal render process when daemon process killed by user forcely. * Move message/status update code to extension space. * File manager can monitor file change now * Add new package: manatee-template, template code to help create manatee application. * Add some utils functions in manatee-core Session manager will bring in 0.1.9, that's mean, Manatee will restore your layout before exit. IDE framework will bring in 0.2.0, support Haskell first, and will support C and Java in the future. Enjoy! :) Any patch and suggestion are welcome! -- Andy

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Re: [Haskell-cafe] GHC.Ptr, Foreign.Storable, Data.Storable.Endian, looking for good examples of usage
by John Lato 12 Jan '11

12 Jan '11

> > From: Aaron Gray <aaronngray.lists(a)gmail.com> > On 9 January 2011 21:30, Henning Thielemann > <lemming(a)henning-thielemann.de>wrote: > > > > > On Sun, 9 Jan 2011, Aaron Gray wrote: > > > > I am trying to work out how to use GHC.Ptr, Foreign.Storable, > >> Data.Storable.Endian, and > >> am looking for good examples of usage. > >> > > > > What do you intend to do with them? > > > > > An (ABC) ActionScript Byte Code backend for Haskell. > > Basically I need to write little-endian binary to a file, and was wondering > the best way to do this; I need various types including a 24bit type. > You could use my "word24" package[1] (GHC only) to provide non-aligned 24-bit word and int types with Storable instances. You should be able to write a binary instance (or whatever blaze-builder needs) fairly simply from this. Little-endian only ATM, but BE could be added if necessary. John Lato [1] http://hackage.haskell.org/package/word24

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ArchLinux binary repository available for beta testing
by Peter Simons 11 Jan '11

11 Jan '11

Hi guys, those of you who use the ArchLinux distribution might be interested to know that a team of volunteers has put together a binary package repository that complements the set of Haskell packages that's already being distributed by ArchLinux. Subscribers of that repository can use Pacman to install all of Haskell Platform 2010.2.0.0 as well as a few other popular packages such as bnfc, hledger, pandoc, sifflet, and yesod on both i686 and x86_64. If you want to use the repository, then append the following two lines at the end of your /etc/pacman.conf file: [haskell] Server = http://andromeda.kiwilight.com/$repo/$arch Please be aware of the fact that this is the very first public announcement of this repository, so you should consider it as being in a kind of beta state. Basically, if your Linux machine is responsible for controlling some large nuclear power plant or something, you probably shouldn't be using this. Everyone else is encouraged to try it out. If you encounter problems, have questions, or would like to make suggestions, then please raise an issue at <https://github.com/archhaskell/habs/issues>. Of course, you're also welcome to provide feedback by posting to the haskell-cafe mailing list or to arch-haskell(a)haskell.org. Many people have contributed to this effort in one way or another. Don Stewart originally wrote the cabal2arch tool that is being used to generate the HABS tree on which this repository is based. Rémy Oudompheng has extended that tool and the underlying ArchLinux library significantly, and he has also written most of the build system that's being used to compile the binary packages. Magnus Therning has compiled all the x86_64 binaries. The i686 binaries were compiled by Yours Truely. Kaiting Chen is kindly hosting the repository on his server. Furthermore, there are many other people who have submitted bug reports, suggestions, and fixes by way of AUR. Have fun, Peter

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Possible participation on parallel programming book
by Michael Oswald 11 Jan '11

11 Jan '11

Hello, Just found the following in the Ada group: There is an open source E-Book about parallel programming, where people can participate (via patches for a git repository): http://lwn.net/Articles/421425/ Since Haskell has very good parallel capabilities, maybe the community would also be interested in participating? lg, Michael

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Re: [Haskell-cafe] [Haskell] School of Expression pages missing from haskell.org
by Henning Thielemann 11 Jan '11

11 Jan '11

On Tue, 11 Jan 2011, Julian Gilbey wrote: > Does anyone know what has happened to the Haskell School of Expression > (SOE) webpages? The link from books for learning Haskell now gives a > 404 Not Found error. Maybe caused by the recent server move.

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Re: [Haskell-cafe] GHC.Ptr, Foreign.Storable, Data.Storable.Endian, looking for good examples of usage
by John Lato 11 Jan '11

11 Jan '11

> > On 10 January 2011 16:36, Antoine Latter <aslatter(a)gmail.com> wrote: > > > On Mon, Jan 10, 2011 at 10:17 AM, Aaron Gray <aaronngray.lists(a)gmail.com > > > > wrote: > > > On 10 January 2011 16:13, Daniel Fischer < > > daniel.is.fischer(a)googlemail.com> > > > wrote: > > >> > > >> On Monday 10 January 2011 16:45:36, Aaron Gray wrote: > > >> > > > >> > This is interesting, what does the following line do :- > > >> > > > >> > data Int24 = I24# Int# deriving (Eq, Ord) > > >> > > > >> > regarding the I24# and Int#, are these inbuilt ? > > >> > > >> Int# is the raw machine int (4 or 8 bytes) and I24# is the > constructor. > > >> GHC > > >> uses the magic hash '#' to denote raw unboxed types (and the > > constructors > > >> making ordinary boxed Haskell types from these, e.g. there's > > >> > > >> data Int = I# Int# > > >> data Word = W# Word# > > >> data Double = D# Double# > > >> > > >> and more defined in base [GHC.Types, GHC.Word]). > > > > > > So the 24 bit value is actually stored as a 32bit value. Meaning I will > > have > > > to do my own IO reader and writer code to a ByteString. > > > Thanks, > > > Aaron > > > > > > > I don't think so - the Storable instance provided for the Int24 type > > peeks and pokes 24-bit values. At least, that what I understand John's > > earlier message to mean. > > > Yes looking at the code it does support 24bit peeks and pokes. > This is correct (at least it's how it's meant to work). 24bit values are represented as 32bit words for ops, but peeks and pokes are 24bit. If all you want to do is read/write 24bit values there's nothing wrong with binary, but if you ever want to use them the Word24 and Int24 types are nice to have. John

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Join Manatee Team!
by Andy Stewart 11 Jan '11

11 Jan '11

Hi all, I'm Andy, the author of Manatee ( http://haskell.org/haskellwiki/Manatee ). You can watch video http://www.youtube.com/watch?v=weS6zys3U8k (or http://www.youtube.com/watch?v=A3DgKDVkyeM ) to understand "What is it?" . :) Manatee is Haskell integrated environment written in Haskell. The goal of the Manatee project is provide a fast, safe and flexible integrated environment for haskell hacking. Some cool feature in Manatee: * It can dynamic update Haskell extension code in runtime, like elisp for Emacs. * It protected every application in sandbox, so it minimum your losses once unexpected exception throw. * It's multi-thread design, it's won't freeze your operation in anytime. * It provide handy window manage, you can mix any application together fastly. * You can use keyboard control everything you want, and same experience in all applications. :) * You can use it develop any application you want, not just text toy. :) ... etc. I'm looking for new developer join Manatee Team, I believe you guys can develop cooler application than me. :) If you interested Manatee, welcome to join manatee-user(a)googlegroups.com (or https://groups.google.com/forum/#!forum/manatee-user ) Thanks, -- Andy PS. I'm working on session manager and IDE feature...

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ghci 6.12.3 segfaults for reasons unknown to me.
by Michael Litchard 11 Jan '11

11 Jan '11

mlitchard@kether:~/projects/perf/autoperf/session_creator/newtry2/strings$ ghci -v GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Glasgow Haskell Compiler, Version 6.12.3, for Haskell 98, stage 2 booted by GHC version 6.8.2 ghci was working a few weeks ago. I may have hidden a package or installed one to cause this segfault now. Could someone help me troubleshoot what is causing this? I did a stack trace but could not discern any useful info from it. Using binary package database: /usr/local/lib/ghc-6.12.3/package.conf.d/package.cache Using binary package database: /home/mlitchard/.ghc/x86_64-linux-6.12.3/package.conf.d/package.cache hiding package parsec-2.1.0.1 to avoid conflict with later version parsec-3.1.0 hiding package base-3.0.3.2 to avoid conflict with later version base-4.2.0.2 wired-in package ghc-prim mapped to ghc-prim-0.2.0.0-5da421112969a971aa3433fdf154b37a wired-in package integer-gmp mapped to integer-gmp-0.2.0.1-67f3940ec8fd509683668f40451c9ca1 wired-in package base mapped to base-4.2.0.2-cc69ae37cc080e111e11df0109986bd2 wired-in package rts mapped to builtin_rts wired-in package haskell98 mapped to haskell98-1.0.1.1-4d2891ad99eae334ff8234bcfbddce06 wired-in package template-haskell mapped to template-haskell-2.4.0.1-e9e9c63092746bd4a3f64cc37ddb1e06 wired-in package dph-seq mapped to dph-seq-0.4.0-5ed11291726022ff6bb22049478399e8 wired-in package dph-par mapped to dph-par-0.4.0-5a5a24e36763903b0b26f03d8d510a95 Hsc static flags: -static Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Segmentation fault

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POPL 2011 Hotel Room
by Michael D. Adams 10 Jan '11

10 Jan '11

Hi all, I'm a graduate student (male) and am looking for a (male) roommate to split the cost of a hotel room at POPL. If anyone is interested in splitting a room, please drop me a line. I will be attending the workshops collocated with POPL so I currently have a reservation at the conference hotel (at the conference rate) with check-in on January 22 (the day before the first workshop) and check-out on January 30 (the day after the last workshop). Michael D. Adams Ph.D. Student, Indiana University

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ANN: nanoparsec 0.1
by Maciej Piechotka 10 Jan '11

10 Jan '11

Nanoparsec is currently simply a port of attoparsec on the ListLike (the abstraction of lists used by iteratee). It allows to achive in parsing a near-attoparsec levels of speed (benchmarks from attoparsec library shown a 0.450 ± 0.028 for attoparsec, 0.479 ± 0.043 for nanoparsec and 1.532 ± 0.084 for parsec 3) combining the flexibility of stream of parsec 3 with the iterative approach and speed of attoparsec. Regards

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How to write such a code elegantly ?
by z_axis 10 Jan '11

10 Jan '11

betterStdGen :: IO StdGen betterStdGen = alloca $ \p -> do h <- openBinaryFile "/dev/urandom" ReadMode hGetBuf h p $ sizeOf (undefined :: Int) hClose h mkStdGen <$> peek p picoSec :: IO Integer picoSec = do t <- ctPicosec `liftM` (getClockTime >>= toCalendarTime) return t The pseudo-code is : if doesFileExist "/dev/urandom" then myGen = betterStdGen else myGen = (mkStdGen . fromTnteger) <$> picoSec How to write these pseudo-code elegantly ? Sincerely! ----- e^(π.i) + 1 = 0 -- View this message in context: http://haskell.1045720.n5.nabble.com/How-to-write-such-a-code-elegantly-tp3… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Draw K-ary forest in dot script
by larry.liuxinyu 10 Jan '11

10 Jan '11

Hi, I wrote a Haskell program to parse K-ary forest and convert it to dot script (Graphviz). Here is the literate program. -- First is some stuff imported: module Main where import System.Environment (getArgs) import Text.ParserCombinators.Parsec import Control.Monad (mapM_) import Data.List (concatMap, intercalate) import System.IO (writeFile) import Data.Char (isSpace) -- For each tree in the forest, it is described in pre-order. -- Example description string of a forest of CLRS[1] Figure 19.5(a): -- (12), (7, (25)), (15, (28, (41)), (33)) -- Definition of K-ary node data Node a = Node { root :: a , children :: [Node a]} deriving (Eq, Show) -- Definition of Forest type Forest a = [Node a] -- parsers -- a forest is a list of trees separate by ',' forest = do ts <- node `sepBy` (char ',') return ts -- a node contains a key then followed by a children forest or nothing (leaf case) node = do char '(' elem <- key ts <- (try (char ',')>>forest) <|> return [] char ')' return (Node elem ts) -- a key is just a plain literate string. key = many (noneOf ",()") -- Command line arguments handling parseArgs :: [String] -> (String, String) parseArgs [fname, s] = (fname, s) parseArgs _ = error "wrong usage\nexample:\nfr2dot output.dot \"(12), (7, (25)), (15, ((28, (41)), 33))\"" -- A simplified function to generate dot script from parsed result. toDot f = forestToDot f "t" True -- a handy function to convert children of a K-ary tree to dot script treesToDot ts prefix = forestToDot ts prefix False -- convert a forest to dot script forestToDot [] _ _ = "" forestToDot [t] prefix _ = nodeToDot t prefix forestToDot ts@(_:_:_) prefix lnk = (concatMap (\t->nodeToDot t prefix) ts) ++ consRoot where consRoot = "{rank=same " ++ ns ++ vis ++ "}\n" ns = intercalate "->" $ map (\t -> prefix ++ root t) ts vis = if lnk then "" else "[style=invis]" -- convert a node to dot script nodeToDot (Node x ts) prefix = prefix'++"[label=\""++x++"\"];\n" ++ (treesToDot ts prefix') ++ (defCons ts prefix') where prefix' = prefix ++ x -- define connections among nodes in dot format defCons ts prefix = concatMap f ts where f (Node x _) = prefix++"->"++prefix++x++";\n" -- generate dot script from a parsed forest genDot fname (Right f) = writeFile fname dots >> putStrLn dots where dots = "digraph G{\n\tnode[shape=circle]\n"++(addTab $ toDot f)++"}" addTab s = unlines $ map ("\t"++) (lines s) main = do args <- getArgs let (fname, s) = parseArgs args genDot fname (parse forest "unknown" (filter (not.isSpace) s)) -- END I tested with the following simple cases: ./fr2dot foo.dot "(12), (7, (25)), (15, (28, (41)), (33))" ./fr2dot bar.dot "(18), (3, (37)), (6, (8, (30, (45, (55)), (32)), (23, (24)), (22)), (29, (48, (50)), (31)), (10, (17)), (44))" Run the following commands can convert to PNG files: ./dot -Tpng -o foo.png foo.dot ./dot -Tpng -o bar.png bar.dot Reference: [1]. Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest and Clifford Stein. ``Introduction to Algorithms, Second Edition''. The MIT Press, 2001. ISBN: 0262032937. Best regards. -- Larry, LIU https://sites.google.com/site/algoxy/home

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Ratification for my understanding of functors
by C K Kashyap 10 Jan '11

10 Jan '11

Hi, Thanks to Luke Palmer's explanation. I got a better idea of functors. I just want to ratify my understanding of what a functor is - according to category theory, functors are arrows between categories ... these arrows map objets and arrows from one category into another. So, would it be right to say that the list type constructor + the map function form the functor? Saying "list is a functor then is a convenient approximation right? In this context, could someone please point me to some writeup on co-variant/contravariant type constructors? Googling for it seems to only give a lot of C++/Java/C# hits. Regards, Kashyap

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GHC.Ptr, Foreign.Storable, Data.Storable.Endian, looking for good examples of usage
by Aaron Gray 10 Jan '11

10 Jan '11

Hi, I am trying to work out how to use GHC.Ptr, Foreign.Storable, Data.Storable.Endian, and am looking for good examples of usage. Many thanks in advance, Aaron

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