- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] The state of binary (de)serialization
by wren ng thornton 28 Feb '13

28 Feb '13

On 2/27/13 2:17 AM, Vincent Hanquez wrote: > Two major problems of lazy bytestrings is that: > > * you can't pass it to a C bindings easily. > * doing IO with it without rewriting the chunks, can sometimes (depending > how the lazy bytestring has been produced) result in a serious degradation of > performance calling syscalls on arbitrary and small chunks (e.g. socket's 'send'). If you're on a POSIX system, you can always make use of unix-bytestring[1]. In particular, for lazy ByteStrings the function you want is System.Posix.IO.ByteString.Lazy.fdWritev which performs a single syscall to write all the chunks, and without manually concatenating them. [1] http://hackage.haskell.org/package/unix-bytestring -- Live well, ~wren

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ANN: cabal-dev 0.9.2 - sandboxed haskell builds
by Rogan Creswick 28 Feb '13

28 Feb '13

The (ever growing) cabal-dev team is happy to announce v. 0.9.2! Cabal-dev is a tool to test development libraries by creating a sandboxed package and dependency build environment. Executing `cabal-dev install` will create a sandbox, named cabal-dev in the current directory, and populate it with the project dependencies, which are built and installed into a package database within the sandbox. This is a bug-fix/maintenance release, but it's worth noting that it is now possible to build cabal-dev from hackage with a recent GHC. We've also decided to focus our efforts to provide support for the current release of GHC, the current Haskell Platform, and the previous Haskell Platform. This is the long-awaited version that actually builds with recent GHC's (7.6, in particular). A great deal of thanks for this release to go to active members of the community who've patiently directed other Haskell users to the github repo for a working cabal-dev, and in particular to the dozens (!) of you who've submitted bug reports and patches that resulted in this release (in particular: Bob Ippolito for not only fixing a race condition that caused the test suite to fail. Thanks Bob!)). Urls: * Hackage: http://hackage.haskell.org/package/cabal-dev * Issue tracker: http://github.com/creswick/cabal-dev/issues * Github repo: http://github.com/creswick/cabal-dev --Rogan

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Help to write type-level function
by Dmitry Kulagin 27 Feb '13

27 Feb '13

Hi, I try to implement typed C-like structures in my little dsl. I was able to express structures using type-level naturals (type Ty is promoted): > data Ty = TInt | TBool | TStruct Symbol [Ty] That allowed to implement all needed functions, including type-level function: > type family Get (n :: Nat) (xs :: [Ty]) :: Ty But it is not very convenient to identify struct's fields using naturals, and I wanted to change Ty definition to: > data Ty = TInt | TBool | TStruct Symbol [(Symbol, Ty)] It is much closer to how C-struct looks, but I was unable to implement required type function: > type family Find (s :: Symbol) (xs :: [(Symbol,Ty)]) :: Ty Which just finds a type in a associative list. Could someone give me a hint, how to do it? Or perhaps, is it just impossible thing to do? Thanks!

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Conflicting bindings legal?!
by Andreas Abel 27 Feb '13

27 Feb '13

To your amusement, I found the following in the Agda source: abstractToConcreteCtx :: ToConcrete a c => Precedence -> a -> TCM c abstractToConcreteCtx ctx x = do scope <- getScope let scope' = scope { scopePrecedence = ctx } return $ abstractToConcrete (makeEnv scope') x where scope = (currentScope defaultEnv) { scopePrecedence = ctx } I am surprised this is a legal form of shadowing. To understand which definition of 'scope' shadows the other, I have to consult the formal definition of Haskell. But I studied computer science to *not* become a lawyer!! -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

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Running out of space while concatinating a list of bytestring
by C K Kashyap 27 Feb '13

27 Feb '13

Hi, I have the following code - It looks like things go okay until concatination is attempted. I get the following output There are 2258 ByteStrings *** Exception: <stdout>: hPutBuf: resource exhausted (Not enough space) I am thinking that I should do strict concatination at each point in the support function - how can I go about doing so? (BS is the lazy.char8 bytestring) connectionGetNBytes :: NC.Connection -> Int -> IO ByteString connectionGetNBytes c n = do bs <- connectionGetNBytes' c n putStrLn ("There are " ++ (show (length bs)) ++ " ByteStrings") return (BS.concat bs) connectionGetNBytes' :: NC.Connection -> Int -> IO [ByteString] connectionGetNBytes' _ 0 = return [] connectionGetNBytes' c n = do l <- NC.connectionGet c n let ll = BS.length l remaining <- connectionGetNBytes' c (n - ll) return (l:crlfStr:remaining) Regards, Kashyap

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Help to write type-level function
by oleg＠okmij.org 27 Feb '13

27 Feb '13

Dmitry Kulagin wrote: > I try to implement typed C-like structures in my little dsl. HList essentially had those http://code.haskell.org/HList/ > I was unable to implement required type function: > type family Find (s :: Symbol) (xs :: [(Symbol,Ty)]) :: Ty > Which just finds a type in a associative list. HList also implemented records with named fields. Indeed, you need a type-level lookup in an associative list, and for that you need type equality. (The ordinary List.lookup has the Eq constraint, doesn't it?) Type equality can be implemented with type functions, right now. http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable (That page also defined a type-level list membership function).

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tplot (out of memory)
by Manish Trivedi 27 Feb '13

27 Feb '13

Hi, I am running into a weird out of memory issue. While running timeplot over an input file having ~800 rows. From below provided info, seems like machine has enough ram (1849MB). Please let me know if anyone has pointers. * # free -m * total used free shared buffers cached Mem: 3825 1975 1849 0 13 71 -/+ buffers/cache: 1891 1934 Swap: 4031 111 3920 *#time tplot -o out.png -or 1024x768 -k 'CurrentPerHour' 'lines' -k 'RequiredPerHour' 'lines' -if adgroup_delivery_chart.input -tf 'date %Y-%m-%d %H:%M:%OS' * tplot: user error (out of memory) real 0m0.026s user 0m0.018s sys 0m0.008s -Manish

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GHC API wiki update
by satvik chauhan 27 Feb '13

27 Feb '13

I was playing with the GHC api and found that examples on the GHC wiki don't seem to work with GHC 7.6.1. Does anybody have the updated examples of the same? Regards, Satvik

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derivatives and integrals
by Christopher Howard 27 Feb '13

27 Feb '13

Hi. The scope of this question is likely bigger that Haskell, but this seems like the right crowd to ask. I'm in Calc II right now, and I'm looking for a FOSS desktop application (I use Gnu/Linux) to replace the functionality of my TI-98 in finding derivatives and integrals. (It's very convenient for double checking my own solutions.) I've heard of some software like Octave, but the manuals are usually very large, and I'm not sure what is the best one to start with. I was wondering if there is anyone on this list who uses free software to do that sort of simpler Calc stuff, and what it is that they use. -- frigidcode.com

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package dependencies should distinguish between API and implementation?
by Johannes Waldmann 27 Feb '13

27 Feb '13

Package dependencies are modelled by a relation "A depends-on B". Shouldn't this in fact be two relations: API-depends-on and implementation-depends-on? (meaning that A API-depends-on B iff some type of B is visible to the user of A) So what we currently have is implementation-depends-on and API-depends-on is a sub-relation of that. The point being the following: assume * A implementation-depends-on B.1 * not (A API-depends-on B.1), * U implementation-depends-on A * U implementation-depends-on B >= 2 Then U (indirectly) implementation-depends on two versions of B but it should still be safe? (e.g., I can install B.3, re-compile U, but keep A) Example: A = template-haskell, B = containers (at least I don't see any mention of Data.Map/Set in th's API, I think the only dependency is in the implementation of PprM http://hackage.haskell.org/packages/archive/template-haskell/2.8.0.0/doc/ht… ) or in general, A (and B.1) are baked into ghc, but there is some B.2/B.3 out there which U wants to use. Or is this what already happens? (ghc would notice that B.1.foo is different from B.2.foo. cabal-install would warn, but proceed? Then the effect of the proposal would just be to switch off these warnings in some cases?) - J.W.

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