February 2013 - Haskell-Cafe

Question about forkIO
by C K Kashyap 28 Feb '13

28 Feb '13

Hi All, Say I have a haskell function 'f' that does a forkIO and starts an action "a". I create a DLL of this haskell code and inovke "f" from C. Can I expect the "a" to continue to run once "f" has returned to C? Regards, Kashyap

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How does one create an input handle bound to a string instead of a file?
by John D. Ramsdell 28 Feb '13

28 Feb '13

How does one create a value of type System.IO.Handle for reading that takes its input from a string instead of a file? I'm looking for the equivalent of java.io.StringReader in Java. Thanks in advance. John

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Implementing name-mangling in GHC?
by Ismael Figueroa Palet 28 Feb '13

28 Feb '13

Hi café, I'm working on a small project, and I need to rename all ocurrences of a data type in a module. For instance, if the program is module Foo where data Bar fun :: Bar -> Bar fun = ... I'd like that during compilation the module looks as if it was written like: module Foo where data _BarXZY fun :: _BarXZY -> _BarXZY fun ... I think the simplest way to do this is using TH to define a special $data splice, which directly mangles the name. But then I'd need another splice every time I want to use the type. I've read other options like implementing a GHC compiler plugin ( http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/compiler-plugins.html ) is it possible to perform the renaming by modifying the ModGuts structure in a compiler pass? Can you please give more pointers on how to perform this task? Thanks -- Ismael

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Problem accessing Hackage
by Roman Cheplyaka 28 Feb '13

28 Feb '13

I have this weird problem accessing hackage, when the first response gets delayed by 30 seconds: http://superuser.com/q/557271/53951 Does anyone else experience it? (No need to reply with "works for me" — I know that it works for most people.) Any clues about what can be the problem? Roman

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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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