March 2012 - Haskell-Cafe

Re: [Haskell-cafe] Unable to call function from DLL using FFI
by rajendra prasad 15 Mar '12

15 Mar '12

Hi, Thanks for very quick reply Claude. Your suggestion resolved the issue. Using extern "C" worked for me. Thank you very much. Thanks to all. Regards, Rajendra > > ---------- Forwarded message ---------- > From: Claude Heiland-Allen <claude(a)goto10.org> > Date: Wed, Mar 14, 2012 at 7:43 PM > Subject: Re: [Haskell-cafe] Unable to call function from DLL using FFI > To: haskell-cafe(a)haskell.org > > > On 14/03/12 14:01, rajendra prasad wrote: > >> My c++ code(HelloWorld.cpp) looks like this: >> > > Try adding extern "C" { ... } to use the C ABI instead of a C++ ABI (which > usually features symbol name mangling to add type information, among other > things). (This may not solve the entire problem, but is an important > puzzle-piece). > > > __declspec(dllexport) int doubleMe(int a) >> { >> return 2*a; >> } >> > > > Claude > > ______________________________**_________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> > >

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Mapping string to a function
by Haisheng Wu 15 Mar '12

15 Mar '12

Hi there, Do you have any comments / suggestions for the following scenario? I have two list and a function over list testdata :: [Int] testdata2 :: [Int] f testdata = map g testdata What I like to do is choosing what test data via command line arguments. i.e. test.hs testdata2 will run against testdata2 I could make it using pattern match between argument and data definition but it is annoy. code here: https://github.com/freizl/dive-into-haskell/blob/master/sandbox/one-in-arit… I'm wondering it can be done simply in haskell. Thanks a lot. -Haisheng

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A new home for haskell-mode
by Johan Tibell 14 Mar '12

14 Mar '12

Hi all, After a discussion with Philip Weaver and a few other interested parties in the community we've moved Philip's repo from https://github.com/pheaver/haskell-mode to the new official location at: https://github.com/haskell/haskell-mode You can file bugs/feature requests at: https://github.com/haskell/haskell-mode/issues Hosting the repo under the Haskell GitHub organization gives us a few benefits: * It's easier to find. (We'll update all the references out there.) * We can add co-maintainers, to offload Philip and make sure pull requests get merged in a timely manner. * The repo doesn't have to move if Philip no longer has time to maintain haskell-mode some time in the future. Cheers, Johan

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using mutable data structures in pure functions
by E R 14 Mar '12

14 Mar '12

Consider the following idea for implementing pure functions: A pure function can allocate and modify memory as long as a) it never returns a reference to the memory or b) it never again modifies the memory once it returns (i.e. it returns an immutable object). This is based on the idea of "value objects" in object-oriented languages - the only mutation of a value object occurs in the constructor. Once the constructor is finished initializing the object it becomes immutable. My first question is whether or not this is accurate or does it need some qualifications / fixing up. Secondly, where does this idea fit into the Haskell philosophy? For example, consider the definition of Data.List.nub: nub l = nub' l [] where nub' [] _ = [] nub' (x:xs) ls | x `elem` ls = nub' xs ls | otherwise = x : nub' xs (x:ls) Clearly the memory allocated to ls never escapes nub', so it seems that ls could be replaced with a mutable data structure (with an eye towards improving performance in special cases). For another example, consider Data.Map.fromList. I kind of expected fromList to build up the map using a mutable data structure and then "seal it up" before returning it, but it seems to call the same insert that one would call to add to the map after it has been constructed. So, again, what is the Haskell philosophy towards using mutable data structures in pure functions? Is it: 1. leave it to the compiler to find these kinds of opportunities 2. just use the immutable data structures - after all they are just as good (at least asymptotically) 3. you don't want to use mutable data structures because of _____ 4. it does happen, and some examples are ______ Thanks, ER

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[ANNOUNCE] JuicyPixels 1.1 - image loading library (maintenance release)
by Vincent Berthoux 14 Mar '12

14 Mar '12

Hello list, This is a maintenance release without new functionality, the central change is the switch from Haskell Array to Data.Vector (to ease it's use for OpenGL texture loading). Juicy.Pixels 1.1 change log : - Fixing automatic bitmap decoding, the decodeImage function wasn't trying to decode bitmap - Converting Haskell array types to Data.Vector (thanks to Jason Dagit) - Some tiny documentation enhancement Hackage : http://hackage.haskell.org/package/JuicyPixels GitHub : https://github.com/Twinside/Juicy.Pixels Thanks Vincent Berthoux

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An idea to document inter department dependencies in Haskell
by C K Kashyap 14 Mar '12

14 Mar '12

My dear Haskell folks, I work in a software company where I develop components that go into a really complex system that's built of several components developed by different teams that are geographically distributed. The components themselves run in a distributed manner across client and several servers. All our design documents are in wiki's (fashionably so). As a result of the above situation and the fact that our code base is not in Haskell, we are almost always dealing with "Oh I did not know this would effect that" and "Oh I have no clue what all this change will impact". I've been wondering if it would be a good idea to try and create a spec for the system in Haskell, such that would could get a better handle on the dependencies. Perhaps an EDSL, that would allow Product Managers to introduce new requirements - compilation failures would indicate the areas that would need to be touched. How is it different from having a spec in a diagram - well, in that case, we are requiring humans to look at the diagram to detect dependencies instead of the compiler telling me about the dependencies. I am about to start off with some implementation to capture my idea - I can articulate it better then. However, I just wanted to throw it out there to check if anyone's had some thought in this direction or if there is some prior art here. Regards, Kashyap

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Is there a better way to subtyping?
by Jeff Shaw 14 Mar '12

14 Mar '12

More specifically, if I have a record type from which I construct multiple sub-record types, and I want to store these in a collection which I want to map over while preserving the ability to get at the sub-fields, is there a better way to do it than to have an enumeration for the sub-types and then use Dynamic? I also have a nastier version that doesn't require the enumeration, which throws an exception when fromDynamic can't return a value with one of the expected types. {-# LANGUAGE Rank2Types, DeriveDataTypeable #-} module Super where import Data.Dynamic import Data.Typeable import Data.Maybe data Super a = Super { commonFields :: (), subFields :: a } deriving Typeable data SubTypes = SubA | SubB | SubC data A = A { aFields :: () } deriving Typeable data B = B { bFields :: () } deriving Typeable data C = C { cFields :: () } deriving Typeable doSomethingWithSubType :: (Super A -> ()) -> (Super B -> ()) -> (Super C -> ()) -> (SubTypes, Dynamic) -> Maybe () doSomethingWithSubType a _ _ (SubA, dynamic) = fromDynamic dynamic >>= return . a doSomethingWithSubType _ b _ (SubB, dynamic) = fromDynamic dynamic >>= return . b doSomethingWithSubType _ _ c (SubC, dynamic) = fromDynamic dynamic >>= return . c doSomethingWithSubType2 :: (Super A -> ()) -> (Super B -> ()) -> (Super C -> ()) -> Dynamic -> () doSomethingWithSubType2 a b c dynamic = let dynamicAsA = fromDynamic dynamic :: Maybe (Super A) dynamicAsB = fromDynamic dynamic :: Maybe (Super B) dynamicAsC = fromDynamic dynamic :: Maybe (Super C) in head $ catMaybes [ dynamicAsA >>= return . a , dynamicAsB >>= return . b , dynamicAsC >>= return . c]

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Adding type annotations to an AST?
by Stephen Tetley 14 Mar '12

14 Mar '12

Hello All How do I add type annotations to interior locations in an abstract syntax tree? I have a small ML language where programs are a single expression, but the expression type has let-rec so it supports recursive function definitions. Source programs have no type annotations. I want to annotate the lec-rec with type annotations, but my type inference is a simple implementation of Algorithm J so it seems "monolithic" to me - i.e. the type it infers is the type of the whole program, I would also like the types of any internal let-rec definitions so I can label my AST. What is the idiom for accomplishing this? I'm using Eijiro Sumii's MinCaml as reference - this embeds mutable references for types in the AST. So this appears a non-starter. My other thought is to uniquely label all type locations during parsing then collect them in a map (location -> type) whilst computing the outermost type - I presume I can identify inner types as they are resolved during Algorithm J's run... Any solutions? Many thanks Stephen

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Unable to call function from DLL using FFI
by rajendra prasad 14 Mar '12

14 Mar '12

Hi, I am new to Haskell and I need to call a c function by loading dynamic link library in Haskell. I started with very simple code to create a dll in visual studio 2008 and then trying to load it in haskell. I am listing here the steps I follwed to achieve this: *Step 1: * My c++ code(HelloWorld.cpp) looks like this: __declspec(dllexport) int doubleMe(int a) { return 2*a; } I created the dll using following command: cl /D_USRDLL /D_WINDLL HelloWorld.cpp /link /dll /out:LoadDLL.dll *Step 2:* My Haskell code(Main.hs) looks like this: {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Foreign.C foreign import ccall "LoadDLL doubleMe" twice :: CInt -> IO (CInt) main = do let x = 4 y <- twice x print y I am compling haskell code using following command: ghci Main.hs -L. -lLoadDLL I am finally getting the following error: During interactive linking, GHCi couldn't find the following symbol: doubleMe This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs(a)haskell.org Please help me in identifying the mistake I am doing. If possible please let me know the exact steps to be followed to solve this issue. Thank you so much in advance, Regards, Rajendra

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Why so many strings in Network.URI, System.Posix and similar libraries?
by Jason Dusek 14 Mar '12

14 Mar '12

The content of URIs is defined in terms of octets in the RFC, and all Posix interfaces are byte streams and C strings, not character strings. Yet in Haskell, we find these objects exposed with String interfaces: > :info Network.URI.URI data URI = URI {uriScheme :: String, uriAuthority :: Maybe URIAuth, uriPath :: String, uriQuery :: String, uriFragment :: String} -- Defined in Network.URI > :info System.Posix.Env.getEnvironment System.Posix.Env.getEnvironment :: IO [(String, String)] -- Defined in System.Posix.Env But there is no law that environment variables must be made of characters: :; export x=$'\xFF' ; echo -n $x | xxd -p ff :; locale LANG="en_US.UTF-8" That the relationship between bytes and characters can be confusing, both in working with UNIX and in dealing with web protocols, is undeniable -- but it seems unwise to limit the options available to Haskell programmers in dealing with these systems. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

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