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Maybe a, The Rationale
by PR Stanley 14 May '08

14 May '08

Hi folks data Maybe a = Nothing | Just a What is the underlying rationale for the Maybe data type? is it the safe style of programming it encourages/ Something tells me this is going to start a lengthy discussion. :-) Cheers, Paul

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Re: Trying to avoid duplicate instances
by oleg＠okmij.org 14 May '08

14 May '08

Eric Stansifer wrote: > I am using a bunch of empty type classes to categorize some objects: > > > class FiniteSolidObject o > > class FinitePatchObject o > > class InfiniteSolidObject o > > Since "solid objects" are exactly "finite solid objects" plus > "infinite solid objects", there is an obvious way to code up this > logical relationship. So I try to write: > > > class SolidObject o > > instance FiniteSolidObject o => SolidObject o > > instance InfiniteSolidObject o => SolidObject o There is an easy way to accomplish your goal in GHC, with a couple of simple tricks. First of all, it pays in this and many related cases do define the full predicate for the classification of objects. That is, rather than defining the constraint (FiniteSolidObject o) that succeeds when 'o' is really a FiniteSolidObject and fails otherwise, define a constraint (FiniteSolidObjectP o f) which always succeeds; it unifies the second argument with HTrue if o is indeed FiniteSolidObject, and unifies f with HFalse otherwise. We can easily define the semi-predicate (FiniteSolidObject o) if so needed. That is the first trick. One can then introduce typeclasses HAnd, HOr, etc. and use them to define more complex predicates like SolidObject (which is the logical OR of FiniteSolidObjectP and InfiniteSolidObjectP). Defining the primitive predicates such as FiniteSolidObject does including the trick, the TypeCast. It may appear bizarre; well, if the point is to use it rather than contemplate it, one can just take the pattern for granted. It works. http://okmij.org/ftp/Haskell/typecast.html But there is even a more uniform way, explained in http://okmij.org/ftp/Haskell/poly2.hs http://okmij.org/ftp/Haskell/poly2.txt Using that file, we can write your code as follows: {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module Mem where -- primitive types data Box = Box data Sphere = Sphere data Mesh = Mesh data Plane = Plane -- classes of types type FiniteSolidObjects = Box :*: Sphere :*: HNil type FinitePatchObjects = Mesh :*: HNil type InfiniteSolidObjects = Plane :*: HNil -- All of finite and infinite solid objects are solid objects type SolidObjects = AllOf FiniteSolidObjects :*: AllOf InfiniteSolidObjects :*: HNil -- membership predicate -- Statically tests if an object of the type x is a member of the class c is_of_class :: forall c x r. Apply (Member c) x r => x -> c -> r is_of_class x t = apply (undefined::Member c) x test1 = is_of_class Box (undefined::FiniteSolidObjects) -- type HTrue test2 = is_of_class Box (undefined::SolidObjects) -- type HTrue test3 = is_of_class Box (undefined::InfiniteSolidObjects) -- type HFalse test4 = is_of_class Plane (undefined::SolidObjects) -- type HTrue -- make a semi-predicate SolidObject class SolidObject c instance Apply (Member SolidObjects) x HTrue => SolidObject x test_solid :: SolidObject x => x -> () test_solid = undefined ts1 = test_solid Plane -- ts3 = test_solid Mesh -- causes the type error -- The following is borrowed verbatim from poly2.hs type Fractionals = Float :*: Double :*: HNil type Nums = Int :*: Integer :*: AllOf Fractionals :*: HNil type Ords = Bool :*: Char :*: AllOf Nums :*: HNil type Eqs = AllOf (TypeCl OpenEqs) :*: AllOfBut Ords Fractionals :*: HNil -- The Fractionals, Nums and Ords above are closed. But Eqs is open -- (i.e., extensible), due to the following: data OpenEqs instance TypeCls OpenEqs () HTrue -- others can be added in the future -- Type class membership testing data AllOf x data AllOfBut x y data TypeCl x -- Classifies if the type x belongs to the open class labeled l -- The result r is either HTrue or HFalse class TypeCls l x r | l x -> r -- the default instance: x does not belong instance TypeCast r HFalse => TypeCls l x r -- Deciding the membership in a closed class, specified -- by enumeration, union and difference data Member tl instance Apply (Member HNil) x HFalse instance TypeCls l x r => Apply (Member (TypeCl l)) x r instance (TypeEq h x bf, MemApp bf t x r) => Apply (Member (h :*: t)) x r instance (Apply (Member h) x bf, MemApp bf t x r) => Apply (Member ((AllOf h) :*: t)) x r instance (Apply (Member exc) x bf, Apply (MemCase2 h t x) bf r) => Apply (Member ((AllOfBut h exc) :*: t)) x r class MemApp bf t x r | bf t x -> r instance MemApp HTrue t x HTrue instance Apply (Member t) x r => MemApp HFalse t x r -- we avoid defining a new class like MemApp above. -- I guess, after Apply, we don't need a single class ever? data MemCase2 h t x instance Apply (Member t) x r => Apply (MemCase2 h t x) HTrue r instance Apply (Member ((AllOf h) :*: t)) x r => Apply (MemCase2 h t x) HFalse r testm1 = apply (undefined::Member Fractionals) (1::Float) testm2 = apply (undefined::Member Fractionals) (1::Int) testm3 = apply (undefined::Member Fractionals) () -- The standard HList stuff, extracted from HList library data HNil = HNil data a :*: b = a :*: b infixr 5 :*: data HTrue data HFalse data Z = Z newtype S n = S n class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b -- A heterogeneous apply operator class Apply f a r | f a -> r where apply :: f -> a -> r apply = undefined -- Normal function application instance Apply (x -> y) x y where apply f x = f x

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HSFFIG compilation problem
by Harri Kiiskinen 14 May '08

14 May '08

Hello, I've tried to compile hsffig-1.0pl2, unsuccesfully, also from the unstable darcs repo. I'm using ghc-6.8 (ghc6-6.8.2 from Debian unstable). HSFFIG comes with its own version of Cabal, but I cannot get past the configuration phase (./cabal-setup configure), when I get the following output: Configuring HSFFIG-1.0... configure: searching for ghc in path. configure: found ghc at /usr/bin/ghc configure: looking for package tool: ghc-pkg near compiler in /usr/bin/ghc configure: found package tool in /usr/bin/ghc-pkg configure: Using install prefix: /usr/local configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.8.2 configure: Using package tool: /usr/bin/ghc-pkg configure: Using haddock: /usr/bin/haddock configure: Using happy: /usr/bin/happy configure: Using alex: /usr/bin/alex configure: Using hsc2hs: /usr/bin/hsc2hs configure: Using cpphs: /usr/bin/cpphs configure: Reading installed packages... configure: Dependency base-any: using base-3.0.1.0 cannot satisfy dependency text-any Does anyone have an idea, how the dependency 'text-any' could be satisfied? Harri K.

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Analog data acquisition
by Tom Nielsen 14 May '08

14 May '08

Hello, I would like to use a lazy, purely functional language to create an experiement description (and execution!) language for cellular neuroscience, i.e. electrical recordings and stimulation. Unfortunately, this means I have to talk to a Analog-to-Digital/Digital-to-Analog converter board, with a precision of about 16 bit at a rate of 50 kHz. I currently have a National Instruments M-series board, but would be happy to try another board. I'm not looking for real-time control right now, but that might be of interest in the future. Has anyone looked at creating bindings to the NI-DAQmx drivers or the open-source COMEDI project, or similar hardware? Would anyone be interested in helping out with this driver binding? Regards Tom

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Trying to avoid duplicate instances
by Eric Stansifer 14 May '08

14 May '08

I am using a bunch of empty type classes to categorize some objects: > class FiniteSolidObject o > class FinitePatchObject o > class InfiniteSolidObject o ... > instance FiniteSolidObject Box > instance FiniteSolidObject Sphere > instance FinitePatchObject Mesh > instance InfiniteSolidObject Plane ... and would like to write a function elsewhere with a type signature like: > intersection :: (SolidObject o1, SolidObject o2) => o1 -> o2 -> Intersection o1 o2 These classes are not exposed to the user (which will be me, but that's besides the point) so the the user (me) does not accidentally try to intersect two meshes, etc. So long as I define my instances correctly the compiler will verify this at compile time. Since "solid objects" are exactly "finite solid objects" plus "infinite solid objects", there is an obvious way to code up this logical relationship. So I try to write: > class SolidObject o > instance FiniteSolidObject o => SolidObject o > instance InfiniteSolidObject o => SolidObject o but inevitably GHC complains that I have duplicate instance declarations. I had mistakenly thought that GHC would only give such an error if it could exhibit a specific type 'o' which satisfied both the context 'FiniteSoildObject o' and the context 'InfiniteSolidObject o' (and I know that there does not exist any such 'o' because I define my instances such that 'FiniteSolidObject' and 'InfiniteSolidObject' will never coincide, and I do not export any of these type classes), but I see now that this is not the case. Adding flags like -fallow-overlapping-instances doesn't convince GHC. Is there a way around this other than manually writing out all the instances I want? That is, > instance FiniteSolidObject Box > instance FiniteObject Box > instance SolidObject Box > instance UnionableObject Box > instance Object Box > > instance FiniteSolidObject Sphere ... Thanks, Eric

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Instance of Eq on cyclic data structure?
by Leonard Siebeneicher 14 May '08

14 May '08

Hi. I am testing a winged edge data structure. > module Main where > > type Point3 = (Float, Float, Float) > > data Body= Body [Vertex] [Edge] [Face] deriving(Eq) > data Face= Face Edge deriving(Eq) > data Edge= Edge (Vertex, Vertex) > (Face, Face) > (Edge, Edge) > (Edge, Edge) deriving(Eq) > data Vertex= Vertex Point3 Edge deriving(Eq) ... > --- Face, Edge, Vertex: Conversion > > getVertsFromFace :: Face -> [Vertex] > getVertsFromFace (Face startEdge) = process_wing_clockwise startEdge > where > process_wing_clockwise (Edge > (vp, _) > _ > (_, eWing1) > _) > | startEdge == eWing1 = > [vp] > | otherwise = > vp : (process_wing_clockwise eWing1) > > > --- Misc procedures > printPointList :: [Vertex] -> IO () > printPointList [] = return () > printPointList ((Vertex p _ ):vs) = putStr (show p) >> > printPointList vs > > --- main > main = > do > let q = createQuad (0,0,0) (0,1,0) (1,1,0) (1,0,0) > let f = (\(Body _ _ (face:faces)) -> face) q > let verts = getVertsFromFace f > printPointList verts > > Here I get a error message. > (0.0,0.0,0.0)(0.0,1.0,0.0)(1.0,1.0,0.0)runhugs: Error occurred > > ERROR - Control stack overflow > I think this come because the derived Eq-Instance "==" get into an infinite loop Alternatively I tried following > --- We want to identify each Body, Face, Edge or Vertex by a number ID > type ID = Int --- A special number for identifying, used for Eq > > data Body = Body ID [Vertex] [Edge] [Face] > data Face = Face ID (Edge) > data Edge = Edge ID > (Vertex , Vertex) > (Face , Face) > (Edge , Edge) > (Edge , Edge) > data Vertex = Vertex ID Edge > > --- Eq Instances > instance Eq Vertex) where > (Vertex i1 _) == (Vertex i2 _) = i1 == i2 > > instance Eq Face where > (Face i1 _ ) == (Face i2 _) = i1 == i2 > > instance Eq Edge where > (Edge i1 _ _ _ _) == (Edge i2 _ _ _ _) = i1 == i2 > > > instance Eq (Body) where > (Body i1 _ _ _) == (Body i2 _ _ _) = i1 == i2 ... This way my code does not hang up. But I have to manage those ugly ID's. Is there a better way to create instances of class Eq, without something like ID? Thanks for reading. Best regards, Leonard > > > -------------------- > -------------------- > ---- Begin ---- > -------------------- > module Main where > > type Point3 = (Float, Float, Float) > > data Body= Body [Vertex] [Edge] [Face] deriving(Eq) > data Face= Face Edge deriving(Eq) > data Edge= Edge (Vertex, Vertex) > (Face, Face) > (Edge, Edge) > (Edge, Edge) deriving(Eq) > data Vertex= Vertex Point3 Edge deriving(Eq) > > {- > implementing simple generative modelling > -} > > --- elementar object generation > createQuad :: Point3 -> > Point3 -> > Point3 -> > Point3 -> > Body > > createQuad p0 p1 p2 p3 = > let > {faceFront = Face edge0 > ;faceBack = Face edge2 > ;vert0 = Vertex p0 edge0 > ;edge0 = Edge > (vert0, vert1) > (faceFront, faceBack) > (edge3, edge1) > (edge1, edge3) > ;vert1 = Vertex p1 edge1 > ;edge1 = Edge > (vert1, vert2) > (faceFront, faceBack) > (edge0, edge2) > (edge2, edge0) > ;vert2 = Vertex p2 edge2 > ;edge2 = Edge > (vert2, vert3) > (faceFront, faceBack) > (edge1, edge3) > (edge3, edge1) > ;vert3 = Vertex p3 edge3 > ;edge3 = Edge > (vert3, vert0) > (faceFront, faceBack) > (edge2, edge0) > (edge0, edge2) > } > in > Body [vert0, vert1, vert2, vert3] [edge0, edge1, edge2, edge3] > [faceFront, faceBack] > > --- Face, Edge, Vertex: Conversion > > getVertsFromFace :: Face -> [Vertex] > getVertsFromFace (Face startEdge) = process_wing_clockwise startEdge > where > process_wing_clockwise (Edge > (vp, _) > _ > (_, eWing1) > _) > | startEdge == eWing1 = > [vp] > | otherwise = > vp : (process_wing_clockwise eWing1) > > > --- Misc procedures > printPointList :: [Vertex] -> IO () > printPointList [] = return () > printPointList ((Vertex p _ ):vs) = putStr (show p) >> > printPointList vs > > --- main > main = > do > let q = createQuad (0,0,0) (0,1,0) (1,1,0) (1,0,0) > let f = (\(Body _ _ (face:faces)) -> face) q > let verts = getVertsFromFace f > printPointList verts > ----------------- > ---- End ----

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hsc2hs: expanding macros in the .hsc file
by Olivier Boudry 14 May '08

14 May '08

Hi all, Is it possible to expand macros defined in includes into the .hsc file? I'm trying to call functions from a library written in C. The library can be used with or without Unicode chars, depending on #define instructions. The library has macros for all the standard functions used to work on strings (malloc, strlen, strcpy, strtok, ...). So that in code using the library one can always call the macro and don't care about the char type (strlenU instead of wcslen or strlen). Macro expansion will define which function will be used depending on the SAPwithUNICODE #define. I put a very simplified example below. The original file contains much more conditions (OS, Unicode or not, little or big endian, ...). #define strlenU SAP_UNAME(strlen) #ifdef SAPwithUNICODE #define SAP_UNAME(name) name##U16 #else #define SAP_UNAME(name) name #endif #if defined(WCHAR_is_2B) #define strlenU16 wcslen #endif I would like to be able to expand strlenU in my .hsc file and get the correct function. foreign import ccall "static sapuc.h strlenU" f_strlenU :: Ptr (#type SAP_UC) -> IO (#type size_t) I would like to be able to expand the strlenU macro to the real function name (wcslen or strlen). Is there a way to do this? Thanks, Olivier.

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M1 + M2 = M3 where both computations in M1 and M2 can be used?
by sam lee 13 May '08

13 May '08

Hi. I want to compose two monads to build another monad where computations of the two monads can be used inside. I have: - MonadTypeInfer : interface (class) for TypeInfer monad - TypeInfer : a monad that has Map String Type (association of names and types) - TypeInferT : transformer of above monad - MonadEval : interface (class) for Eval monad - Eval : a monad that has Map String Expr (association of names and code/function body) - EvalT : transformer of Eval - tInfer :: Expr -> TypeInfer Type -- given expr, returns type of it in TypeInfer monad - eval :: Expr -> Eval Expr -- given expr, returns normalized expr in Eval monad Problem: in repl, when user defines a function, it should type check and register type of the function to TypeInfer monad's Map String Type. Also, it should store the expression of the function in Eval monad. I build REPL monad using TypeInferT and EvalT. > newtype REPL a = REPL { runREPL :: TypeInferT (EvalT IO) a } > deriving(Monad, Functor, MonadIO, MonadTypeInfer, MonadEval) > repl :: REPL () > repl = do > input <- prompt ">>> " > case parse input of > Left err -> -- handle error > Right expr -> do > t <- tInfer expr -- BAD!! tInfer :: TypeInfer Type > println (show t) > result <- eval expr -- BAD!! eval :: Eval Expr > println (show result) > repl Should I make tInfer :: REPL Type, eval :: REPL Expr? Is there a way to build a monad where you could use sub-monads' (monads used to build current monad) computations? I prefer keeping tInfer :: TypeInfer Type, eval :: Eval Expr because tInfer never uses actions in Eval monad and vice versa. It seems like what I am asking is to break the type system. Maybe I should just make them run in REPL monad. Thank you. Sam.

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Re: [Haskell-cafe] List concat
by Lennart Augustsson 13 May '08

13 May '08

Well, if you want to be picky evaluating (calling) (xs ++ ys) takes whatever time it takes to evaluate xs to WHNF and if xs is null then it in addition takes the time it takes to evaluate ys to WHNF. Saying anything about time complexity with lazy evaluation requires a lot of context to be exact. -- Lennart On Mon, May 12, 2008 at 7:36 AM, <ajb(a)spamcop.net> wrote: > G'day all. > > Quoting Andrew Coppin <andrewcoppin(a)btinternet.com>: > > The function (++) :: [x] -> [x] -> [x] has O(n) complexity. >> > > That's not entirely true. > > When you call (++), it does O(1) work. If you evaluate k cons cells. > it takes O(min(k,n)) work. > > Cheers, > Andrew Bromage > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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On syntactic sugar
by Lauri Oksanen 13 May '08

13 May '08

Hi, I'm writing my first real Haskell program and I came up with the following code snippet. --- let x' = x \+ dist \* dir nx' = normal geometry wi = (-1) \* dir in do (p, wo) <- brdfCosSampling reflector nx' wi let color' = p \** color q = min 1 (scalarContribution p) in do sampler <- biasedCoinSampler q (radianceSampler surfaces x' wo (q \* color')) (terminalRadianceSampler surfaces x' nx' ((1-q) \* color')) sampler --- This works just fine but I don't like the way I had to indent the code because of alternating lets and dos. I would like to indent the code more like an imperative code i.e. like this --- let { x' = x \+ dist \* dir ; nx' = normal geometry ; wi = (-1) \* dir ; } in do { (p, wo) <- brdfCosSampling reflector nx' wi ; let { color' = p \** color ; q = min 1 (scalarContribution p) ; } in do { sampler <- biasedCoinSampler q (radianceSampler surfaces x' wo (q \* color')) (terminalRadianceSampler surfaces x' nx' ((1-q) \* color')) ; sampler ; }} --- but without braces and semicolons. Following works also, but is ugly (and probably less efficient?). --- do x' <- return $ x \+ dist \* dir nx' <- return $ normal geometry wi <- return $ (-1) \* dir (p, wo) <- brdfCosSampling reflector nx' wi color' <- return $ p \** color q <- return $ min 1 (scalarContribution p) sampler <- biasedCoinSampler q (radianceSampler surfaces x' wo (q \* color')) (terminalRadianceSampler surfaces x' nx' ((1-q) \* color')) sampler --- Is there some nice trick to do this?

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Re: [Haskell-cafe] List concat
by Derek Elkins 13 May '08

13 May '08

On Mon, 2008-05-12 at 02:36 -0400, ajb(a)spamcop.net wrote: > G'day all. > > Quoting Andrew Coppin <andrewcoppin(a)btinternet.com>: > > > The function (++) :: [x] -> [x] -> [x] has O(n) complexity. > > That's not entirely true. > > When you call (++), it does O(1) work. If you evaluate k cons cells. > it takes O(min(k,n)) work. I think we can safely simplify that to O(k) (at least for sequential programs).

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help(!!)
by Galchin, Vasili 12 May '08

12 May '08

Hello, As I said in a Friday posting, I changed the version # of the cabal file that I am using, unix.cabal. Now my build environment is broken and I am dead in the water: vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs configure Configuring unix-2.3.0.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs build Setup.hs: unix.cabal has been changed, please re-configure. vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ It seems like "runhaskell Setup.hs configure" isn't doing much after the unix.cabal change. How can I force the configure to really run? Do something with some of the config files?? Thanks, Vasili

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Re: [Haskell-cafe] saner shootout programs
by PR Stanley 12 May '08

12 May '08

>I don't know Haskell very well, but Paul: "I'm not racist but . . ." :-) >even I can tell, looking at, for >example, the N-body benchmark, that the Haskell code is probably not >type-safe, and the tricks used in it would not be usable in a larger >program (see below). > >The task is essentially a pure computation: take a list of bodies >having mass, position and velocity; apply Newton laws at discrete >intervals for a large number of times; return new positions and >velocities. > >I could write a C++ procedure that performs this task and have some >piece of mind regarding its type correctness, exception safety and >functional purity: side effects would be local to the procedure, >arguments passed as const or by value, the result returned by value, >no type casts or new/delete operators used. > >On the other hand, the Haskell code makes assumptions about the size >of double-precision floats (obviously not type-safe). Further, the >simulation is not a pure function. > >It is often argued that one only needs these dirty tricks in the most >time-consuming functions. However, if using imperative programming in >these "inner loop" procedures places them in IO monad, the "outer >loops" (the rest of the program - procedures that call it) will have >to go there as well. This makes me doubt the Haskell approach to >functional programming. > >If anyone has a version of the N-body benchmark, where the simulation >is a type-safe pure function, I would very much like to see and time >it. >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

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List concat
by Andrew Coppin 12 May '08

12 May '08

The function (++) :: [x] -> [x] -> [x] has O(n) complexity. If somebody were to invent some type that looks like [x] but actually uses some sort of tree rather than a linked list, you should be able to get O(1) concatenation. Has anybody ever implemented such a thing?

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Interesting critique of OCaml
by Don Stewart 12 May '08

12 May '08

An interesting critique of OCaml. http://enfranchisedmind.com/blog/2008/05/07/why-ocaml-sucks/ One phrase that stood out, regarding GHC's support for deforestation transformations like build/foldr and stream fusion: "Haskell is doing data structure level optimizations with the ease that most other compiler do peephole instruction optimization. This is a non-trivial result." Which I think really captures the joy of being able to write algebraic and data structure transformations, via rewrite rules, without having to extend the compiler -- all thanks to purity, laziness, and static typing. -- Don

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dropping hyphens and \n in words
by Ivan Amarquaye 12 May '08

12 May '08

Generally a hyphen is written at the end of the sentance when moving on to the next line and i managed to achieve this in haskell by using the "\n"- newline which places an index word in the next line i.e. if the words appear indexed like this...([1]),[mangoes] and a hyphen is applied, it becomes ([1],[mang-oes]) and it is valid in my function as i made it accept hyphens as part of a single word. Now my problem is this...I'm assuming that the hyphen normally comes at the end of a sentence like this: "there are so many guys ravis-hing our women" and this can be demonstrated in haskell by "\n" which places the words or characters following it in a new line like this: input: makeIndex"there are so many guys ravis\nhing our women" and output is: (([1],[there]),([1],[ravis]),([2],[hing])) where 1 means the first line and 2 the next. Now i want to write a function that would take away the hyphen and \n from all the words supposed to end on the first line and continue on the next and make all appear on the first line like this: all words in this form: "chip-\nheater" should become "chipheater". hope i can get some guidance on doing this. _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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ANN: xmonad-utils 0.1
by Gwern Branwen 12 May '08

12 May '08

Just dropping in a quick note: I've uploaded to Hackage the 0.1 release of xmonad-utils here: <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-utils>. There is of course a Darcs repo available as well: <http://gorgias.mine.nu/repos/xmonad-utils>. What is xmonad-utils? It's a couple of small Xlib programs which might be useful for Xmonaders. In particular: * hxsel can replace xclip, and was the foundation for the XMonad.Util.XSelection module in XMonadContrib/; it gets a copy of the copy-paste buffer to manipulate * hhp is a replacement for unclutter, the famous utility which hides your mouse when not in use and so cleans up your screen (but which is also famously unmaintained and buggy). I've personally found hhp to be very helpful, so I packaged it all up. Miscellaneous notes: * There is also a 'heval' program, which tries to provide an X prompt to a GHC eval through the GHC API, but that may or may not work for you (it doesn't compile for me, but I'm fairly sure that's because my 'directory' package is b0rked in an unrelated manner). * There is a requirement on cabal >= 1.4; this was needed because older Cabals didn't properly include in the sdist tarball one of the .hsc files. I don't believe it's actually needed to build. -- gwern nuclear Monarch NSADS Scud foundation World or Kilo SADMS M-x

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Cryptographic hash uniquness (was [Haskell-cafe] Simple network client)
by Peter Verswyvelen 11 May '08

11 May '08

>winds up having a write cache, which is mutable in practice. The >interesting thing is that the block's location is the cryptographic >hash of its contents, which leads to all sorts of neat properties (as >well as requiring immutability). That's interesting. When I developed a version control system for a customer, I also used a cryptographic hash as the database key of file+content in question, but I was afraid I might have clashes (two files with different content generating the same hash)... My intuition told me that the odds of two cryptographic hashes (on meaningful content) colliding was much less than the earth being destroyed by an asteroid... But this is just intuition... What does computer science tell us about this? Thank you, Peter

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Error: Improperly terminated character constant
by Ivan Amarquaye 11 May '08

11 May '08

I'm writing a function dRop to accept words ending in 'es' and drop the last two characters i.e. 'es'.eg. mangoes -> mongo but i keep on getting this error: "Improperly terminated character constant" after running this code which i have left below. Can i get any form of help from anyone in here...? ----------------------------------------- dRop :: String -> String dRop word = if drop (length word - 2) word == 'es' then take (length word - 2) word else word -------------------------------------------------- regards Amarquaye.Ivan _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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Newbie Question: Using Haskell Functions in a C Program
by Philip Müller 11 May '08

11 May '08

Hi, I'm in the process of writing a C program, but I can't stop thinking about how some functions would be much nicer implemented in Haskell. Is there a way to write some of the functions in Haskell and then use them in my C code via some kind of interface? BTW yes, I have been thinking about writing the whole program in Haskell, but I just don't have the level of experience in Haskell programming for that, since it's really heavy on IO. Thanks in advance for the answers! Regards Philip

5 7

Order of Evaluation
by PR Stanley 11 May '08

11 May '08

Hi (take 4 . map (>0)) (f s t) where s = 2 : t t = 3 : s f = zipWith (-) What would be the order of evaluation for the above code? How would I illustrate the evaluation step-by-step? I'm guessing that the code necessitates lazy evaluation and as such it starts with take then it applies f which in turn applies s and t and zipWith until the first element satisfies the predicate in map and This is repeated 4 times What does the list think? Many thanks, Paul P.S. I'm not done with induction. I'm just letting it rst for a bit.

10 14

Stack vs Heap allocation
by Edsko de Vries 11 May '08

11 May '08

Hi, How can I know whether something will be stack or heap allocated? For example, in the standard example of why foldl (+) 0 will fail to evaluate a long list of integers due to a stack overflow, but foldl' won't, it is pointed out that foldl starts building up unevaluated thunks. So, apparently, these thunks are allocated on the stack rather than on the heap with a pointer to the thunk on the stack. (I understand that foldl' is asymptotically better than foldl space-wise; that is irrelevant to my question.) On the other hand, in this version that sums all the values in a tree data Tree = Leaf Int | Node Tree Tree sum :: Tree -> Int sum t = sum' [t] 0 where sum' [] acc = acc sum' (Leaf i : ts) acc = sum' ts $! (i + acc) sum' (Node l r : ts) acc = sum' (l : r : ts) acc we are building up a (potentially) large lists of trees yet to be processed, but never run out of stack space. Apparently, the list is built up on the heap rather than on the stack. What is the fundamental difference between these two examples? Why is the list of trees yet to be processed allocated on the heap (with a pointer on the stack, presumably) but the unevaluated thunks in the foldl example allocated on the stack? Thanks, Edsko

5 6

seeking advice on my POSIX wrapper
by Galchin, Vasili 11 May '08

11 May '08

Hello, Last night I sent out an announcement about some POSIX work that I have been doing. In any case, one of the FFI wrappers is driving me crazy, i.e. the one for mq_receive: http://opengroup.org/onlinepubs/007908799/xsh/mq_receive.html . When I call this function (mqReceive), I get "message too long". In my test cases I am sending and receiving messages that are only 11 bytes! The wrapper seems really straightforward. Perhaps I am looking right at the problem and don't see. I need other eyes on the wrapper to help me ;^). Please see below. Regards, V. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -- mqReceive is still being debugged!!!!!!!!!! -- | Retrieve a message from mqueue designated by "mqd" -- mqReceive :: Fd -> ByteCount -> Maybe Int -> IO (String, Int) mqReceive (Fd mqd) len (Just prio) = do allocaBytes (fromIntegral len) $ \ p_buffer -> do with (fromIntegral prio) $ \ p_prio -> do rc <- throwErrnoIfMinus1 "mqReceive" (c_mq_receive mqd p_buffer (fromIntegral len) p_prio) case fromIntegral rc of 0 -> ioError (IOError Nothing EOF "mqReceive" "EOF" Nothing) n -> do s <- peekCStringLen (p_buffer, fromIntegral n) return (s, n) mqReceive (Fd mqd) len Nothing = do allocaBytes (fromIntegral len) $ \ p_buffer -> do rc <- throwErrnoIfMinus1 "mqReceive" (c_mq_receive mqd p_buffer (fromIntegral len) nullPtr) case fromIntegral rc of 0 -> ioError (IOError Nothing EOF "mqReceive" "EOF" Nothing) n -> do s <- peekCStringLen (p_buffer, fromIntegral n) return (s, n) foreign import ccall unsafe "mqueue.h mq_receive" c_mq_receive :: CInt -> Ptr CChar -> CSize -> Ptr CInt -> IO CInt

3 4

Old editions of The Monad.Reader lost
by Henk-Jan van Tuyl 11 May '08

11 May '08

L.S., Now that all hawiki pages have been removed, we have lost some valuable information. For example The Monad.Reader; on http://www.haskell.org/haskellwiki/The_Monad.Reader it says: Older editions can be found on the old Haskell wiki – they haven't been included here for licensing reasons. The page it links to, a hawiki page, has dissappeared. I propose to bring at least the The Monad.Reader pages back. Backups of these pages can be found at the Wayback Machine, but there is no guarantee that will always be there. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

3 6

Type unions
by Eric Stansifer 11 May '08

11 May '08

I have been trying to write a DSL for Povray (see www.povray.org) in Haskell, using the technique of: http://okmij.org/ftp/papers/tagless-final-APLAS.pdf with some inspiration taken from http://okmij.org/ftp/Haskell/DSLSharing.hs The Povray Scene Description Language is a very declarative language, with few high level constructs (even loops take a bit of work) -- which is why I'm putting it in Haskell. At one point, I needed a "varargs" function for the DSL, a function f :: b -> a -> b dressed up to take a variable number of 'a's, known at compile time. This was easy enough: > data Nil a = Nil > data Cons b a = a ::: b a > infixr 1 ::: > > class VarArgs v where > apply_args :: (s -> a -> s) -> s -> v a -> s > > instance VarArgs Nil where > apply_args _ start _ = start > > instance VarArgs b => VarArgs (Cons b) where > apply_args f start (a ::: b) = apply_args f (f start a) b The solution is quite workable: I can simply write the following, and I believe the summation is expanded out at compile-time: > apply_args (+) 0 (2 ::: 3 ::: 8 ::: 1 ::: (-3) ::: Nil) But I found I also needed a function to take a union type -- that is, the function would either take an argument of type T1, or of type T2, known at compile time. I tried a similar technique as I tried with varargs, and unfortunately ended up with this: > data LeftOf a b = L a > data RightOf a b = R b > > class Union u where > apply_union :: (a -> c) -> (b -> c) -> (u a b) -> c > > instance Union LeftOf where > apply_union f _ (L a) = f a > > instance Union RightOf where > apply_union _ g (R b) = g b > > type A = Integer > type B = String > type C = () > > type Union_ABC u1 u2 = u1 A (u2 B C) > > f_A = show . (+ 3) > f_B = reverse > f_C = const "unit" > > f :: (Union u1, Union u2) => Union_ABC u1 u2 -> String > f = apply_union f_A (apply_union f_B f_C) > > main = do > putStrLn $ f $ (L 6 :: Union_ABC LeftOne LeftOne) > putStrLn $ f $ R (L "hello, world") > putStrLn $ f $ R (R ()) Notice a lot of ugliness in my example: e.g., the definition of f, the type signature of f (I can't move the context into the type-synonym Union_ABC), creating objects of the union type, and the unpleasant surprise that I needed to provide the type of 'L 6'. This solution is very not-scalable: the Povray SDL is a "messy" language, and for my DSL I would need approximately 20 or 30 such unions, each a union of about 20 types (admittedly with a lot of overlap from union to union). I think the solution is to abandon the lofty ideal of statically determining argument types; instead have a universal type with tags to distinguish types dynamically: > data Universal = UA A | UB B | UC C > f :: Universal -> String > f (UA a) = f_A a > f (UB b) = f_B b > f (UC c) = f_C c > > main2 = do > putStrLn $ f $ UA 6 > putStrLn $ f $ UB "hello, world" > putStrLn $ f $ UC () ...but I'm not ready to give up hope yet. Suggestions please? Eric

3 3

I am new to haskell
by Ambrish Bhargava 10 May '08

10 May '08

Hi All, I am new to Haskell. Can anyone guide me how can I start on it (Like getting binaries, some tutorials)? Thanks in advance. -- Regards, Ambrish Bhargava

8 11

unsafeInterleaveIO and OpenGL
by Peter Verswyvelen 10 May '08

10 May '08

I'm experiencing strange behavior when executing some Haskell OpenGL code inside unsafeInterleaveIO. For example, vp <- get GL.viewport returns garbage, as if the opengl context is not correctly set. Is this to be expected? Or is it likely a bug on my side? Thanks, Peter

2 2

inserting values in a binary tree
by PR Stanley 10 May '08

10 May '08

Hi data Ord a => Tree a = Nil | Node (Tree a) a (Tree a) How would one go about inserting a value in a binary search tree of the above description? Cheers Paul

5 6

build version problem?
by Galchin, Vasili 10 May '08

10 May '08

Hello, I changed the version # in a cabal file and now I get ... runhaskell Setup.hs configure Configuring unix-2.4.0.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs build Setup.hs: unix.cabal has been changed, please re-configure. Now no matter what I change the version #, e.g. 2.3.0.0, things are not happy. ? Thanks, Vasili

2 1

[ANN] Twitter Client
by Chris Eidhof 10 May '08

10 May '08

Hey everyone, I was tired of all those graphical Twitter clients that aren't usable from my Terminal, so I wrote my own. It's still very much alpha, but comments or improvements are more than welcome. You can install it by downloading the twitter-package from hackage. In order to get it to work, create a file "~/.twitter" with the contents username:password, and you should be set. The source code isn't polished yet, but it makes heavy use of the new curl, json and xml libraries from Galois, so it might be interesting to browse the source. -chris

3 3

ANN: unix-2.4.0.0 additional POSIX real time support
by Galchin, Vasili 10 May '08

10 May '08

Hello, Additional POSIX realtime (POSIX 1003.1b) support has been added: 1) MQueue.hsc - POSIX message queues Supported FFI bindings for: - mq_open - mq_close - mq_unlink - mq_getattributes - mq_setattributes - mq_send Not yet supported bindings: - mq_rcv .. still debugging a mysterious problem - mq_notify - will support when asynchronous I/O is implemented 2) RTsched.hsc - real time scheduling Supported bindings for: - sched_yield - sched_get_param - sched_set_param - sched _get_scheduler - sched_set_scheduler - sched_get_priority_min - sched_get_priority_max Unsupported currently: - sched_rr_get_interval - still looking for the TimeSpec data type in order to get "peek" working 3) Files.hsc Added bindings for: - fsync - fdatasync I am new to Haskell but would like to make a lot of contributions to insure Haskell success in the marketplace (of ideas ;^)). I apologize for any glitches and look for feedback. Kind regards, Vasili

2 1

ANN: Roguestar 0.2
by Christopher Lane Hinson 10 May '08

10 May '08

website: http://roguestar.downstairspeople.org darcs 2: darcs get --lazy http://www.downstairspeople.org/darcs/roguestar tarball: http://www.downstairspeople.org/roguestar/roguestar-0.2.tar.gz After much delay, I'm happy to announce Roguestar 0.2. Roguestar is a science fiction themed roguelike (turn-based, chessboard-tiled, role playing) game written in Haskell. Roguestar uses OpenGL for graphics. This is still a very early release and several important things don't work. This initial release allows you to play one of six alien races. You begin the game stranded on an alien planet, fighting off an endless hoard of hostile robots. RSAGL, the RogueStar Animation and Graphics Library, includes a domain-specific monad for 3D modelling of arbitrary parametric surfaces, as well as an animation monad and arrow, which is more or less like YAMPA as a stack of arrow transformers. RSAGL was specifically designed for roguestar, but every effort has been made (including the license) to make it accessable to other projects that might benefit from it. Performance is an issue, but there is a lot of low-hanging fruit in this area. Roguestar is released under the Affero General Public License version 3. However, RSAGL uses a permissive license. I would be absolutely thrilled to receive any feedback anyone may have on the design and aesthetics of the game, especially in the form of source code.

1 0

Re: [Haskell-cafe] inserting values in a binary tree
by PR Stanley 10 May '08

10 May '08

>Are there any invariants you wish to maintain when inserting? If >not, it's rather trivial. Paul: The idea is to find a value in the tree greater than the new value and then placing the new value before it on the tree. duplicates are ignored and if no value greater than he new value is found then it is appended as a new node to the end of the last node checked. in C you'd fiddle with pointers and Bob's your uncle. Here I'm not sure how to piece that tree back together again with the new element after having expanded it recursively. Cheers Paul

2 1

(Num t) => [t] or [Int]
by PR Stanley 10 May '08

10 May '08

Hi I thought [1, 2, 3] :: [Int] GHC says it's (Num t) > [t] Okay, when it comes to 3.3:[1,2,3] (Num t) => [t] makes more sense. Is that the only reason? Cheers, Paul

3 2

Seeking advice on best practices with FFI
by PJ Durai 10 May '08

10 May '08

Hello, I am trying to write FFI wrappers for a dynamic library in windows. I would like to get some help in how to go about doing things. For example for this C function, the import and wrapper I came up with are listed below. -- STATUS LNPUBLIC NSFDbOpen (char far *PathName, DBHANDLE far *rethDB); foreign import stdcall "nsfdb.h NSFDbOpen" nsfDbOpen' ::CString -> Ptr CInt -> IO CInt type DbHandle = CInt nsfDbOpen :: String -> IO DbHandle nsfDbOpen dbname = alloca $ \ptr -> do db <- newCString dbname rc <- nsfDbOpen' db ptr handle <- peek ptr if rc /= 0 then error "dbopen failed" else return handle Q: Does that look ok? Is the db <- newCString leaking ? Q: How would I import and call a function with signature like this calling C code: char str1[MAX_PATH]; char str2[MAX_PATH]; GetStringValues( str1, str2); called C function signature is: void GetStringValues( char* s1, char* s2); i.e. how would the haskell import (and the wrapper function) look like in situation like this (where I have to pass preallocated char array to C code and get it filled up)? Appreciate your time. thanks pj

2 1

MonadPlus
by Andrew Coppin 10 May '08

10 May '08

OK, so I feel I understand monads fine. I regularly use Maybe, [] and IO, and I've even constructed a few monads of my own. But here's a question: what is the purpose of the MonadPlus class? Clearly it defines a binary operation over monadic values and an identity element for that operation. But... what is this supposed to *do*? (For example, (>>) has an almost identical signature to mplus. But presumably they don't both do the same thing...) What functionallity is this supposed to give you? [In a somewhat unrelated question... I saw some code the other day that used Either as if it were a monad. And yet, I don't see an instance given in the standard libraries - even though there should be one. I can see Functor (Either a), but not Monad (Either a) or even Monad (Either String)...]

9 12

Haskell PNG Writer
by Nahuel Rullo 10 May '08

10 May '08

Hi list, i am new in Haskell. I need to make images (PNG, JPEG) with haskell, if you can give me a tutorial, thanks! -- Nahuel Rullo

8 7

Re: [Haskell-cafe] Induction (help!)
by PR Stanley 10 May '08

10 May '08

You've got the right idea. Paul: At long last! :-) I should point out that it doesn't make sense to say p(Succ n) = Succ(p(n)), p(x) represents some statement that is either true or false, so it doesn't make sense to say Succ(p(n)). . Paul: okay, da capo: We prove/test through case analysis that the predicate p holds true for the first/starting case/element in the sequence. When dealing with natural numbers this could be 0 or 1. We try the formula with 0 and if it returns the desired result we move onto the next stage. If the formula doesn't work with 0 and so the predicate does not hold true for the base case then we've proved that it's a nonstarter. In the inductive step we'll make a couple of assumptions: we'll imagine that p(j). We'll also assume that p holds true for the successor of j - p(j+1). Then with the help of rules and the protocol available to us we'll try to establish whether the formula (f) gives us f(j) = f(j+1) - f(1) So, we know that the predicate holds for 0 or at least one element. By the way, could we have 3 or 4 or any element other than 0? Anyway, p(0). Then we set out to find out if p holds for the successor of 0 followed by the successor of the successor of 0 and so forth. However, rather than laboriously applying p to every natural number we innstead try to find out if f(j+1) - f(1) will take us back to fj). I think this was the bit I wasn't getting. The assumptions in the inductive step and the algebraic procedures are not to prove the formula or premise per se. That's sort of been done with the base case. Rather, they help us to illustrate that f remains consistent while allowing for any random element to be succeeded or broken down a successive step at a time until we reach the base/starting element/value. Okay so far? Cheers Paul

3 5

[Haskell-cafe] Announce: SCC, Streaming Component Combinator library
by Mario Blazevic 09 May '08

09 May '08

I'm pleased to announce the 0.1 release of Streaming Component Combinators in Haskell, based on my earlier work done in OmniMark and published in paper Mario Blažević, Streaming component combinators, Extreme Markup Languages, 2006. http://www.idealliance.org/papers/extreme/proceedings/html/2006/Blazevic01/… The release consists of the following: - Yet another monad transformer with coroutine functionality. Its central feature is the function pipe :: forall context x m r1 r2. Monad m => Producer m x r1 -> Consumer m x r2 -> Pipe context m (r1, r2) - Based on the above, streaming component types Transducer and Splitter are defined, as well as a number of primitive component instances. - Next are component combinators that can be used to synthesize larger components. - Finally, there is a rudimentary executable, shsh (Streaming Haskell SHell), that exposes most of the above within a command-line shell. It's useful for experiments and testing of new components. Here are a few simple command examples: wc -c count | show | concatenate wc -l foreach line then substitute x else suppress end | count | show | concatenate grep "foo" foreach line having substring "foo" then append "\n" else suppress end sed "s:foo:bar:" foreach substring "foo" then substitute "bar" end sed "s:foo:[\\&]:" foreach substring "foo" then prepend "[" | append "]" end The SCC library and shell are available on Hackage, at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/scc-0.1

1 0

The Monad.Reader (11) - Call for Copy
by Wouter Swierstra 09 May '08

09 May '08

Call for Copy The Monad.Reader - Issue 11 Please consider writing something for the next issue of The Monad.Reader. The deadline for Issue 11 is ** August 1, 2008 ** It doesn't matter if you're a respected researcher or if you have only just started learning Haskell, get your thoughts together and write an article for The Monad.Reader! The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but more enduring than a wiki-page or blog. There have been a wide variety of articles, including: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. * Submission Details * Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (wss at cs.nott.ac.uk). Articles should be written according to the guidelines available from http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out. All the best, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

6 10

FFI: Creating a Storable for a C-struct composed of char arrays
by Olivier Boudry 09 May '08

09 May '08

Hi all, I'm trying to make RFC calls to SAP using the nwsaprfc library. Some structs defined in the library contains arrays (byte or word arrays). For example: typedef struct _RFC_ATTRIBUTES { SAP_UC dest[64+1]; /* RFC destination */ SAP_UC host[100+1]; /* Own host name */ SAP_UC partnerHost[100+1]; /* Partner host name */ SAP_UC sysNumber[2+1]; /* R/3 system number */ SAP_UC sysId[8+1]; /* R/3 system name */ SAP_UC client[3+1]; /* Client */ SAP_UC user[12+1]; /* User */ SAP_UC language[2+1]; /* Language */ ... continued }}RFC_ATTRIBUTES, *P_RFC_ATTRIBUTES; I would like to create a Haskell Storable counterpart of this structure. Using http://therning.org/magnus/archives/tag/hsc2hs as an starting point, I could create Storable for structures containing fields with basic types. Before working on the real data structures I wrote a more simple example to play with: -- File: ArrayStruct.h -- typedef struct _ArrayStruct { char a[10+1]; char b[20+1]; char c[30+1]; } ArrayStruct, *P_ArrayStruct; -- File: ArrayStruct.c -- #include "ArrayStruct.h" void print_array_struct(ArrayStruct *f) { printf("%s\n", __FUNCTION__); printf("f->a: %s\n", f->a); printf("f->b: %s\n", f->b); printf("f->c: %s\n", f->c); } -- File: HArrayStruct.hsc {-# OPTIONS -ffi #-} module Main where import Foreign import Foreign.C.Types #include "ArrayStruct.h" data HArrayStruct = HArrayStruct { a :: String, b :: String, c :: String } type HarrayStructPtr = Ptr HArrayStruct foreign import ccall "static ArrayStruct.h print_array_struct" f_print_array_struct :: ArrayStructPtr -> IO () instance Storable HArrayStruct where sizeOf _ = (#size ArrayStruct) alignment _ = alignment (undefined :: CInt) peek _ = error "peek is not implemented" poke ptr (HArrayStruct a' b' c') = do (#poke ArrayStruct, a) ptr a' (#poke ArrayStruct, b) ptr b' (#poke ArrayStruct, c) ptr c' printArrayStruct as = with as f_print_array_struct main = printArrayStruct $ HArrayStruct { a="some", b="test", c="data" } -- End of files Of course it won't work as HArrayStruct in file HArrayStruct.hs uses Strings and String is not an instance of Storable. Ideally I would need some sort of Storable array of char. Is Data.Storable.Array the type I'm looking for? Could someone point me to some code using the same kind of structures? Thanks, Olivier.

3 3

GHC API: how to get the typechecked AST?
by Gerbo Engels 09 May '08

09 May '08

Hello all, In the GHC Commentary [1] I can read that the type checker adds type information to the syntax tree. So I would expect that there is a way to get a syntax tree of type HsModule Id. The GHC API provides functions checkModule and checkAndLoadModule which both return a CheckedModule that contains the parsedSource :: Located (HsModule RdrName) and typecheckedSource :: LHsBinds Id. But it doesn't include anything of type HsModule Id. I don't see how to unify the parsedSource and typecheckedSource to get the syntax tree parametrised by Id instead of RdrName (and containing the type information), and the API doesn't seem to provide such functionality (or did I miss something?). Can anyone help? Or indicate where in the parsedSource the LHsBinds should be placed? Regards, Gerbo Engels [1] http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/HsSynType#Deco…

1 0

"resource exhausted"
by Galchin, Vasili 09 May '08

09 May '08

Hello, I am running some monadic code that I have written. I double checked my code and it seems to be ok (no guarantee though). I am getting a "resource exhausted (Message too long)". I just did a google on "resource exhausted" and saw a few posts on the number of open files which is obviously not my case. Any ideas? Kind regards, Vasili

2 2

REMINDER: Haskell Communities and Activities Report
by Janis Voigtlaender 09 May '08

09 May '08

Dear Haskellers, It is not yet too late to contribute to the 14th edition of the HC&A Report. The deadline is this weekend, Saturday, 10 May 2008. If you haven't already, please write an entry for your new project or update your old entry. Please mail your entries to <hcar at haskell dot org> in plain text or LaTeX format. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2008-April/020378.html We look forward to receiving your contributions. Thanks a lot, Andres and Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

1 0

A Cabal problem
by Mario Blazevic 09 May '08

09 May '08

I have a problem with Cabal: it doesn't seem to pick up the "Main-is:" option from the configuration file. Here's my "scc.cabal" file. Note the "Main-is:" line: > Name: scc > Version: 0.1 > Cabal-Version: >= 1.2 > Build-Type: Simple > License: GPL > License-file: LICENSE.txt > Copyright: (c) 2008 Mario Blazevic > Author: Mario Blazevic > Maintainer: blamario(a)yahoo.com > Synopsis: Streaming component combinators > > Executable shsh > Main-is: Shell.hs > Other-Modules: Foundation, ComponentTypes, Components, Combinators > Build-Depends: base, containers, process, readline, parsec > > Library > Exposed-Modules: Foundation, ComponentTypes, Components, Combinators > Build-Depends: base, containers And below are the outputs of Cabal configure and build commands. Note the absence of "--main-is" ghc command-line option. I'm doing this on Ubuntu 8.04 Linux, but I get the same problem with Gentoo. Is there something wrong with my configuration file, or is this a bug in Cabal? > ~/scc/pipe$ runhaskell Setup.lhs configure --verbose --prefix=~ > Configuring scc-0.1... > Dependency base-any: using base-3.0.1.0 > Dependency containers-any: using containers-0.1.0.1 > Dependency process-any: using process-1.0.0.0 > Dependency readline-any: using readline-1.0.1.0 > Dependency parsec-any: using parsec-2.1.0.0 > Using compiler: ghc-6.8.2 > Using install prefix: ~ > Binaries installed in: ~/bin > Libraries installed in: ~/lib/scc-0.1/ghc-6.8.2 > Private binaries installed in: ~/libexec > Data files installed in: ~/share/scc-0.1 > Documentation installed in: ~/share/doc/scc-0.1 > No alex found > Using ar found on system at: /usr/bin/ar > No c2hs found > No cpphs found > No ffihugs found > Using ghc version 6.8.2 found on system at: /usr/bin/ghc > Using ghc-pkg version 6.8.2 found on system at: /usr/bin/ghc-pkg > No greencard found > Using haddock version 0.8 found on system at: /usr/bin/haddock > No happy found > No hmake found > Using hsc2hs version 0.66-ghc found on system at: /usr/bin/hsc2hs > No hscolour found > No hugs found > No jhc found > Using ld found on system at: /usr/bin/ld > No nhc98 found > No pfesetup found > Using pkg-config version 0.22 found on system at: /usr/bin/pkg-config > Using ranlib found on system at: /usr/bin/ranlib > Using tar found on system at: /bin/tar > > ~/scc/pipe$ runhaskell Setup.lhs build --verbose > Creating dist/build (and its parents) > Creating dist/build/autogen (and its parents) > Preprocessing library scc-0.1... > Preprocessing executables for scc-0.1... > Building scc-0.1... > Building library... > Creating dist/build (and its parents) > Creating dist/build (and its parents) > /usr/bin/ghc -package-name scc-0.1 --make -hide-all-packages -i > -idist/build/autogen -idist/build -i. -Idist/build -odir dist/build > -hidir dist/build -stubdir dist/build -package base-3.0.1.0 -package > containers-0.1.0.1 -package process-1.0.0.0 -package readline-1.0.1.0 > -package parsec-2.1.0.0 -O Foundation ComponentTypes Components Combinators > [1 of 4] Compiling Foundation ( Foundation.hs, > dist/build/Foundation.o ) > [2 of 4] Compiling ComponentTypes ( ComponentTypes.hs, > dist/build/ComponentTypes.o ) > [3 of 4] Compiling Components ( Components.hs, > dist/build/Components.o ) > [4 of 4] Compiling Combinators ( Combinators.hs, > dist/build/Combinators.o ) > Linking... > /usr/bin/ar q dist/build/libHSscc-0.1.a dist/build/Foundation.o > dist/build/ComponentTypes.o dist/build/Components.o dist/build/Combinators.o > /usr/bin/ar: creating dist/build/libHSscc-0.1.a > /usr/bin/ld -x -r -o dist/build/HSscc-0.1.o.tmp dist/build/Foundation.o > dist/build/ComponentTypes.o dist/build/Components.o dist/build/Combinators.o > Building executable: shsh... > Creating dist/build/shsh (and its parents) > Creating dist/build/shsh/shsh-tmp (and its parents) > Creating dist/build/shsh/shsh-tmp (and its parents) > /usr/bin/ghc -o dist/build/shsh/shsh --make -hide-all-packages -i > -idist/build/autogen -idist/build/shsh/shsh-tmp -i. > -Idist/build/shsh/shsh-tmp -odir dist/build/shsh/shsh-tmp -hidir > dist/build/shsh/shsh-tmp -stubdir dist/build/shsh/shsh-tmp -package > base-3.0.1.0 -package containers-0.1.0.1 -package process-1.0.0.0 > -package readline-1.0.1.0 -package parsec-2.1.0.0 -O ./Shell.hs > [1 of 5] Compiling Foundation ( Foundation.hs, > dist/build/shsh/shsh-tmp/Foundation.o ) > [2 of 5] Compiling ComponentTypes ( ComponentTypes.hs, > dist/build/shsh/shsh-tmp/ComponentTypes.o ) > [3 of 5] Compiling Components ( Components.hs, > dist/build/shsh/shsh-tmp/Components.o ) > [4 of 5] Compiling Combinators ( Combinators.hs, > dist/build/shsh/shsh-tmp/Combinators.o ) > [5 of 5] Compiling Shell ( ./Shell.hs, > dist/build/shsh/shsh-tmp/Shell.o ) > Warning: output was redirected with -o, but no output will be generated > because there is no Main module. Name: scc Version: 0.1 Cabal-Version: >= 1.2 Build-Type: Simple License: GPL License-file: LICENSE.txt Copyright: (c) 2008 Mario Blazevic Author: Mario Blazevic Maintainer: blamario(a)yahoo.com Synopsis: Streaming component combinators Executable shsh Main-is: Shell.hs Other-Modules: Foundation, ComponentTypes, Components, Combinators Build-Depends: base, containers, process, readline, parsec Library Exposed-Modules: Foundation, ComponentTypes, Components, Combinators Build-Depends: base, containers

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Re: [Haskell] [Haskell-cafe] Help with polymorphic functions
by Reinier Lamers 09 May '08

09 May '08

Op Thursday 08 May 2008 21:10:08 schreef Wei Yuan Cai: > shift is defined as "a -> Int -> a" It's not. It's defined as "(Bits a) => a -> Int -> a" or something along those lines. So there is a restriction that the type a must be a member of the Bits typeclass. Because "test" is essentially just "shift", its type must also be "(Bits a) => a -> Int -> a". Reinier

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Re: [Haskell] [Haskell-cafe] Help with polymorphic functions
by Bulat Ziganshin 09 May '08

09 May '08

Hello Wei, Thursday, May 8, 2008, 11:10:08 PM, you wrote: > test :: a -> Int -> a > shift is defined as "a -> Int -> a" not exactly ;) this type signature is given inside class Bits, where 'a' isn't a free variable (as in standalone signature declaration), but means 'a' from type class header: class Num a => Bits a where shift :: a -> Int -> a so, this declaration is equivalent to the following standalone one: shift :: (Bits a) => a -> Int -> a from the common sense POV, you can't shift *ANY* type 'a', but only types that belong to the Bits class. so, shift cannot have signature w/o class, and the same remains true for `test` -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Workshop on Generic Programming: Call for Papers (co-located w/ ICFP08)
by Matthew Fluet (ICFP Publicity Chair) 09 May '08

09 May '08

CALL FOR PAPERS Workshop on Generic Programming 2008 Victoria, Canada, 20th September 2008 http://www.comlab.ox.ac.uk/ralf.hinze/wgp2008/cfp.{html,pdf,ps,txt} The Workshop on Generic Programming is sponsored by ACM SIGPLAN and forms part of ICFP 2008. Previous Workshops on Generic Programming have been held in Marstrand (affiliated with MPC), Ponte de Lima (affiliated with MPC), Nottingham (informal workshop), Dagstuhl (IFIP WG2.1 Working Conference), Oxford (informal workshop), Utrecht (informal workshop), and Portland (affiliated with ICFP). Scope ----- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, but only recently have generic programming techniques become a specific focus of research in the functional and object-oriented programming language communities. This workshop will bring together leading researchers in generic programming from around the world, and feature papers capturing the state of the art in this important emerging area. We welcome contributions on all aspects, theoretical as well as practical, of o adaptive object-oriented programming, o aspect-oriented programming, o component-based programming, o generic programming, o meta-programming, o polytypic programming, o programming with modules, and so on. Submission details ------------------ Deadline for submission: 30th June 2008 Notification of acceptance: 14th July 2008 Final submission due: 28th July 2008 Workshop: 20th September 2008 Authors should submit papers, in PostScript or PDF format, formatted for A4 paper, to Ralf Hinze (ralf.hinze(a)comlab.ox.ac.uk) or Don Syme (Don.Syme(a)microsoft.com) by 30th June 2008. The length should be restricted to 12 pages in standard (two-column, 9pt) ACM. Accepted papers are published by the ACM and will additionally appear in the ACM digital library. Programme committee ------------------- Ralf Hinze (co-chair) University of Oxford Patrik Jansson Chalmers University Andrew Lumsdaine Indiana University Conor McBride University of Nottingham Adriaan Moors Universiteit Leuven Fritz Ruehr Willamette University Tim Sheard Portland State University Don Syme (co-chair) Microsoft Research Todd Veldhuizen University of Waterloo

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Random numbers / monads - beginner question
by Madoc 09 May '08

09 May '08

Hello, I am just learning Haskell. Now, I encountered something that I cannot solve by myself. Your advice will be greatly appreciated. Given a list of numbers, I want to modify each of those numbers by adding a random offset. However, each such modified number shall stay within certain bounds, given by the integers minValue and maxValue. After that, I want to continue computation with the resulting list of type [Int]. But for demonstration, I made a program that just prints out the list: import IO; import Random minValue = 0::Int maxValue = 1000::Int normalize a | a < minValue = minValue | a > maxValue = maxValue | otherwise = a modify a = do offset <- randomRIO(-100::Int, 100) return(normalize(a + offset)) main = putStrLn $ show $ map (modify) [0, 200, 400, 600, 800, 1000] This program will not compile. GHC complains: test.hs:14:18: No instance for (Show (IO Int)) arising from a use of `show' at test.hs:14:18-21 Possible fix: add an instance declaration for (Show (IO Int)) In the first argument of `($)', namely `show' In the second argument of `($)', namely `show $ map (modify) [0, 200, 400, 600, ....]' In the expression: putStrLn $ show $ map (modify) [0, 200, 400, 600, ....] I understand that the result of the modify function is not an Int, as I would like to have it, but instead IO Int, and that cannot be applied to show. (I also did not quite understand why I need those brackets around the return value of the modify value. It won't compile if I leave them out, but I can accept that for now.) I also figured out how to generate a modified list of type [IO Int] and of type IO [Int]. However, I could not find out how to completely get rid of the IO monad and just get a mofied list of type [Int], which is what I really want. Please, do You have any advice for me? I tried for some hours, and now I am really angry at that IO monad that sticks to my pretty integers like glue! Also, any comment on the programming style and how I could achive my goals easier would be appreciated. (I left out comments and function types for the sake of brevity.) Thanks a lot in advance. Madoc. -- View this message in context: http://www.nabble.com/Random-numbers---monads---beginner-question-tp1712438… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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IO Help
by Mark Wallsgrove 08 May '08

08 May '08

Hey, I am studying Haskell as a unit at University. I find the concept and design idea's of Haskell interesting but I am finding my self struggling to understand the relationship between the normal state and IO state of Haskell. This is my situation: I have created 12 functions for a program which take in two types: type Catalogue = [Track] type Playlist = [Track] The definition for track is as follows: -- Track = ArtistDetails Title Length PCount data Track = Track ArtistType String Float Int deriving (Show,Eq,Read) -- Popular = Artist | Composor Performer data ArtistType = Popular String | Classical String String deriving (Show,Eq,Read) I have managed to save the data to a file using this code: --Saving Data saveData :: String -> Catalogue -> IO() saveData fileName catalogue = writeFile fileName (show catalogue) Problem now is reading the data back into the program. When I read the data back into the program it comes as IO [Track]. This is the code I have been using to load the data: loadData :: String -> Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue) I think I have missed a trick some where or I am using IO wrong, I really don't know. I believe it is the latter. I have been told that my definition for the function is wrong and that I should use the lazy approach to fix the problem. But this still leaves me with IO [Track]. Could someone inform me on what I am doing wrong? Thank you in advance Smoky

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