- Haskell-Cafe - Haskell.org

"with" and "preserving" for local state
by Jules Bean 03 Oct '07

03 Oct '07

Lots of external libraries contain state, but one that really contains a *lot* of state is the OpenGL libraries, since OpenGL is specified as a statemachine. This means that when you're writing structured code you quite often want to save and restore chunks of state 'automatically'. For the very most common case (coordinate transformations) Sven gives us 'preservingMatrix' which is extremely handy. Unless I've missed something there's no similar API for saving/restoring arbitrary state variables. It's not hard to write: > {-# OPTIONS -fglasgow-exts #-} > import Graphics.Rendering.OpenGL > import Graphics.UI.GLUT > > preserving :: (HasSetter g, HasGetter g) => g a -> IO t -> IO t > preserving var act = do old <- get var > ret <- act > var $= old > return ret This enables us to write preserving lighting $ do ..... Note that, since IORef is an instance of HasGetter and HasSetter, you can do 'preserving' on any old IORef, not just an openGL StateVar. Also note that the 'makeStateVar' interface that Graphics.Rendering.OpenGL.GL.StateVar exports allows you to make a statevar out of any appropriate action pair (not entirely unrelated to http://twan.home.fmf.nl/blog/haskell/overloading-functional-references.deta…) Sometimes you don't only want to preserve a value, but set a specific temporary value, so: > with :: (HasSetter g, HasGetter g) => g a -> a -> IO t -> IO t > with var val act = do old <- get var > var $= val > ret <- act > var $= old > return ret with lighting Enabled $ do .... (of course, with could be written as with var val act = preserving var $ var $= val >> act ) But this gets really clumsy if you have multiple variables to save/restore, which is really what lead me to write this message in the first place. A cute syntax for doing multiple save/restores at once is given by an existential: > data TemporaryValue = forall a g. > (HasGetter g,HasSetter g) => > g a := a > > with' :: [TemporaryValue] -> IO t -> IO t > with' tvs act = do olds <- mapM (\(a := b) -> do old <- get a > return (a := old)) > tvs > ret <- act > mapM_ (\(a := b) -> a $= b) tvs > return ret so we can then write: with' [lighting := Enabled, currentColor := Color4 1 0 1 0] $ do ... and have a type safe list of temporary assignments passed as an argument. And, amazingly, you get decent error messages too: *Main> :t with' [lighting := Enabled, currentColor := Color4 1 0 1 0] with' [lighting := Enabled, currentColor := Color4 1 0 1 0] :: IO t -> IO t *Main> :t with' [lighting := Enabled, currentColor := "Foo"] <interactive>:1:44: Couldn't match expected type `Color4 GLfloat' against inferred type `[Char]' In the second argument of `(:=)', namely `"Foo"' In the expression: currentColor := "Foo" In the first argument of `with'', namely `[lighting := Enabled, currentColor := "Foo"]' Hope someone else finds that useful, Jules

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Parsing R5RS Scheme with Parsec
by Pasqualino 'Titto' Assini 03 Oct '07

03 Oct '07

Hi Alex, I hope not to spoil your fun but have you had a look at this: Write Yourself a Scheme in 48 Hours http://halogen.note.amherst.edu/~jdtang/scheme_in_48/tutorial/overview.html Regards, titto

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Re: [Haskell-cafe] Assignment, Substitution or what?
by PR Stanley 03 Oct '07

03 Oct '07

> > > f x = x + x > > > Is the "x" use to create a pattern in the definition and when f is > > > called it's replaced by a value? > > > >Those equation-like definitions are syntactic sugar for lambda > >abstractions. f could as well be defined as f = \x -> x + x. > >Please elaborate > > >First, the > >f x = > >part says that f is a function which takes a single parameter, >called x. The other side of the = sign gives the function body: in >this case, x + x. This is exactly the same thing that is expressed >by the lambda expression > >\x -> x + x > >This expression defines a function that takes a single parameter >called x, and returns the value of x + x. The only difference is >that with the lambda expression, this function is not given a >name. But you can easily give the function a name (just as you can >give any Haskell expression a name) by writing > >f = \x -> x + x > >In general, writing > >g x y z = blah blah > >is just a shorthand for > >g = \x -> \y -> \z -> blah blah. > >That is, it simultaneously creates a function expression, and >assigns it a name. > >Does that help? Yes and thanks for the reply. When a function is declared in C the argument variable has an address somewhere in the memory: int f ( int x ) { return x * x; } any value passed to f() is assigned to x. x is the identifier for a real slot in the memory (the stack most likely) made available for f(). Is this also what happens in Haskell? Thanks, Paul

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build problems with hscurses
by brad clawsie 03 Oct '07

03 Oct '07

hoping someone here can help me with a build problem when going through the hackage "build" stage, i get numerous errors like: HSCurses/Curses.hs:XXXX:0: invalid preprocessing directive #def where XXXX ranges in lines from 1597 to 1621. is there a special directive i need for runhaskell? any info appreciated. (on freebsd6.2, ghc6.6.1, ncurses-5.6_1)

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Installation of GLFW package
by Immanuel Normann 02 Oct '07

02 Oct '07

Hello, I have just read the thread about "installation of GLUT package" started at 9/3/2007 by Ronald Guida. Installation of the GLFW package is very much related to that. However, I haven't found the solution for installing the GLFW successsfully. I have downloaded http://www.cs.yale.edu/homes/hl293/download/GLFW-20070804.zip and followed the compile and installation instructions from the README.txt file. Actually I haven't noticed any error messages. Then I tried SimpleGraphics from SOE (downloaded from http://www.cs.yale.edu/homes/hl293/download/SOE-20070830.zip) with ghci-6.6. Again loading the file did not raise any error. But when I tried to start main = runGraphics ( do w <- openWindow "My First Graphics Program" (300,300) drawInWindow w (text (100,200) "Hello Graphics World") k <- getKey w closeWindow w ) I got this error message: "During interactive linking, GHCi couldn't find the following symbol: GLFWzm0zi1_GraphicsziUIziGLFW_initializze_closure 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." Here I am lost. What is the missing library? Regards, Immanuel

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Parsing R5RS Scheme with Parsec
by Alex Queiroz 02 Oct '07

02 Oct '07

Hallo, For fun and learning I'm trying to parse R5RS Scheme with Parsec. The code to parse lists follows: -- -- Lists -- parseLeftList :: Parser [SchDatum] parseLeftList = do char '(' many parseDatum >>= return . filter (/= SchAtmosphere) parseDottedList :: [SchDatum] -> Parser SchDatum parseDottedList ls = do char '.' many1 parseAtmosphere d <- parseDatum many parseAtmosphere char ')' return $ SchDottedList ls d parseProperList :: [SchDatum] -> Parser SchDatum parseProperList ls = do char ')' return $ SchList ls parseList :: Parser SchDatum parseList = do ls <- parseLeftList (parseDottedList ls) <|> (parseProperList ls) I've factored out the common left sub-expression in parseLeftList. The problem is that "..." is a valid identifier so when inside the left of the list the parser sees a single dot, it tries to match it with "...", which fails. Can anybody give advice on how to rewrite these list parsing functions? Cheers, -- -alex http://www.ventonegro.org/

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Using C-c C-l to load .hsc file into ghci in Emacs
by Johan Tibell 02 Oct '07

02 Oct '07

How can I get C-c C-l to first run cpp on a .hsc file and then load the .hs file? I checked out the network package from darcs and then did: Start ghci, C-c C-z, then: Prelude> :set -cpp And then pressed load, C-c C-l: Prelude> :load "/Users/tibell/src/haskell/network-bytestring/Network/Socket.hsc" <command line>: Could not find module `/Users/tibell/src/haskell/network-bytestring/Network/Socket.hsc': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> :load "/Users/tibell/src/haskell/network-bytestring/Network/Socket.hsc" <command line>: Could not find module `/Users/tibell/src/haskell/network-bytestring/Network/Socket.hsc': Use -v to see a list of the files searched for. Failed, modules loaded: none.

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Unfriendly hs-plugins error
by Björn Buckwalter 02 Oct '07

02 Oct '07

Dear all, I'm getting a rather unfriendly error when trying to load a plugin with hs-plugins: my_program: Ix{Int}.index: Index (65536) out of range ((0,7)) The exact numbers in the message vary depending on what I'm trying to do. I'm using GHC-6.6.1 on MacOS X. Here are three files that exhibit the behaviour for me: API.lhs ======= > module API where > data Config = Config { param :: String } Main.lhs ======== > module Main where > import System (getArgs) > import System.Plugins > import API > getConfig :: [String] -> IO Config > getConfig [file] = do > status <- make file [] > obj <- case status of > MakeSuccess _ o -> return o > MakeFailure es -> mapM_ putStrLn es >> error "make failed" > putStrLn $ "### loading " ++ file > m_v <- load_ obj ["."] "config" > putStrLn $ "### checking " ++ file > val <- case m_v of > LoadSuccess _ v -> return v > LoadFailure es -> mapM_ putStrLn es >> error "load failed" > return val > main = getArgs >>= getConfig >>= putStrLn . param CustomConfig.hs =============== >module CustomConfig where > import API > config = Config { param = "Doomed to fail!" } I compile the above with "ghc --make -o my_program PluginTest.lhs" and execute with "./my_program CustomConfig.lhs". Any hints welcome. Thanks, Bjorn Buckwalter

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Assignment, Substitution or what?
by PR Stanley 01 Oct '07

01 Oct '07

Hi f x = x + x Is the "x" use to create a pattern in the definition and when f is called it's replaced by a value? What's actually happening here? Thanks, Paul

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Compose
by PR Stanley 01 Oct '07

01 Oct '07

Hi sumSquareEven = (sum.map (^2)).filter even I wrote this thinking i was being very clever. The question is asking to spot the error in f = compose [sum, map (^2), filter even] I've only seen [] used in list comprehension and initialisation. I conducted a mini search on compose earlier but to be honest it just made matters even less clear. So the question is, does compose as a function or keyword exist in Haskell and if so how can the above code frag be corrected? O and, is compose in any way related to curry and uncurry? Thanks v mucho, Paul

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