September 2007 - Haskell-Cafe

Data types and field labels and "Show"
by bbrown 27 Sep '07

27 Sep '07

I am trying to print the data from a data type and also get the field values. How would I reference those values if I am declaring a Show function. I should probably use a class for this, but so far it is working. I have something along the lines of this. data SimplePlayer = SimplePlayer { shape :: MechShape, angle :: GLfloat, posX :: GLfloat, posY :: GLfloat } and then to use "Show", I was declaring this. instance Show SimplePlayer where show a = "<Simple> posX [" ++ show a{posX} ++ "]" Of course, this didn't compile. How should I change this code to get the field label values of the type. Tests/GLTests.hs:20:42: parse error on input `}' -- Berlin Brown [berlin dot brown at gmail dot com] http://botspiritcompany.com/botlist/?

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Desugaring of infix operators is (always?) the wrong way round
by Brian Hulley 27 Sep '07

27 Sep '07

Hi, I'm in the process of designing a little language inspired by Haskell but imperative, and have hit an issue regarding infix syntax which may be of interest also to anyone thinking about future revisions of Haskell or the problem of consistent parameter order in libraries. I'm wondering if anyone can shed light on the reason why x # y gets desugared to (#) x y and not (#) y x since the first desugaring always seems to lead to (#) having to be defined in a way that is unnatural for functional programming. This is why a lot of functions in the standard libraries have their arguments the wrong way round. For example in Data.Bits we have: shiftL :: a -> Int -> a whereas the natural argument order for a functional language would be to make it so that in an application one has the verb first followed by the noun as in: shiftL' :: Int -> a -> a so we could write (shiftL' 3) to denote the operation of shifting something left by 3 eg: let shiftLeftByThree = shiftL' 3 in map shiftLeftByThree [10, 78, 99, 102] (This choice of argument order is explained at http://www.haskell.org/haskellwiki/Parameter_order ) Similarly, considering arithmetic, it would make a lot more sense to me to define: x - y === (-) y x since the "action" here is "take away y" and the noun is "x" so the natural functional reading of (-) a b is "subtract a from b". To be consistent this would also have to apply to the use of (->) in types to get: a -> b === (->) b a Can anyone think of an example where the current desugaring of infix arguments gives the correct order when the function is used in a postfix application? (apart from commutative functions of course!) Thanks, Brian

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distghc possible?
by wp 27 Sep '07

27 Sep '07

hi, i am a newbie in haskell. i have read about the tool distcc, http://distcc.samba.org/. so i was wondering, does something like this already exist for haskell? would it be possible to implement a similar tool based on ghc for haskell or does this not make sense as a haskell program has to be compiled all in one go? how are haskell programs compiled? it would be great if someone would have some suggestions on this. walter

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Re: [Haskell-cafe] Haskell Cheat Sheet?
by Jonathan Cast 27 Sep '07

27 Sep '07

On Wed, 2007-09-26 at 17:09 -0500, Derek Elkins wrote: > On Wed, 2007-09-26 at 14:12 -0700, Jonathan Cast wrote: > > On Wed, 2007-09-26 at 11:43 -0700, Dan Weston wrote: > > > It seems no one liked idea #2. I still think fix is the wrong name for > > > this, maybe limit would be better. > > > > It calculates least fixed points. `fix' is as good a name as any. > > > > `limit' is terrible; the argument to fix, a -> a, is neither a sequence > > nor diagram nor net type, and hence its values don't have limits... > > > > jcc > > > > PS Yes, I know fix a = sup_{i=0}^inf f^i(bot). That sequence is rather > > different than the input function... > > Actually, f :: a -> a -is- a diagram and its limit -is- fix f. In what way? I'm not disputing you, but I don't see how to interpret it as such. > That > said, limit is still a horrible name for it. fix isn't much better, and > Y is even worse. I'm not sure what an immediately intuitive name would > be, so might as well go with the historical one. recursive (at least for my usage). jcc

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Re: [Haskell-cafe] Re: Very crazy
by Andrew Coppin 27 Sep '07

27 Sep '07

Chaddaï Fouché wrote: > 2007/9/25, Andrew Coppin <andrewcoppin(a)btinternet.com>: > >> This is why I found it so surprising - and annoying - that you can't use >> a 2-argument function in a point-free expression. >> >> For example, "zipWith (*)" expects two arguments, and yet >> >> sum . zipWith (*) >> >> fails to type-check. You just instead write >> >> \xs ys -> sum $ zipWith(*) xs ys >> >> > > (sum . zipWith (*)) xs ys > == (sum (zipWith (*) xs)) ys > > so you try to apply sum :: [a] -> Int to a function (zipWith (*) xs) > :: [a] -> [b], it can't work ! > > (sum.) . zipWith (*) > works, but isn't the most pretty expression I have seen. > I'm still puzzled as to why this breaks with my example, but works perfectly with other people's examples... So you're saying that (f3 . f2 . f1) x y z ==> f3 (f2 (f1 x) y) z ? In that case, that would mean that (map . map) f xss ==> map (map f) xss which *just happens* to be what we want. But in the general case where you want f3 (f2 (f1 x y z)) there's nothing you can do except leave point-free.

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Sending email from a Haskell program
by Peter Gammie 26 Sep '07

26 Sep '07

Hello, Does anyone have a library for sending email from a Haskell program? I'd like something portable and cabalised. I note there is code in darcs to send email on Windows and UNIX-y systems, and also something in WASH (I know not what). Apparently there is an SMTP server of some kind (?) in HAppS, but I want to re- use the system's MTA. BTW is anyone else looking at the darcs codebase and thinking that there are many useful libraries lurking there? cheers peter

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Haskell Cheat Sheet?
by Evan Klitzke 26 Sep '07

26 Sep '07

Has anybody made (or have a link to) a Haskell reference cheat sheet? I'm thinking of a nice LaTeXed PDF in the 1-10 page range (e.g. something like this http://www.tug.org/texshowcase/cheat.pdf) with the basics of the language syntax, the type declarations for the common type classes, the type signatures of the most commonly used functions in the Prelude and other common modules, and so forth? The Haskell standard library is very large for a newcomer (even just the Prelude!), and as a learner of the language I find myself spending a lot of time looking up Prelude functions and syntax details -- having all of this in a short PDF document that I could have offline would be very useful. -- Evan Klitzke <evan(a)yelp.com>

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Troubles understanding memoization in SOE
by Peter Verswyvelen 26 Sep '07

26 Sep '07

Hi, in SOE, the following memoization function is implemented: memo1 :: (a->b) -> (a->b) memo1 f = unsafePerformIO $ do cache <- newIORef [] return $ \x -> unsafePerformIO $ do vals <- readIORef cache case x `inCache` vals of Nothing -> do let y = f x writeIORef cache [(x,y)] -- ((x,y) : -- if null vals then [] else [head vals]) return y Just y -> do return y inCache :: a -> [(a,b)] -> Maybe b x `inCache` [] = Nothing x `inCache` ((x',y'):xys) = if unsafePtrEq x x' then Just y' else x `inCache` xys This is then used in type Time = Float type UserAction = G.Event data G.Event = Key Char Bool | Button Point Bool Bool | MouseMove Point | Resize | Closed deriving Show newtype Behavior a = Behavior (([Maybe UserAction],[Time]) -> [a]) newtype Event a = Event (([Maybe UserAction],[Time]) -> [Maybe a]) Behavior fb `untilB` Event fe = memoB $ Behavior (\uts@(us,ts) -> loop us ts (fe uts) (fb uts)) where loop (_:us) (_:ts) ~(e:es) (b:bs) = b : case e of Nothing -> loop us ts es bs Just (Behavior fb') -> fb' (us,ts) memoB :: Behavior a -> Behavior a memoB (Behavior fb) = Behavior (memo1 fb) If I understand it correctly, the memoization is required because otherwise recursive "streams" wouldn't work. For example, in the Pong game example, a ballPositionX stream is generated by integrating a ballVelocityX stream, but the ballVelocityX stream changes sign when the ball hits the left or right walls, and to determine that event, the ballPositionX stream is required. So both streams are mutually recursive, and without memoization, the program would be stuck (at least my own FRP experiments, which don't use memoization yet, gets stuck :-)). Another trick to prevent this, is the "b : case e of" code in untilB, which causes the event to be handled a bit too late, to avoid cyclic interdependencies. I hope I got that right. Now my questions. So, the keys (x) and values (y) in (memo1 fb) are streams (aka infinite lists)? More correctly, memo1 uses a pointer to the head of the list as a key, for fast comparing (as you can't compare infinite lists)? But since both key and value are infinite streams, won't this approach cause a serious space leak because the whole list cannot be reclaimed by the garbage collector? So the full ballPositionX and ballVelocityX streams would remain in memory, until the program exits? Since this doesn't happen when I run the SOE examples (I guess!), I clearly misunderstand this whole thing. I could explain it when the pointer to the list is actually a pointer to the delayed computation (a "thunk"?) of the tail, but the code doesn't seem to do that. Thanks for any help, I hope I explained the problem well enough. Peter Verswyvelen

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YI on Windows?
by Peter Verswyvelen 26 Sep '07

26 Sep '07

I retrieved the latest version of YI, but I failed to compile it for Windows (GHC 6.6.1, and I got alex-2.1.0, haddock-0.8, HsColour, GTK, but not VTY) Before even pasting the error log, is it supported in Windows? I see that "make" gave the error "ghc.exe: unknown package: unix" and some more errors. "make emacs" worked better, but failed starting "dist/build/yi/yi -B. -fgtk --as=emacs" with the error "\driver\package.conf.inplaceonf as c:\app\ghc" Thanks, Peter BTW: I was using MinGW/Msys

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Does Haddock respect hiding?
by Adrian Hey 26 Sep '07

26 Sep '07

Hello folks, Using Haddock 0.8, if I use haddock to document a module like this.. module MyModule ( module MyOtherModule ) where import MyOtherModule hiding (foo) I still get foo documented in the API for MyModule, despite the fact that foo is not really made available by importing MyModule. This looks like a bug to me, or maybe there's something else I should be doing. Anyone care to comment? Thanks -- Adrian Hey

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