- Haskell-Cafe - Haskell.org

Name overloading
by Cristiano Paris 14 Jan '10

14 Jan '10

Hi, these days I'm thinking about name scoping in Haskell and a question built up silently but steadily in my mind. Many times I see code like this: data Foo = { fooBar :: Int, fooSay :: String, fooClose :: String } which reminds me of "Ye Olde Times" of C where you prepend the structure name (or an abbreviation) to the structure's fields so as to avoid clashes with other (possibly included) structures (here Haskell qualified imports don't help as they just let you cut down the size of the prefix which is still present though and is module-scoped anyway). One of the most appreciated (at least by me) features of OOP languages like C++, Java, Python and co. is the possibility to name instance methods the same in different classes and have the compiler resolve to the correct implementation (at least indirectly through a virtual table when inheritance is in place) simply looking at the type of the variable which "->" is applied to. The most fulgid example of this is the "open" method, which is likely to be present in lots of classes. Now, in Haskell we have type inference, which is "The Good Thing" as it allows to "validate" your program at compile time. Hence, the idea coming up to my mind is that type inference actually forbids a type-directed resolution of names as in C++ or Java. Is this correct? Think about this piece of code: data DB = { open :: DBHandle } data FS = { open :: File } foo x = open x or, worse: foo x = open (open x) if the result of the first "open" application should typecheck with the second. Notice that, in my understanding, adding a signature doesn't help as the type checker must guarantee that "foo" code complies with its signature, which is actually impossible if open is left undetermined in its type. Thank you for any comment. Cristiano

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Strategies
by Alexei Kitaev 14 Jan '10

14 Jan '10

Dear Ryan, Thanks a lot for your support and for pointing out a problem with my approach to forcing the state. I was aware of this issue but I didn't know there was a standard solution. It is indeed very natural to let each type specify how to evaluate itself. I will try this technique on the nearest occasion. Incidentally, the link to Control.Parallel.Strategies from the latest GHC User Guide is broken... --Alexei Ryan Ingram wrote: > On Sat, Jan 9, 2010 at 2:23 AM, Alexei Kitaev <kitaev(a)iqi.caltech.edu> wrote: >> Reading the discussion related to your blog, I >> realized that strict State is different in that it does not actually >> force the state. But forcing can be achieved by wrapping all actions >> with the following function: >> >> sState :: (s -> (a,s)) -> State s a >> sState f = State $ \s -> case f s of >> (a,s') -> s' `seq` (a,s') >> >> I hope that somebody will answer my other questions about the >> operational semantics and optimizations. > > Hi Alexei, you have a ton of great points but I wanted to discuss an > issue with this one. > > It's unusual that this is what you want either; since it only reduces > the state to WHNF. For example, if your state is a string, this only > evaluates enough to know whether or not the string is empty at each > step, and you can still get into trouble with code like this: > > put ("xxx" ++ some_bad_computation) > > which leave bottoms inside of your state which won't show up until later. > > Several attempts to solve this problem exist, but the most commonly > used one is the "rnf" strategy from Control.Parallel.Strategies, which > uses a typeclass to allow each type to specify how to evaluate itself > completely. > > -- ryan

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How to fulfill the "code-reuse" destiny of OOP?
by Magicloud Magiclouds 13 Jan '10

13 Jan '10

Hi, I think when people talk about OOP, especially the inheriting, their focus mainly is on functions (methods). My concern here is about the data member inheriting. In OOP, when I inherit a class, I also got the members of it. But in haskell, how to inherit a "data"? -- 竹密岂妨流水过山高哪阻野云飞

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status of wash?
by Günther Schmidt 13 Jan '10

13 Jan '10

Hi, is WASH still active, what's the current status of it? I'm looking for a Haskell version of Smalltalk's Seaside, at first glance WASH seems to come close. Günther

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ghc -e
by Tony Morris 13 Jan '10

13 Jan '10

Can I import a module when using ghc -e? e.g. ghc -e "import Control.Monad; forM [[1,2,3]] reverse" -- Tony Morris http://tmorris.net/

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what is *hack*?
by Günther Schmidt 13 Jan '10

13 Jan '10

Hi, References to a Hack. module came in the responses to my posts on HTML-GUIs. What is Hack then? Günther

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What is the difference between Strachey diff from Scott Semantic Domain?
by Daryoush Mehrtash 13 Jan '10

13 Jan '10

On Stack overflow page Conal Elliot says: > Beware that denotational semantics has two parts, from its two founders > Christopher Strachey and Dana Scott: the easier & more useful Strachey part > and the harder and less useful (for design) Scott part. > http://stackoverflow.com/questions/1028250/what-is-functional-reactive-prog… > Any more information on how the Scott version is different from Starchey? On Tue, Jan 12, 2010 at 3:24 PM, Raoul Duke <raould(a)gmail.com> wrote: > http://lambda-the-ultimate.org/node/1665#comment-55086 > <http://perlustration.blogspot.com/> Daryoush

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Xhtml?
by Günther Schmidt 13 Jan '10

13 Jan '10

Hi, apologies upfront. As time presses I decided to post questions immediately as soon as I run into dead-ends. I just don't want to give the impression that I'm not willing to do my homework. I'm trying to find documentation on Xhtml, the site on hackage refers to http://www.cse.ogi.edu/~andy/html/intro.htm for an introduction to the library, the link is dead. I managed to locate Andy Gill's current homepage (http://www.cse.ogi.edu/~andy/html/intro.htm) but there is no mention of his html combinatory library. Does anyone else know where an introduction to this library can be retrieved from? Günther

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QuickCheck: test passes in GHCi, fails when compiled
by Lauri Pesonen 13 Jan '10

13 Jan '10

I provided a Java solution to a problem of returning the first digit of an integer on StackOverflow and someone wondered if there were any floating point point problems with the solution, so I though I'd implement the algorithm in Haskell and run QuickCheck on it. Everything works fine on GHCi, but if I compile the code and run the test, it fails with -1000, 1000, -1000000. Any ideas why? In any case it seems that the commenter was right and there are some subtle problems with my solution. I'd just like to know more details... import Data.Char import Test.QuickCheck -- my solution getFirstDigit :: Int -> Int getFirstDigit 0 = 0 getFirstDigit x = let x' = abs x digits = (floor $ logBase 10 $ fromIntegral x') in x' `div` (floor $ 10 ** (fromIntegral digits)) -- two reference implementation that agree with each other getFirstDigitRef1 :: Int -> Int getFirstDigitRef1 x = digitToInt $ head $ show $ abs x getFirstDigitRef2 :: Int -> Int getFirstDigitRef2 x | x < 0 = getFirstDigitRef2 (-x) | x < 10 = x | otherwise = getFirstDigitRef2 $ x `div` 10 myTest x = getFirstDigit x == getFirstDigitRef1 x --myTest x = getFirstDigitRef2 x == getFirstDigitRef1 x myCheck n = check (defaultConfig { configMaxTest = n }) myTest main = myCheck 1000000000 -- ! Lauri

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overloaded overloading?
by Alberto G. Corona 13 Jan '10

13 Jan '10

Hi, I sometimes strumble on the same quiestion that forces me to insert functions that process objects of a certain class inside their class definition. This occurs when a computation uses the object internally, neiter as parameter or as a return value or in the case of existential types. An example of the first: class Example a where irec :: IO a pr :: a → IO String sample2 :: a → IO () sample2 _ = do x ← irec :: IO a pr x return () sample :: Example a ⇒ a → IO () sample _ = do x ← irec :: IO a pr x return () With the flag -fglasgow-exts, the following error below appears in sample. without the flag, the error appears in both sample and sample2. I´m too lazy to find what concrete extension is involved and why, anyhow, in the case of sample, the compiler must generate a new type a1 with no context. Could not deduce (Example a1) from the context () arising from a use of `irec' at Control\Workflow\Users.hs:73:7-10 Possible fix: add (Example a1) to the context of an expression type signature In a stmt of a 'do' expression: x <- irec :: IO a In the expression: do x <- irec :: IO a pr x return ()

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