October 2006 - Haskell-Cafe

casting
by Thomas Conway 10 Oct '06

10 Oct '06

On 10/11/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > it's a sort of problem that bites me many times when i start to wrote > Streams library :) although you said that you discovered the > dictionaries mechanism, i propose you to read > http://haskell.org/haskellwiki/OOP_vs_type_classes page where you can > find something that you don't yet know Yes, I did actually know this stuff, it was just paged out, and I posted before thinking. :-) Something to do with having two small kids, one of whom gets up waaaaay too early in the morning (i.e. 5 - 5:30am). :-) In theory, however, there is no particular reason why a runtime type cast couldn't be implemented to allow downcasting, but it does require the runtime system to have a repository of instance declarations, or associate with static data for a type the classes of which it is an instance. It's been a few years, but if my recallection is correct, the Mercury runtime system does this. T.

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Is Haskell a 5GL?
by Ch. A. Herrmann 10 Oct '06

10 Oct '06

Dear Haskellers, do you think that Haskell is a 3GL (third generation language) or a 5GL or that the hierarchy of programming language generations is useless? I see a problem if the potential of a language is evaluated according to the generation it belongs to. 1GLs: machine languages 2GLs: assembler languages 3GLs: programming based on algorithms (C, Java, ...) 4GSs: domain-specific languages (SQL,...) 5GLs: automatic problem solving (Mathematica, Prolog,...) and/or visual programming (Visual Basic,...) Haskell's laziness would in principle permit programming in a style akin to Prolog, e.g., for parser combinators. The question is: how strong is the influence of laziness in everyday Haskell programming? Cheers -- Christoph

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casting
by Thomas Conway 10 Oct '06

10 Oct '06

Hi All I'm having some difficulty with typeclasses. What I'm trying to do should be obvious, but it's still giving me trouble. I want to take a packaged item, and strengthen the constraints on its type. Rather than being just any type that is an instance of A, I want to do a runtime check and do something different if it is a type that is also an instance of B (which implies that it is an instance of A also). I've attached some code that attempts this, but ghci (er, 6.4.2 with -fglasgow-exts) complains: Compiling Main ( Cast.hs, interpreted ) Cast.hs:15:22: Ambiguous type variable `b' in the constraint: `B b' arising from use of `qux' at Cast.hs:15:22-24 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. So, can anyone suggest how I can achieve my goal? thanks, Tom

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How would you replace a field in a CSV file?
by Pete Kazmier 10 Oct '06

10 Oct '06

The other day at work an opportunity arose where I was hoping to sneak some Haskell into the pipeline of tools used to process call detail records (CDRs). In the telecommunications industry, CDRs are used for billing. Each CDR is a single line record of 30 comma-separated values. Each line is approximately 240 characters in length. The task at hand is to replace field number 10 if a new value can be found in a hashmap which is keyed using the contents of the field. My colleague was going to write a C program (that's all he knows), but I whipped up a trivial python program instead. I was curious if a haskell version could be faster and more elegant , but I have not been able to beat my python version in either case. So, I'm curious as to how you would go about this task in Haskell. The input files are generally 300-400MB, and the hashmap will contain perhaps 20-30 items. For those that know python, here is a very simple implementation that happens to be very fast compared to my Haskell version and very short: for line in sys.stdin: fields = line.split(',') fields[9] = tgmap.get(fields[9], fields[9]) print ",".join(fields), For each line in standard input: - Splits the string on the comma: "field0,field1,...,field29" => ["field0", "field1", ..., "field29"] to obtain a list of strings. - Gets the value associated with the key of field9 from tgmap, if it does not exist, it returns a default value which is the original value. I.e., if it's not in the map, then don't replace the field. - Joins the list of fields with a comma to yield a string again which is printed out to standard output. The join method on the string is a bit odd: ",".join([1,2,3]) => "1,2,3" Here is my first Haskell attempt: import Data.ByteString.Lazy.Char8 as B hiding (map,foldr) import Data.List (map) import Data.Map as M hiding (map) -- This is just a placeholder until I actually populate the map tgmap = M.singleton (B.pack "Pete") (B.pack "Kazmier") main = B.interact $ B.unlines . map doline . B.lines where doline = B.join comma . mapIndex fixup . B.split ',' fixup i s = if i==9 then M.findWithDefault s s tgmap else s comma = B.pack "," -- f is supplied the index of the current element being processed mapIndex f xs = m f 0 xs where m f i [] = [] m f i (x:xs') = f i x : m f (i+1) xs' After talking with dons on #haskell, he cleaned my version up and produced this version which gets rid of 'if' statement and makes mapIndex stricter: import Data.ByteString.Lazy.Char8 as B hiding (map,foldr) import Data.List (map) import Data.Map as M hiding (map) -- This will be populated from a file dict = M.singleton (B.pack "Pete") (B.pack "Kazmier") main = B.interact $ B.unlines . map doline . B.lines where doline = B.join comma . mapIndex fixup . B.split ',' comma = B.singleton ',' fixup 3 s = M.findWithDefault s s dict fixup n s = s -- f is supplied the index of the current element being processed mapIndex :: (Int -> ByteString -> ByteString) -> [ByteString] -> [ByteString] mapIndex f xs = m xs 0 where m [] _ = [] m (x:xs') i = f i x : (m xs' $! i+1) That helped things a bit, but I must confess I don't understand how the strictness improved things as I had assumed things were going to be evaluated in a reasonable amount of time due to the printing of output. I thought IO was interlaced with the execution and thus I wasn't going to have to concern myself over laziness. In addition, the function is able to generate new elements of the list on demand so I thought it was a good citizen in the lazy world. Could anyone help explain? And then he came up with another version to avoid the 'unlines', but that did not that really speed things up significantly. So, with all that said, is there a better approach to this problem? Perhaps a more elegant Haskell solution? Thanks, Pete

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Re: [Haskell-cafe] casting
by Thomas Conway 09 Oct '06

09 Oct '06

Thanks Misha & Matthias. I now get what's going on. The mention of the word "dictionary" revealed it all. I've spent the last 7 years programming in C++, and had dynamic_cast<> firmly fixed in my head. I totally forgot that Fergus Henderson and I independently reinvented dictionary passing for the implementation of Mercury circa 1995. :-) I also realized that the concrete signatures from my actual application don't quite satisfy the requirements anyway, and that the problem evaporates when I get the signatures right.:-) T.

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Announce: Monad Transformer Tutorial
by Martin Grabmueller 09 Oct '06

09 Oct '06

Hallo all, I hereby announce a small tutorial on using monad transformers. In contrast to others found on the web, it concentrates on using them, not on their implementation. I'd like to hear comments, suggestions, etc. about it! Both PDF and Literate Haskell source available here: http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html Have fun, Martin

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Bizarre garbage collection behaviour
by Guillaume Theoret 09 Oct '06

09 Oct '06

I'm using Hugs98 for .NET and I'm running into some bizarre garbage collection issues. I hope I'm posting at the right spot. I didn't want to post this in Hugs bugs since I'm pretty new to Haskell and it's entirely possible I'm doing something a way I shouldn't. Back to the problem at hand. My script (link available at the bottom of this email) fetches a webpage through the .NET Net library and processes it. If I output the result of this processing (a list with the player's name, goals, assists and points) to the screen with putStrLn I can process about two dozen before I hit a .NET runtime exception (which is a different issue entirely). If I don't ouput the result to the screen (I delete 2 lines that only do putStrLn, Ln 148 & Ln 161) I get "ERROR - Garbage collection fails to reclaim sufficient space" after 2 players processed which seems completely counter-intuitive to me. Does someone have an idea as to what's going on here? The script (and its dependencies) are available here: http://smokinn.tengun.net/comp348/week6/ Thanks, Guillaume Theoret

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Trying to serialize HList
by Matthias Fischmann 08 Oct '06

08 Oct '06

... and here is the code I am giving up on for today: Serialization of HLists. Questions below. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module Foo where import Char import List import Monad import Permutation import HListPrelude -- [1] http://web.engr.oregonstate.edu/~erwig/pfp/ -- Serializable is like Show, but -- (a) it carries explicit type information, and -- (b) it allows for serializing IORefs etc. class Serializable s where serialize :: s -> IO String deSerialize :: String -> IO s instance Serializable Int where serialize = return . ("Int::" ++) . show deSerialize s | isPrefixOf "Int::" s = return . (read :: String -> Int) . drop 5 $ s instance Serializable Char where serialize = return . ("Char::" ++) . show deSerialize s | isPrefixOf "Char::" s = return . (read :: String -> Char) . drop 6 $ s -- SList is a list of heterogenous serializable elements... class HList l => SList l instance SList HNil instance (Serializable s, SList ss) => SList (HCons s ss) -- ... so it should be possible to write instantiate Serializable, right?: instance (SList s, HMapOut Serialize s (IO String)) => Serializable s where serialize = liftM show . (sequence :: [IO String] -> IO [String]) . hMapOut Serialize deSerialize = error "Not yet. (I am not even done with serialize yet.)" -- Seems we need the trick from the paper that oleg pointed out to me earlier in this thread: data Serialize = Serialize instance (Serializable s) => Apply Serialize s (IO String) where apply _ = serialize -- Example: slist = HCons (1 :: Int) (HCons ('c' :: Char) HNil) test1 = serialize slist -- (This is where -fallow-overlapping-instances helps. There is a -- section in [1] on how to get rid of it, which I haven't read yet.) test2 :: IO (HCons Int (HCons Char HNil)) test2 = test1 >>= deSerialize Two questions: (1) Do you see any reasons why it should be impossible in principle to write deSerialize for the SList instance of Serializable? (I think the answer is "it's possible to write it, but you need to add quite some type information by hand".) (2) The problem with test2 is that I need to know its precise object-level type, ie which types occur at which positions in the SList. I am pretty sure this is a restriction I have to live with. Please tell me I am wrong. (-: (I think my application will make it possible for ghc to infer the type, which is fixed at compile time anyways, so it's not a severe restriction.) (3) (bonus question :) Who wants to write deSerialize for SLists for me? And another one: Why do I need to list the HMapOut instance in the context of the instance declaration of Serializable for SLists? (It's not a big deal, but I can't see why it can't be inferred automatically from the rest of the code.) Possibly back with another issue of my HList diary tomorrow. (Please tell me if you find this interesting or if you would like me to stop being so verbose.) cheers, Matthias

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collection monads
by tpledger＠ihug.co.nz 08 Oct '06

08 Oct '06

Matthias Fischmann wrote: > > Do you expect the contained type x to change during a > > sequence of monadic actions? e.g. would you ever use (>>=) > > at the type 'Permutation Int -> (Int -> Permutation Bool) -> > > Permutation Bool'? > > no, i don't need that. but aside from > the fact that > > > data Permutation k v = > > Permutation [(k, v)] > > instance (Ix k) => > > Monad (Permutation k) > > is redundant (i think of the permutation > as a function applicable to arbitrary > lists): how would that change anything? > my definition of return still doesn't > work. or how would you redefine > 'return'? Ah. Yes, my approach falls over because it lacks two things. #1: a distinguished value of the Ix-constrained type k, to pair off with return's argument. #2: a purpose. I don't have a clear idea of what a do-block in a permutation monad ought to mean. Whoops! <font color=red>:-]</font> Regards, Tom (crawling back under his rock)

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Haskell speaking Spanish
by Luis F. Araujo 08 Oct '06

08 Oct '06

Hello Haskell folks! I have recently started editing a page on the Haskell.org wiki site dedicated to our spanish-speaker community.[0] The main idea is to collect spanish information about Haskell. For example, tutorials, news, comments, etc. And that way to spread such a beautiful language and technology among our spanish-speakers. I also expect we could get some of the main Haskell wiki pages translated, at least the most popular pages.[1] After talking with Don Stewart on IRC; we have also decided it would be a nice idea to offer a spanish translation for the HWN (nice!), which i expect to post every week on this web page too. Please, feel free to start cooperating , and let me know if you got any doubt or suggestion about this new initiative. You are also invited to visit our irc channel #haskell.es on irc.freenode.net and say "hola" to us and idle for a while! Thanks! [0] http://haskell.org/haskellwiki/Haskell.es [1] http://haskell.org/haskellwiki/Special:Popularpages

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