- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] safe lazy IO or Iteratee?
by John Lato 06 Feb '10

06 Feb '10

> Subject: Re: [Haskell-cafe] safe lazy IO or Iteratee? > > Downside: iteratees are very hard to understand. I wrote a > decently-sized article about them trying to figure out how to make > them useful, and some comments in one of Oleg's implementations > suggest that the "iteratee" package is subtly wrong. Oleg has written > at least three versions (non-monadic, monadic, monadic CPS) and I've > no idea why or whether their differences are important. Even dons says > he didn't understand them until after writing his own iteratee-based > IO layer. More significant than, and orthogonal to, the differences between non-monadic and monadic are the two primary implementations Oleg has written. They are[1]: Design 1: newtype Iteratee el m a = Iteratee{runIter:: Stream el -> m (IterV el m a)} data IterV el m a = IE_done a (Stream el) | IE_cont (Iteratee el m a) (Maybe ErrMsg) Design 2: newtype Iteratee el m a = Iteratee{runIter:: m (IterV el m a)} data IterV el m a = IE_done a (Stream el) | IE_cont (Stream el -> Iteratee el m a) (Maybe ErrMsg With the first design, it's impossible to get the state of an iteratee without feeding it a chunk. There are other consequences too. The second design seems to require some specialized combinators, that is (>>==) and ($$), which are not required for the first version. Neither situation is ideal. The CPS version appears to remedy both flaws, but at the expense of introducing CPS at a low level (this can be hidden from the end user in many cases). I already think of iteratees as holding continuations, so to me the so-called "CPS version" is to me a double CPS. Both designs appear to offer similar performance in aggregate, although there are differences for particular functions. I haven't yet had a chance to test the performance of the CPS variant, although Oleg has indicated he expects it will be higher. The monadic/non-monadic issue is related. Non-monadic iteratees are iteratees that can't perform monadic effects when they're running (although they can still be fed from a monadic enumerator). Essentially it's the difference between "fold" and "foldM". They are simpler and more efficient because of this, but also much less powerful. Any iteratee design can support both non-monadic and monadic, but *I* don't want to support both. At least, I don't want to have double modules for everything for nearly identical functions, and polymorphic code that can handle non-monadic and monadic iteratees is non-trivial[2]. Much of my recent work has been in the consequences of these various design considerations for the next version of the iteratee library. Currently undecided, although I'm leaning towards CPS. It seems to solve a lot of problems, and the implementation details are generally cleaner too. Cheers, John [1] Both taken from http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs. Design 1 is commented out on that page. [2] At least for me. Maybe others can provide a better solution.

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Simulation of interconnect network
by Roger King 06 Feb '10

06 Feb '10

I am building a simulator for an interconnect network for a multiprocessor computer. I would like to develop it in Haskell as an opportunity to learn Haskell. The network will have a number of routers with input ports and output ports and crossbars between them. I would like to simulate the protocol. This would be an event driven simulator. It would be at a high level, leaving out many details. I would like it to be fast and be able to run it on several processors. I would like to know if you have any advice. Has anyone done this before? Are there any discrete event simulators written in Haskell? R

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a beginner question: decorate-op-undecorate
by Aran Donohue 06 Feb '10

06 Feb '10

Hi Haskell-Cafe, Consider a data type such as data Binding = Binding Var (Either Value [Value]) representing a variable bound either to a fixed value or that has a list of possible values. I'd like to perform an operation on say, the fixed-value members of a list of bindings. Data.Either has "partitionEithers"---I'd essentially like to use partitionEithers, but in a way that it "peeks" into the value field of the binding. For the sake of argument, let's say I can't or can't modify Binding to move the Either to the outside. What would be an idiomatic Haskell way to accomplish this? Currently I've got "liftedPartitionEithers :: [a] -> (a -> Either b c) -> ([a], [a])" which is my own version of partitionEithers that calls a selector first. Another option would be to map each Binding to a new datatype that has the Either on the outside, use partitionEithers, and map back. Thanks, Aran

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Re: [Haskell-cafe] Re: programmatic DB interface?
by Magnus Therning 06 Feb '10

06 Feb '10

On Fri, Feb 5, 2010 at 10:38, Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de> wrote: > >> As I posted earlier, using 'T' to separate date and time is correct >> ISO8601 according to wikipedia[1]. > > So you're saying "2009-12-20 16:55:57.297" > (returned by the psql server) is wrong, > and it should instead send "2009-12-20T16:55:57.297Z" ? Well, it isn't necessarily *wrong* ;-) "2009-12-20 16:55:57.297" is made up of two pieces, one for date and one for time. "2009-12-20T16:55:57.297Z" is made up of one piece, which specifies both date and time. This is why I believe that the function ISO8601DateFormat is correct when it provides a format for a combined date and time. > What would be the query to obtain this format? > > I don't see this mentioned anyplace on > http://www.postgresql.org/docs/8.3/interactive/functions-datetime.html Neither do I, I don't find anything relating to ISO8601 dates (they seem more concerned with ISO DOW and year). > Perhaps we take this back to haskell-cafe (or libraries?) as it might be > of general interest. Sure, why not. It was most likely a mistake on my side to drop haskell-cafe in the first place. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Trapping getChar before echo
by Mark Spezzano 05 Feb '10

05 Feb '10

Hi, Is there any way of trapping keystrokes in Haskell, modifying them, and then echoing? Basically I want to give the user a prompt like: > and then have whatever they type appear in UPPERCASE regardless of whether caps lock was on or not. By default Haskell seems to echo characters in whatever case they were typed. I want to sneak in a toUpper in before the Chars get echoed. Thanks Mark Spezzano

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Generating repeatable arbitrary values with QuickCheck 2
by Sean Leather 05 Feb '10

05 Feb '10

I would like to generate an arbitrary (large) value to benchmark the performance of constructing that value with isomorphic types. It seems like QuickCheck might be useful in this regards. Has anyone done something similar? In versions 1.*, there was a generate function: generate :: Int -> StdGen -> Gen a -> a > generate n rnd (Gen m) = m size rnd' > where (size, rnd') = randomR (0, n) rnd > That seems to have disappeared in versions 2.*, and I didn't find a clear replacement. I came up with using the destructor for Gen: unGen :: Gen a -> StdGen -> Int -> a > The function generate seems to have a little something extra, though I'm not sure if it's necessary. Is this true, or should I write an equivalent generate function? As an aside, it would be nice to have a generate function in the library, even if it is only a wrapper for unGen. In the end, I would write something like the following: unGen arbitrary (mkStdGen 11) 5 :: [Int] > This produces, for example, [5,1,-2,-4,2]. I also want to generate the same value for a type isomorphic to [Int]. unGen arbitrary (mkStdGen 11) 5 :: List Int > Unfortunately, this produces Cons 4 (Cons 3 (Cons (-2) (Cons 0 (Cons (-1) Nil)))): same length but different values. The Arbitrary instances are the same. I had similar results with generate from QC 1. Any suggestions on how to do this? With another library perhaps? Thanks, Sean

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online Real World Haskell - tool used?
by Sophie 05 Feb '10

05 Feb '10

Not a Haskell question, pardon me. I believe the online Real World Haskell book with commenting system was generated with an open-source tool (written by one of the authors?). Any idea where I might find this? [I think I had gotten the link long ago, lost it]. I am also curious what authoring tools were used to write the book. Thanks! Sophie

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could we get a Data instance for Data.Text.Text?
by Jeremy Shaw 05 Feb '10

05 Feb '10

Hello, Would it be possible to get a Data instance for Data.Text.Text? This would allow us to create a Serialize instance of Text for use with happstack -- which would be extremely useful. We (at seereason) are currently using this patch: http://src.seereason.com/haskell-text-debian/debian/patches/add_Data_instan… which basically adds: +textType = mkStringType "Data.Text" + +instance Data Text where + toConstr x = mkStringConstr textType (unpack x) + gunfold _k z c = case constrRep c of + (CharConstr x) -> z (pack [x]) + _ -> error "gunfold for Data.Text" + dataTypeOf _ = textType + This particular implementation avoids exposing the internals of the Data.Text type by casting it to a String in toConstr and gunfold. That is similar to how Data is implemented for some numeric types. However, the space usage of casting in Float to a Double is far less than casting a Text to a String, so maybe that is not a good idea? Alternatively, Data.ByteString just does 'deriving Data'. However, bytestring also exports Data.ByteString.Internal, wheres Data.Text.Internal is not exported. Any thoughts? I would like to get this handled upstream so that all happstack users can benefit from it. - jeremy

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Stack ADT?
by michael rice 05 Feb '10

05 Feb '10

Can't find a Stack datatype on Hoogle? Where should I look? Michael

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programmatic DB interface?
by Johannes Waldmann 05 Feb '10

05 Feb '10

Hello. What is currently the recommended higher level data base interface for Haskell? I want to construct queries in a programmatic/algebraic way, and I want typed queries and results. - I guess I want LINQ. The examples in http://book.realworldhaskell.org/read/using-databases.html seem to construct queries as strings. That's horrible! Looks exactly like PHP! Well, no, but it's a lower abstraction level than that I want to work at. I'm sure I'm just missing some package/framework here. Would takusen help? Where's its home page? this does not work: http://darcs.haskell.org/takusen/ Best, Johannes.

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