July 2010 - Haskell-Cafe

Template Haskell sees into abstract data types
by Serguey Zefirov 28 Jul '10

28 Jul '10

Data.Map.Map and Data.Set.Set are exported abstractly, without exposing knowledge about their internal structure. I cannot directly create my own class instances for them because of that. But I found that I can write Template Haskell code that could do that - those data types could be reified just fine. This is somewhat strange situation. Was it a design decision?

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Class instances on class constraints
by Dušan Kolář 28 Jul '10

28 Jul '10

Hello all, For school purposes, I would like to define something like this: module SetOverList( SetFromList , SetOL(..) ) where type SetFromList a = [a] addEq :: Eq a => a -> SetFromList a -> SetFromList a addEq v [] = [v] addEq v l@(x:xs) = if x==v then l else x : addEq v xs addOrd :: Ord a => a -> SetFromList a -> SetFromList a addOrd v [] = [v] addOrd v l@(x:xs) = case compare x v of GT -> v : l EQ -> l LT -> x : addOrd v xs class SetOL a where addElem :: a -> SetFromList a -> SetFromList a instance (Eq a) => SetOL a where addElem = addEq instance (Ord a) => SetOL a where addElem = addOrd which does not work, of course (Flexible or Undecidable instances won't help). The aim is to have addElem function that works differently according to situation whether a type, which is base of the list/set, is a member of class Eq or Ord. Could you point me or hint me how to get as close as possible to the required solution? Maybe I'm not able to see an obvious way... Thanks, Dušan

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Haskell Forum
by Daniel Díaz 28 Jul '10

28 Jul '10

Hi all, I want to open a Haskell forum based on phpBB, but I need some collaborators for organize its content, and moderate its use. When we have finished, I will open this forum for the entire community of Haskell! If you are interested, mail me: danieldiaz(a)asofilak.es Thanks in advance.

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Int and ByteStrings
by wren ng thornton 28 Jul '10

28 Jul '10

Hey all, Is there a library function (f :: [Int] -> ByteString) such that it's prefix-preserving, and platform independent? GHC-only is fine for now. There are a bunch of functions of that type, but I need those guarantees and I'm hoping someone else has already done it. If there isn't one for Int, has someone already made a package that offers IntMap on Word32, Word64, etc? -- Live well, ~wren

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Need Control.Monad.State
by michael rice 28 Jul '10

28 Jul '10

How do I import Control.Monad.State? I see this note in http://en.wikibooks.org/wiki/Haskell/Understanding_monads/State Note: in some package systems used for GHC, the Control.Monad.State module is in a separate package, usually indicated by MTL (Monad Transformer Library). Michael ===============rand.hs ============== import Control.Monad.State type GeneratorState = State StdGen rollDie :: GeneratorState Int rollDie = do generator <- get let( value, newGenerator ) = randomR (1,6) generator put newGenerator return value ======================= [michael@localhost ~]$ ghci rand GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. rand.hs:1:7: Could not find module `Control.Monad.State': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude>

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fine control of bytestring streaming
by Alberto G. Corona 27 Jul '10

27 Jul '10

Hi, I´m streaming content using lazy bytestrings in a web application. The problem is that the output comes in huge blobs (presumably of 32k) one at a time. This is good for some purposes, but not for console-like interfaces or runtime log visualization. (one of my purposes is to web-alize ghci. I know that this can be done without streaming the standard output, but I want it that way). I can not control when the IO bytestring primitives flush the data to the stream, because this is not part of my application (nor does it should do it). I suppose here that it is in a chunk by chunk basis. But maybe I misinterpreted something or everything. I use the package hack with the server hack-handler-simpleserver. this serves does Data.ByteString.hPut the bytestring content... I use mappend to add content to the stream. mappend uses "append" which uses. "foldrChunks" which seems not to compact two bytestrings in max size chunks . So I do not know what happens. The question is: are there some way to control bytestring streaming?. Can It be done without the stream handler? Alberto.

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Haskell at CERN
by Alexey Khudyakov 27 Jul '10

27 Jul '10

Hello I'm wondering is there any haskellers in CERN? Given quality and usability of software there must be people looking for better alternatives.

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transform function for a GADT
by Ozgur Akgun 27 Jul '10

27 Jul '10

Café, I've tried several things already, but I am not including any of them for now. My question is, how would you define the 'transform' function for a GADT, say the one in the linked gist: http://gist.github.com/492364 (also attached to this e-mail) To be concise, I want 'transform' to apply the transformation function (its first parameter) to every immidiate child of its second parameter as long as the types match. Similar to what the 'tranform' function of Uniplate does for normal ADTs. (But just one level, so I guess it is more similar to the 'descend' of Uniplate. See http://hackage.haskell.org/packages/archive/uniplate/1.2.0.1/doc/html/Data-… ) I think I got closest to a sensible solution using multi-param type classes, and defining many instances for different combinations of ExprType's but still there were problems. Waiting for suggestions and/or insights. Best, Ozgur Akgun

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Actors and message-passing a la Erlang
by Yves Parès 27 Jul '10

27 Jul '10

Hello ! I've been studying Erlang and Scala, and I was wondering if someone has already implemented an actors and message passing framework for concurrent and distributed programs in Haskell.

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An experimental Zipper using Thrists and first-class Labels. Help or thoughts?
by Brandon Simmons 27 Jul '10

27 Jul '10

I had the idea for a simple generic Zipper data structure that I thought would be possible to implement using type-threaded lists provided by Gabor Greif's thrist package: http://hackage.haskell.org/package/thrist ...and the fclabels package by Sebastiaan Visser, Erik Hesselink, Chris Eidhof and Sjoerd Visscher: http://hackage.haskell.org/package/fclabels It would (ideally) work as follows: - the zipper would consist simply of a tuple: (type threaded list of constructor sections , current "context") - in the type threaded list we store functions (constructor with hole -> complete constructor), so the "one hole context" is represented as a lambda expression where the free variable will be filled by the current "context" (the snd of the tuple) - we "go down" through our structure by passing to our `moveTo` function a first-class label corresponding to the constructor we want to descend into. `moveTo` uses this both as a "getter" to extract the next level down from the current level, and as a "setter" to form the lambda expression which acts as the "constructor with a piece missing" - "going up" means popping the head off the thrist and applying it to the current context, making that the new context, exiting the zipper would be a fold in the same manner After throwing together a quick attempt I realized that I'm not sure if it would be possible to make the `moveUp` function type-check and be usable. I'm still new to GADTs, existential types, template haskell etc. and am stuck. Here is the code I wrote up, which doesn't currently compile: ---------------------------------- START CODE ------------------------------- {-# LANGUAGE TypeOperators, GADTs #-} module ZipperGenerator ( viewC --lets user pattern match against context , moveTo , moveUp , genZippers , zipper , unzipper , (:->) , ZipperGenerator , Zipper ) where -- these provide the secret sauce import Data.Record.Label import Data.Thrist import Language.Haskell.TH type ZipperGenerator = [Name] -> Q [Dec] -- the Template Haskell function that does the work of generating -- first-class labels used to move about the zipper: genZippers :: ZipperGenerator genZippers = mkLabels -- hide the innards: newtype Zipper t c = Z (Thrist (->) c t, c) -- returns the current "context" (our location in the zipper) for pattern -- matching and inspection: viewC :: Zipper t c -> c viewC (Z(_,c)) = c -- takes a first-class label corresponding to the record in the current context -- that we would like to move to: moveTo :: (c :-> c') -> Zipper t c -> Zipper t c' moveTo lb (Z(thr,c)) = Z (Cons (\a-> set lb a c) thr , get lb c) -- backs up a level in the zipper, returning `Nothing` if we are already at the -- top level: moveUp :: Zipper t c -> Maybe (Zipper t b) moveUp (Z (Nil,_)) = Nothing moveUp (Z (Cons f thr,c)) = Just $ Z (thr, f c) -- create zipper with focus on topmost constructor level: zipper :: t -> Zipper t t zipper t = Z (Nil,t) -- close zipper unzipper :: Zipper t c -> t unzipper (Z(thr,c)) = undefined --foldThrist ($) id thr c ---------------------------------- END CODE ------------------------------- Thanks, Brandon Simmons http://coder.bsimmons.name/blog/

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