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FW: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell?
by Peter Verswyvelen 02 Jan '08

02 Jan '08

I'm also new to Haskell, but I'm a C++/C# veteran, so I'll give it a shot. C#'s interfaces look a bit like Haskell's type classes. Although not exactly the same, you could try something like this: -- C#: interface IX1 { String foo1(int); } class IX1 obj where foo1 :: Int -> obj -> String -- C#: interface IX2<A> { String foo2(A); } class IX2 obj a where foo2 :: a -> obj -> String --C#: interface IX3<A> where A : IY { String foo3(A); } class IY a where {- ... -} class IY a => IX3 obj a where foo3 :: a -> obj -> String --C#: interface IX4<A> : IZ where A : IY class IZ a where {- ... -} class (IY a, IZ obj) => IX4 obj a where foo4 :: a -> obj -> String The C# "class" is called an "instance" in Haskell, so to implement an interface, you would do e.g. data X2 = X2 { x2Bla :: String, ... } instance IX2 X2 String where foo2 s o = (x2Bla o) ++ s This assumes your "objects" are immutable, otherwise you would have to return (obj,String) instead of just String and then you most likely want to use the state monad and "do notation" to make functional programming look more like imperative programming. You really have to drop the OO way of thinking, which I find the hardest :) Haskell's type classes are more powerful in some sense than C# interfaces; for example, in C# you can't attach an interface to any class (take for example the Point struct), it has to be your own class, while in Haskell, you can implement an instance of a type class for any datatype! Hope this helps a bit. As I'm new myself to Haskell, so take this with a "grain of salt". Once you bite the bullet, I found Haskell a lot more fun than C#, although C# of course comes with a gigantic .NET framework and tools... Peter -----Original Message----- From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Nicholls, Mark Sent: Wednesday, January 02, 2008 5:41 PM To: haskell-cafe(a)haskell.org Subject: [Haskell-cafe] Is there anyone out there who can translate C# generics into Haskell? I'm trying to translate some standard C# constucts into Haskell... some of this seems easy.... Specifically 1) Interface IX { } 2) Interface IX<A> { } 3) Interface IX<A> Where A : IY { } 4) Interface IX<A> : IZ Where A : IY { } I can take a punt at the first 2....but then it all falls apart _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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I am looking for the source code for STM implementation ...
by Galchin Vasili 02 Jan '08

02 Jan '08

Hello, I found STM.c(lower layer), but cannot find the top layer code written Haskell.?? Vasya

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Newbie question: can laziness lead to space compression?
by Brian Hurt 02 Jan '08

02 Jan '08

My apologies if this has been beat to death before, I'm still new to Haskell. But I was wondering if it is possible that lazy evaluation could lead to space compression, especially under heavily persistant usage patterns? Here's the argument I'm making. Say we have a tree-based Set with, say, 1024 values in it. For ease of math we'll assume that it's perfectly balanced. Say each node in the tree takes 5 words of memory. So in an eager language (for example, Ocaml), adding a new node to this tree requires the allocation of 10 new nodes, or 50 words of memory. In Haskell, what would happen (as I understand it) is that just a new lazy thunk would be allocated- say, 10 words of memory. Conceptually, we could think of the returned value as the original tree plus a small delta. The lazy implementation is using 40 fewer words of memory than the eager implementation. Note that the benefit isn't *big*- we're talking about 40 words of memory when the main data structure is taking up 5K plus words of memory- so it's less than 1% different. But there is a (small) upside in memory usage at least occassionally, right? Brian

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[RFC] benchmarks of bytestrings, teaser
by Peter Lund 02 Jan '08

02 Jan '08

What do you think the relative speeds are of the six small haskell programs at the end of this email? All they do is read from stdin and count the number of spaces they see. There are two that use strict bytestrings, two that use lazy bytestrings, and two that use the standard Haskell strings. Three use a recursive function with an accumulator parameter and three use a foldl with a lambda function. Say the fastest one takes the time 1. How much time will the others take? And how about memory? How much memory do you think they require? Let's say we feed a 150MB(*) file into each of them, how many megabytes do you think they end up using (as seen from the OS, not in terms of how big the live heap is)? I'm going to post full benchmarks + analysis on Wednesday. -Peter *) hardddisk megabytes. The file is 150000034 bytes ≈ 143 mebibytes. PS: For extra credit, what do you think is the peak memory use for this program when given an input file of 150MB? {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Lazy.Char8 as B import GHC.Int (Int64) -- note that D.BS.Lazy.Char8.length is ByteString -> Int64 -- D.BS.C8.length is ByteString -> Int cnt :: B.ByteString -> Int64 cnt bs = B.length (B.filter (== ' ') bs) main = do s <- B.getContents print (cnt s) ============================== hs/space-bs-c8-acc-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Char8 as B cnt :: Int -> B.ByteString -> Int cnt !acc bs = if B.null bs then acc else cnt (if B.head bs == ' ' then acc+1 else acc) (B.tail bs) main = do s <- B.getContents print (cnt 0 s) ============================== hs/space-bslc8-acc-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Lazy.Char8 as B cnt :: Int -> B.ByteString -> Int cnt !acc bs = if B.null bs then acc else cnt (if B.head bs == ' ' then acc+1 else acc) (B.tail bs) main = do s <- B.getContents print (cnt 0 s) ============================== hs/space-xxxxx-acc-1.hs: {-# LANGUAGE BangPatterns #-} cnt :: Int -> String -> Int cnt !acc bs = if null bs then acc else cnt (if head bs == ' ' then acc+1 else acc) (tail bs) main = do s <- getContents print (cnt 0 s) ============================== hs/space-bs-c8-foldlx-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Char8 as B cnt :: B.ByteString -> Int cnt bs = B.foldl' (\sum c -> if c == ' ' then sum+1 else sum) 0 bs main = do s <- B.getContents print (cnt s) ============================== hs/space-bslc8-foldlx-1.hs: {-# LANGUAGE BangPatterns #-} import qualified Data.ByteString.Lazy.Char8 as B cnt :: B.ByteString -> Int cnt bs = B.foldl' (\sum c -> if c == ' ' then sum+1 else sum) 0 bs main = do s <- B.getContents print (cnt s) ============================== hs/space-xxxxx-foldl.hs: {-# LANGUAGE BangPatterns #-} cnt :: String -> Int cnt bs = foldl (\sum c -> if c == ' ' then sum+1 else sum) 0 bs main = do s <- getContents print (cnt s) ==============================

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Problem building hdbc-sqlite3 with ghc 6.8.2
by manu 02 Jan '08

02 Jan '08

hello, has anybody managed to build hdbc-sqlite3 with ghc 6.8.2 ? I get the following error : Macintosh:HDBC-sqlite3-1.1.3.0 manu$ runhaskell Setup.lhs build Preprocessing library HDBC-sqlite3-1.1.3.0... ghc-6.8.2: unrecognised flags: -F/Users/manu/Library/Frameworks Usage: For basic information, try the `--help' option. compiling dist/build/Database/HDBC/Sqlite3/Statement_hsc_make.c failed command was: /usr/local/bin/ghc -c -package base-3.0.1.0 -package mtl-1.1.0.0 -package HDBC-1.1.3 -I. -F/Users/manu/Library/Frameworks dist/build/Database/HDBC/Sqlite3/Statement_hsc_make.c -o dist/build/ Database/HDBC/Sqlite3/Statement_hsc_make.o if somebody has an idea... Thanks E.D

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Knowledge
by jlw501 02 Jan '08

02 Jan '08

I'm new to functional programming and Haskell and I love its expressive ability! I've been trying to formalize the following function for time. Given people and a piece of information, can all people know the same thing? Anyway, this is just a bit of fun... but can anyone help me reduce it or talk about strictness and junk as I'd like to make a blog on it? contains :: Eq a => [a]->a->Bool contains [] e = False contains (x:xs) e = if x==e then True else contains xs e perfectcomm :: Bool perfectcomm = undefined knowself :: Bool knowself = undefined allKnow :: Eq a => [a]->String->Bool allKnow _ "" = True allKnow [] k = False allKnow (x:[]) k = knowself allKnow (x:xs) k = comm x xs k && allKnow xs k where comm p [] k = False comm p ps k = if contains ps p then knowself else perfectcomm -- View this message in context: http://www.nabble.com/Knowledge-tp14423007p14423007.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Missing join and split
by Mitar 02 Jan '08

02 Jan '08

Hi! I am really missing the (general) split function built in standard Haskell. I do not understand why there is something so specific as words and lines but not a simple split? The same goes for join. Yes, I can of course define them but ... in an interactive mode it would be quite handy to have them there. Or am I wrong and are those hidden somewhere? So what are common ways to get around this? What are elegant definitions? Inline definitions? Mitar

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Fwd: Attempt at defining typeful primary key/foreign key relationships
by Justin Bailey 31 Dec '07

31 Dec '07

Small correction - the extensions listed at the bottom weren't necessary. I had attempted to define a relationship among tables automatically as: instance (HasField fk child, HasField pk parent, HasRelation pk fk) => HasRelation parent child But that was redundant with the signature on relation. Removing it allowed me to remove all extensions except MultiParameterTypeClasses. Justin ---------- Forwarded message ---------- From: Justin Bailey <jgbailey(a)gmail.com> Date: Dec 31, 2007 12:25 PM Subject: Attempt at defining typeful primary key/foreign key relationships To: haskelldb-users(a)lists.sourceforge.net All, As part of my data model, I'd like to say there is a primary key/foreign key relationship between two tables/queries. Right now I only want to do this to capture some extra-query information that I'm using for code generation - I don't intend it to actually affect queries generated, though I'd like to go there eventually. I define a class which says there is a relation: class HasRelation pk fk I define a type which holds relation values: data Relation a pk fk p c = Relation { ... } Relation values can only be constructed by a function with appropriate constraints on it: relation :: (HasRelation pk fk, HasField pk parent, HasField fk child) => Attr pk a -> Attr fk a -> Rel parent -> Rel child -> Relation a pk fk parent child relation primaryKey foreignKey prnt chld = Relation .... As an example, if I have tables Orders and LineItems, both with field order_id, I can establish a relationship by first defining an instance: instance HasRelation Orders.Order_id LineItems.Order_id I can then create a relation value between the two: orders <- table Orders.orders lineItems <- table LineItems.lineItems let r1 = relation Orders.order_id LineItems.order_id orders lineitems Note these types would be defined using DBDirect, so Order_id as a type with one constructor, lineItems has type "Table ...", etc. If I try to define an incorrect relation (wrong fields, fields in the wrong order, etc), then the above will not compile, which is just what I want. If you've read this far, I've not done any type-level programming or even played much with classes and instances before so I'd like to know if the approach makes sense. I took most of my inspiration from the HasField class in the HDBRec module. 1) Could this be simpler? Any obvious deficiencies? 2) I had to enable a number of extensions (though all of these are also enabled by haskelldb) - FlexibleInstances, FlexibleContexts, UndecidableInstances and OverlappingInstances. Should I be worried? 3) It's possible to define relationships among any two types. How can I limit that to 1) attributes with 2) the same type? Would I have to define an Attr typeclass, parallel to the Attr type? Thanks for any and all feedback. Happy New Year! Justin

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Trying to fix space leak
by Ben Challenor 31 Dec '07

31 Dec '07

Hi I'm learning Haskell through writing a compiler. I'm seeing huge memory use in a function which converts the dataflow graph to the form required by Data.Graph. It needs to return a map from dataflow nodes to Vertexs, a map in the other direction, and the list of edges (as Vertex pairs). > total time = 18.78 secs (939 ticks @ 20 ms) > total alloc = 6,616,297,296 bytes (excludes profiling overheads) > > COST CENTRE MODULE %time %alloc > > j Dataflow 87.6 99.9 > graphvizCompile Dataflow 6.4 0.0 > f Dataflow 2.9 0.0 > con2tag_DFReal# Representation 2.1 0.0 I assume the allocation is being garbage-collected pretty quickly, because a) 6,616,297,296 bytes is stupid (!) and b) Process Explorer informs me that the peak private bytes of the program is not more than a couple of MB. The offending code looks like this: > -- (next Vertex available for use, edges so far, map so far, map so far) > type DependencyEdgesAcc = (Vertex, [Edge], Map.Map DFNode Vertex, Map.Map Vertex DFNode) > > -- Process a list of DFNodes in the same context. > dependencyEdges :: DependencyEdgesAcc -> [DFNode] -> DependencyEdgesAcc > dependencyEdges acc [] = {-# SCC "a" #-} acc > dependencyEdges acc@(_, _, mnv1, _) (n:ns) = > -- Check whether this node has been done before. > {-# SCC "j" #-} case {-# SCC "i" #-} Map.lookup n mnv1 of > Just _ -> {-# SCC "b" #-} dependencyEdges acc ns > Nothing -> > -- Find the list of dependencies. > let ndeps = {-# SCC "c" #-} nodeDependencies n in > -- Process them first. > let (v2, es2, mnv2, mvn2) = {-# SCC "d" #-} dependencyEdges acc ndeps in > -- Now we can claim v2 as the label for this node n. > -- Look up vdep for each dependency ndep and add an edge from the vdep to v2. > let {(v4, es4, mnv4, mvn4) = {-# SCC "e" #-} List.foldl' ( > \(v3, es3, mnv3, mvn3) ndep -> > case {-# SCC "f" #-} Map.lookup ndep mnv3 of > Just vdep -> (v3, (vdep, v2):es3, mnv3, mvn3) > Nothing -> error $ "this node should have been added already: " ++ show ndep > ) (v2, es2, mnv2, mvn2) ndeps } in > assert (v2 == v4) > assert (mvn2 == mvn4) > assert (mnv2 == mnv4) > -- Finally, add this node to the accumulator and then recurse. > dependencyEdges (v2+1, es4, {-# SCC "g" #-} Map.insert n v2 mnv2, {-# SCC "h" #-} Map.insert v2 n mvn2) ns with the following profile: > dependencyGraph Dataflow 4395 1 0.0 0.0 93.4 99.9 > getRootDFNodes Dataflow 4397 5 0.0 0.0 0.0 0.0 > dependencyEdges Dataflow 4396 282 0.0 0.0 93.4 99.9 > a Dataflow 4407 104 0.0 0.0 0.0 0.0 > j Dataflow 4399 178 87.6 99.9 93.4 99.9 > b Dataflow 4417 75 0.0 0.0 0.0 0.0 > g Dataflow 4405 103 0.1 0.0 0.2 0.0 > con2tag_DFBool# Representation 4419 1972 0.0 0.0 0.0 0.0 > con2tag_DFReal# Representation 4414 186890 0.1 0.0 0.1 0.0 > con2tag_DFNode# Representation 4413 1204 0.0 0.0 0.0 0.0 > h Dataflow 4404 103 0.0 0.0 0.0 0.0 > e Dataflow 4403 103 0.0 0.0 4.8 0.0 > f Dataflow 4412 174 2.9 0.0 4.8 0.0 > con2tag_DFBool# Representation 4420 245794 0.0 0.0 0.0 0.0 > con2tag_DFReal# Representation 4416 25772482 1.9 0.0 1.9 0.0 > con2tag_DFNode# Representation 4415 2068 0.0 0.0 0.0 0.0 > d Dataflow 4402 103 0.0 0.0 0.0 0.0 > c Dataflow 4401 103 0.0 0.0 0.0 0.0 > nodeDependencies Dataflow 4406 103 0.0 0.0 0.0 0.0 > i Dataflow 4400 178 0.6 0.0 0.7 0.0 > con2tag_DFBool# Representation 4418 48230 0.0 0.0 0.0 0.0 > con2tag_DFReal# Representation 4410 5045646 0.1 0.0 0.1 0.0 > con2tag_DFNode# Representation 4409 1928 0.0 0.0 0.0 0.0 Is this a space leak, or is my algorithm just silly? (It could well be the latter...) I've tried adding random bangs, to no effect. So, can anyone help me? :) Cheers Ben

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research topics
by Henrique Ferreiro 31 Dec '07

31 Dec '07

Hi haskellers! I am a postgraduate student interested in functional programming and Haskell. I would appreciate if someone could elaborate on which are the current hot topics of research in this area, and related to Haskell, of course. In particular, I am interested in compiler implementation. Thanks in advance. -- Henrique Ferreiro <henrique.ferreiro(a)gmail.com>

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