- Haskell-Cafe - Haskell.org

22 Aug '07

for sake of argument, suppose an enterprising haskell newbie wanted to code up concurrent b-trees (really b-link trees) in haskell. if i am understanding STM correctly, it will NOT help in any way with the implementation, because of the IO-intensive nature of the algorithms? so i will have to resort to the usual games with locks and latches? (using Ibrahim Jaluta, Seppo Sippu and Eljas Soisalon-Soininen. Concurrency control and recovery for balanced B-link trees. The VLDB Journal, Volume 14, Issue 2 (April 2005), Pages: 257 - 277, ISSN:1066-8888. as a source for b-tree algorithms.) take care, B

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I'm stuck in my thought experiment
by Levi Stephen 22 Aug '07

22 Aug '07

Hi, Apologies for a long post that may not be totally clear. I was thinking through a problem and how the data might be represented in Haskell. I'm now stuck and frustrated. Now, I'm not even sure whether I'm on the right track (I might still be thinking too OO). Suggestions/ideas would be much appreciated. I was imagining a drag and drop web page designer. There are a bunch of Widgets (e.g., BlogWidget, TextWidget, MenuWidget, etc) that the user can place on the page. Along with these are layout/container widgets (e.g., ColumnLayoutWidget) that can contain other widgets. I'm looking at a data structure that would allow this to be represented in Haskell, so I'm keeping in mind that these won't be written in code, but generated on the fly somehow (e.g., from a database or file). So, my thoughts were along the lines of something like: class Widget a where render :: a -> Html -- A page has a title and a Widget. -- I know this isn't valid Haskell, but I'm not sure how to specify what I -- want here. (existential types?) data Page = Page String Widget data TextWidget = TextWidget String instance Widget TextWidget .... -- An example layout widget data ColumnLayoutWidget = ColumnLayoutWidget [Widget] instance Widget ColumnLayoutWidget ... etc... So, entire pages might be represented something like: Page "Main" (ColumnLayoutWidget [MenuWidget, TextWidget mainPageText]) Page "About" (ColumnLayoutWidget [MenuWidget, TextWidget aboutPageText]) Where I get stuck, is I want to extract layout information into a parent page. This would allow global changes such as adding a header image to the above pages to be done once only. So I want to be able to have something like: layout = Page "Main" (ColumnLayoutWidget [MenuWidget, ??? ]) mainPage = ChildPage layout [TextWidget mainPageText] aboutPage = ChildPage layout [TextWidget aboutPageText] So, each page is it's layout/parent page, with additional widgets inserted/added. The issue becomes, given a parent page and the customized content for the child page, what is the best way to insert the customized content at the right point? Might a tree like structure be useful? But, how do you work out where in the tree child content gets added? Store a traversal with each sub tree of child page content that basically says 'insert here'? It might be simple to have a PlaceHolderWidget. Then insertions of the child page content happens at each of those widgets. This just gets trickier if I start considering multiple extension points for child pages and what happens when the layout/parent page changes. This is why I'm thinking I may be going along a bad path here. Thanks, Levi lstephen.wordpress.com

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Re: [Haskell-cafe] STM, IO and b-trees
by Thomas Conway 22 Aug '07

22 Aug '07

On 8/21/07, Ben <midfield(a)gmail.com> wrote: > some basic questions: you have limits on key and value size? fixed > page sizes per tree? I have chosen Yes and Yes respectively, though these are *choices*. Personally, I think fixed sized pages are fine. Handling unlimited sized keys is not hard, especially if you think they will be rare: you use a bit to flag an immediate key vs a remote key, and keep an overflow file for long keys. Another idea that I've had, though I have not had the opportunity to implement it is to use arithmetic coding on the keys. If you carve up the zero-to-one interval in sorted order, then the arithmetic coded bitstrings sort (lexicographically) in the same order as the uncompressed strings. From the top of my head, most *english* words can compress in this fashion with a 0 order model to about 2-3 bits per character. This doesn't eliminate a key length by itself, but it should increase the limit dramatically in practice. My implementation uses 64K pages, with a limit of 32K - 8 bytes on the size of an individual key. > on point 2: it sounds like you'r suggesting using STM after all. not > sure if i understand what you've said: something like type PagePtr t = TVar (Address, Maybe t) data Node = Node (PagePtr Node) [(Key,PagePtr Node)] | Leaf [(Key,Data)] > work with a page cache. do the various b-tree algorithms in STM > transactions on the cached pages. retry STM transactions when pages > aren't loaded. on successful STM transaction completion, write out the > dirty pages. Yes, except you might want to be clever about flushing dirty pages more lazily. My implementation isn't crash-proof (i.e. doesn't support crash recovery - the environment in which my code operated means that if something bad happens, I can rebuild from external data). > probably use the trick where the STM transaction returns > an IO action which you then perform. probably use ordinary page-level > locks to deal with concurrency on IO -- STM doesn't help. Maybe. See the SPJ video on STM. His basic point is that STM helps get rid of the stupid concurrency bugs, leaving just the "more interesting" ones for you to deal with. :-) Actually, using relaxed balance and making all the tree updates local, means that my btree code successfully supports a very high degree of concurrency. > as far as implementing the page cache: using a TVar Map would > essentially be creating a global lock on the entire cache. Exactly, which is why you want to push the TVars down. > so you > suggest using other implementations. your suggestion sounds like an > array with linear search, or probably something more sophisticated. i > would imagine a balanced binary tree where the nodes are TVars might > work nice, though rebalacing would touch a lot of nodes. (it does > seem strange haskell doesn't have more concurrent data structures.) Yes, I've chatted with Andrew Bromage about the need for Data.Map.Concurrent Data.Set.Concurrent etc. I have a concurrent hash table which works very nicely. Think class Hashable t where hash :: t -> Word64 type HashTable k v = TArray Word64 [(k,v)] Another alternative that others have suggested are Tries (radix trees). Something along the lines: type Trie v = .... insert :: [Bit] -> v -> Trie v -> STM () > on point 1: you're saying relaxed balance is better for STM-style > optimistic transactions? i'm using > > B-Trees with Relaxed Balance (1993) > Kim S. Larsen, Rolf Fagerberg > IPPS: 9th International Parallel Processing Symposium Yes. There's a tech report version which includes details of deletion which IIRC the one you mention does not. citeseer... google.... The reason I believe relaxed balance works particularly well with STM is that all the operations (insert, delete, rebalance) operate *locally*. That is, they only modify a single node or a couple of proximate nodes. In STM terms, this means a given operation only *writes* a couple of TVars close to where the operation took place. One of the cool things is that Larsen et al prove that as you progress up the tree the number of rebalancing operations drops geometrically. Effectively, this means you very rarely need to get a write-lock on the root (or top few nodes) of the tree, so randomly dispersed operations are unlikely to conflict with one another. Also, by separating the insert/delete bits from the rebalance bits, both are simpler to code, and you have fewer edge cases to think about. cheers, T -- Dr Thomas Conway drtomc(a)gmail.com Silence is the perfectest herald of joy: I were but little happy, if I could say how much.

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ANN: SF Bay Area Functional Programmers Group
by Keith Fahlgren 21 Aug '07

21 Aug '07

Hi All, I'd like to announce the formation of the Bay Area Functional Programmers group. This group is for anyone using or interested in functional programming and functional programming languages, particularly strongly typed languages such as Haskell, OCaml, SML, etc. The first meeting will be Thursday, September 13th at 7:30pm somewhere in San Francisco. Please join the mailing list at http://groups.google.com/group/bayfp and suggest a location. The initial meeting will be a casual pizza and beer get together, although going forward we'd like to also include speakers, reading and discussion of technical papers, and some hands on coding. Future announcements and the location of the first meeting will be posted to the BayFP mailing list. More information will be available on the website: http://bayfp.org/. Keith (+ Mike Wells)

1 0

"Tying the knot" with unknown keys
by David Ritchie MacIver 21 Aug '07

21 Aug '07

I was playing with some code for compiling regular expressions to finite state machines and I ran into the following problem. I've solved it, but I'm not terribly happy with my solution and was wondering if someone could come up with a better one. :-) Essentially I have data FSM = State { transitions :: (Map Char FSM) } and transitions' :: Regexp -> Map Char Regexp I want to lift this so that the Regexps become states of the finite state machine (while making sure I set up a loop in the data structure). Tying the knot is the traditional way of doing such things, but we couldn't figure out a way to make it work without the set of keys known in advance because of the strictness of Map in its keys (an association list was suggested, and that would probably work, but it seemed a bit ugly and would be fairly inefficient). In the end what I did was just work out the set of reachable regexps in advance and use a standard tying the knot trick, but it felt vaguely unsatisfactory (and does some repeat work which I felt should be unneccessary). Anyone have a more elegant suggestion? Regards, David

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can't build haxml under ghc 6.7, says HughesPJ is hidden... but ghc-pkg doesn't say it's hidden...
by Thomas Hartman 21 Aug '07

21 Aug '07

Can I get some help building HaXml (from hackage) under ghc 6.7? I'm hoping to get HAppS running under 6.7, and use the new debugger to better understand what's going on under the hood. Eg, when I'm in the h function, I can take a look at the args and just see what types they are. (I am finding building an intuition from the documentation alone to be overwhelming. Too many types, too many instances... total confusion.) HaXMl is a dependendency for happs, that's why I need it. Anyway building haxml this is what I get. The subject of my message more or less summarizes my confusion. hartthoma@linuxpt:~/installs/HaXml-1.13.2>runghc Setup.hs build Setup.hs: Warning: The field "hs-source-dir" is deprecated, please use hs-source-dirs. Preprocessing library HaXml-1.13.2... Preprocessing executables for HaXml-1.13.2... Building HaXml-1.13.2... src/Text/XML/HaXml.hs:30:7: Could not find module `Text.PrettyPrint.HughesPJ': it is a member of package pretty-1.0, which is hidden hartthoma@linuxpt:~/installs/HaXml-1.13.2>ghc-pkg list /usr/local/lib/ghc-6.7.20070719/package.conf: Cabal-1.1.7, base-2.1, directory-1.0, filepath-1.0, ghc-6.7.20070719, haskell98-1.0, hpc-0.5, old-locale-1.0, old-time-1.0, pretty-1.0, process-1.0, random-1.0, readline-1.0, rts-1.0, template-haskell-0.1, unix-2.0 hartthoma@linuxpt:~/installs/HaXml-1.13.2>ghc-pkg list pretty-1.0 /usr/local/lib/ghc-6.7.20070719/package.conf: pretty-1.0 hartthoma@linuxpt:~/installs/HaXml-1.13.2>ghc --version The Glorious Glasgow Haskell Compilation System, version 6.7.20070719 hartthoma@linuxpt:~/installs/HaXml-1.13.2>ghc-pkg describe pretty-1.0 name: pretty version: 1.0 license: BSD3 copyright: maintainer: libraries(a)haskell.org stability: homepage: package-url: description: This package contains a pretty-printing library. category: author: exposed: True exposed-modules: Text.PrettyPrint Text.PrettyPrint.HughesPJ hidden-modules: import-dirs: $topdir/lib/pretty-1.0 library-dirs: $topdir/lib/pretty-1.0 hs-libraries: HSpretty-1.0 extra-libraries: extra-ghci-libraries: include-dirs: $topdir/lib/pretty-1.0/include includes: depends: base-2.1 hugs-options: cc-options: ld-options: framework-dirs: frameworks: haddock-interfaces: $topdir/share/ghc/doc/html/pretty/pretty.haddock haddock-html: $topdir/share/ghc/doc/html/pretty hartthoma@linuxpt:~/installs/HaXml-1.13.2> hartthoma@linuxpt:~/ProjectRepos/miscAdmin>ghc-pkg list /usr/local/lib/ghc-6.7.20070719/package.conf: Cabal-1.1.7, base-2.1, directory-1.0, filepath-1.0, ghc-6.7.20070719, haskell98-1.0, hpc-0.5, old-locale-1.0, old-time-1.0, pretty-1.0, process-1.0, random-1.0, readline-1.0, rts-1.0, template-haskell-0.1, unix-2.0 --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

4 5

Re: [Haskell-cafe] How can I pass IOUArrays to FFI functions?
by Ryan Ingram 21 Aug '07

21 Aug '07

Thanks to everyone, especially Bulat Ziganshin. In http://haskell.org/haskellwiki/Modern_array_libraries there is enough information to do what I want. It specifically mentions that it's OK to pass ByteArray# and MutableByteArray# to an "unsafe foreign" procedure as long as that procedure doesn't save the pointer, and that worked for me. Here is what I ended up using, which worked great and the FFI usage for a couple of key functions sped up my code by a large factor: import Data.Array.Base import Data.Array.IO.Internals import GHC.Exts {-# INLINE unsafeByteArrayToPtr #-} unsafeByteArrayToPtr :: IOUArray Int Word32 -> Ptr Word32 unsafeByteArrayToPtr (IOUArray (STUArray _ _ array#)) = Ptr (unsafeCoerce# array#) Possibly a better thing to do would be to declare that the call takes a MutableByteArray# directly in the foreign import statement, which I believe would let me avoid using unsafeCoerce# at all, but this was good enough for my purposes. Afterwards I used -ddump-simpl to check on the generated Core for the foreign call and it looked good. -- ryan

4 8

is there a way to patch the build-depends line of a cabal file without breaking backwards compatibility?
by Thomas Hartman 21 Aug '07

21 Aug '07

cafe, is there a way to patch the build-depends line of a cabal file without breaking backwards compatibility? I just patched HDBC head to compile under ghc 6.7. Unfortunately it now won't compile in 6.6.1. is there a way for build-depends to detect which version of ghc you're on? also I seem to recall that -fglasgow-exts was deprecated under 6.7. is there a better way to beat back the error message below than this? thanks, thomas. { hunk ./Database/HDBC/Statement.hs 1 +{-# LANGUAGE TypeSynonymInstances #-} hunk ./Database/HDBC/Types.hs 1 +{-# OPTIONS_GHC -fglasgow-exts #-} +{- +-- without -fglasgow-exts you get: [_$_] +Database/HDBC/Types.hs:202:0: + Illegal polymorphic or qualified type: forall conn. + (IConnection conn) => + conn -> b + In the type signature for `withWConn': + withWConn :: forall b. + ConnWrapper -> (forall conn. (IConnection conn) => conn -> b) -> b +-} hunk ./HDBC.cabal 13 -Build-Depends: base, mtl + +Build-Depends: base, mtl, old-time, bytestring, containers +-- breaks backwards compability with ghc 6.6.1 + } --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

3 4

How can I pass IOUArrays to FFI functions?
by Ryan Ingram 21 Aug '07

21 Aug '07

I have a C function of type void f ( HsWord32* p0, HsWord32* p1, HsWord32 size ); along with the FFI declaration: foreign import ccall unsafe f :: Ptr Word32 -> Ptr Word32 -> Word32 -> IO () In my Haskell code I have an unboxed IO array of Word32; IOUArray Int Word32. I want to pass the pointer to this array to f(). How can I get the pointer out of the array? Or, is there a better way to declare f() to do this? I'm open to using GHC hackery; using v6.6.1 right now. -- ryan

4 3

Diagnosing stack overflow
by Justin Bailey 20 Aug '07

20 Aug '07

I am trying to determine why my stack overflows in my medium sized program (it has several modules but maybe only 1000 LOC total). On Windows, at least, the ghcprof visualization tool doesn't work. Any suggestions besides an output trace? It may be the function below, which tries to determine if a list of strict bytestrings is longer than the count given. I have tried to make it strict but am not sure if it's too lazy. Any hints are appreciated. -- Determines if the length of the strings in the list is longer than the given -- count. If not, amount the list falls short is returned. Otherwise, -- -1 indicates the prefix list is at least that long. If the count is zero and -- the list is empty or just null strings, -1 is also returned. prefixesAtLeast :: Int -> [S.ByteString] -> Int prefixesAtLeast !0 !ss | null ss = 0 | all S.null ss = 0 | otherwise = -1 prefixesAtLeast !n !ss = prefixesAtLeast' n ss where prefixesAtLeast' !n ss | n < 0 = -1 | null ss = n | otherwise = let (!s : (!rest)) = ss in prefixesAtLeast' (n - (S.length s)) rest

5 10

IO in HApps handler ?
by Luc TAESCH 20 Aug '07

20 Aug '07

Subject: IO in HApps handler ? I am trying to add a handler that would run an external command in HApps 0.8.8, and I got a type issue I do not know how to get around.. can we have IO in a handler ? testcmdpost.hs:52:8: Couldn't match expected type `Ev st Request' against inferred type `IO' Expected type: ServerPart (Ev st Request) Request IO Result Inferred type: ServerPart IO Request im Result In the expression: (h ["xxx"] GET) $ (ok $ (\ () () -> do (MySt val) <- get runCommand "ls" ["."] respond (show "dfdf"))) here is the handler iI am adding : ,h ["xxx"] GET $ ok $ \() () -> do (MySt val) <- get; runCommand "ls" ["."]; respond (show "dfdf" ) in there : import HAppS.Util.Common... ... main :: IO () main = stdHTTP [debugFilter -- we want to see debug messages in the console ,h [""] GET $ ok $ val "GETting root hello" -- ,h (Prefix ["s"]) GET $ respIO $ fileServe staticPath , hs (Prefix ["s"]) GET $ basicFileServe staticPath -- 0.8.8 -- /val shows us the current value ,h ["val"] GET $ ok $ \() () -> do (MySt val) <- get; respond (show val) -- /set with the POST data "val"=56 would set the value to 56 ,h ["xxx"] GET $ ok $ \() () -> do (MySt val) <- get; runCommand "ls" ["."]; respond (show "dfdf" ) ,h ["set"] POST $ ok $ \() newVal -> do put newVal; respond ("New value is " ++ show newVal) -- The first one is FromReqURI and the second one is FromMessage -- The cryptic comment about is referring to the arguments () and newVal -- to the method. The type of newVal being MyState is what -- invokes our custom FromMessage instance above. ] this is the runcommand from HAppS.Util.Common , not from defined as -- | Run an external command. Upon failure print status -- to stderr. runCommand :: String -> [String] -> IO () runCommand cmd args = do (_, outP, errP, pid) <- runInteractiveProcess cmd args Nothing Nothing let pGetContents h = do mv <- newEmptyMVar let put [] = putMVar mv [] put xs = last xs `seq` putMVar mv xs forkIO (hGetContents h >>= put) takeMVar mv os <- pGetContents outP es <- pGetContents errP ec <- waitForProcess pid case ec of ExitSuccess -> return () ExitFailure e -> do hPutStrLn stderr ("Running process "++unwords (cmd:args)++" FAILED ("++show e++")") hPutStrLn stderr os hPutStrLn stderr es hPutStrLn stderr ("Raising error...") fail "Running external command failed"

3 2

building a regular polygon
by dbastos＠toledo.com 20 Aug '07

20 Aug '07

I'm new to functional programming and to haskell. I'm reading ``The Haskell School of Expression'' by Paul Hudak and I'm studying how to solve exercise 2.2. My difficulty here is more algorithmic than Haskellian, but perhaps many here have solved this exercise already, or would know how to solve it; I hope it's okay to post. (*) Exercise 2.2 Define a function regularPolygon :: Int -> Side -> Shape such that regularPolygon n s is a regular polygon with n sides, each of length s. (Hint: consider using some of Haskell's trigonometric functions, such as sin :: Float -> Float, cos :: Float -> Float, and tan :: Float -> Float.) My idea is to start with the vertices (0,0) and (0,s). Then I ``connect a line'' from (0,0) to (0,s) and I then I need to compute the next vertex of the polygon. The angles of a regular polygon are alpha = ((n - 2) * pi)/n, where n is the number of sides of the polygon. So the third vertex of the polygon is x = cos (180 - alpha) * s y = sin (180 - alpha) * s, but I don't know how to teach the computer to get to the forth vertex. Once I figure that in a general way, I would just call myself until I get to the last vertex; if I start at n, we end at at n = 2 in which I return []. Something like: > data Shape = Polygon [Vertex] deriving Show > > type Side = Float > type Vertex = (Float, Float) > > regularPolygon :: Int -> Side -> Shape > regularPolygon n s = Polygon ((0,0) : (0,s) : buildList n s (fromIntegral n)) > where buildList :: Int -> Side -> Float -> [Vertex] > buildList 2 _ _ = [] > buildList n s m = let x = cos(pi - alpha) * s > y = sin(pi - alpha) * s > alpha = ((m - 2) * pi)/m > in (x,y) : buildList (n-1) s m but right now it just repeats all the vertices after the second. Any help is appreciated.

4 4

trouble compiling "import GHC.Prim(MutableByteArray#, ....." (building regex-tdfa from darcs) -- what's that # sign doing?
by Thomas Hartman 20 Aug '07

20 Aug '07

trying to compile regex-tdfa, I ran into another issue. (earlier I had a cabal problem but that's resolved.) there's a line that won't compile, neither for ghc 6.6.1 nor 6.7 import GHC.Prim(MutableByteArray#,RealWorld,Int#,sizeofMutableByteArray#,unsafeCoerce#) so the fresh darcs regex tdfa package won't build. This line (line 16 below) causes this error for ghc -e '' RunMutState.hs for both ghc 6.1 and 6.7 Much obliged for any help, Thomas. ********************************* hartthoma@linuxpt:~/installs/regex_darcs/regex-tdfa>runghc Setup.hs build Preprocessing library regex-tdfa-0.93... Building regex-tdfa-0.93... Text/Regex/TDFA/RunMutState.hs:16:32: parse error on input `#' hartthoma@linuxpt:~/installs/regex_darcs/regex-tdfa>head -n20 Text/Regex/TDFA/RunMutState.hs | cat -n 1 {-# LANGUAGE CPP #-} 2 module Text.Regex.TDFA.RunMutState(TagEngine(..),newTagEngine,newTagEngine2 3 ,newScratch,tagsToGroupsST 4 ,toInstructions,compareWith,resetScratch 5 ,SScratch(..),MScratch,WScratch) where 6 7 import Control.Monad(forM_,liftM,liftM2,liftM3,foldM) 8 --import Control.Monad.ST.Strict as S (ST) 9 --import qualified Control.Monad.ST.Lazy as L (ST) 10 import Control.Monad.State(MonadState(..),execState) 11 12 import Data.Array.Base(unsafeRead,unsafeWrite,STUArray(..)) 13 #ifdef __GLASGOW_HASKELL__ 14 import GHC.Arr(STArray(..)) 15 import GHC.ST(ST(..)) *** 16 import GHC.Prim(MutableByteArray#,RealWorld,Int#,sizeofMutableByteArray#,unsafeCoerce#) 17 #else 18 import Control.Monad(when) 19 import Control.Monad.ST(ST) 20 import Data.Array.ST(STArray) hartthoma@linuxpt:~/installs/regex_darcs/regex-tdfa> --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

3 5

ghc 6.7 /6.8
by Luc TAESCH 20 Aug '07

20 Aug '07

Date: Sun, 19 Aug 2007 12:39:50 +0200 Subject: Fwd: ghc 6.7 /6.8 hello. will 6.7 be released, or will only 6.8 be ? ( i.e do you use an even/uneven relesing number convention) -in that case, does the 6.8 branch means the freeze is on, and what would be the target for 6.8 ? Q3/2007 ? Q4 ? ( context: I am impatient to test the new debbuger, but too newby to go and fix all the lib..hoping it may help me undertand haskell execution.. )

2 1

Re: Interval Arithmetics
by Henning Thielemann 20 Aug '07

20 Aug '07

The page http://www.haskell.org/haskellwiki/Applications_and_libraries/Mathematics#N… lists several implementations of several flavours of computable reals, which may be useful for you.

1 0

How do I simulate dependent types using phantom types?
by DavidA 19 Aug '07

19 Aug '07

Hi, I am trying to implement quadratic fields Q(sqrt d). These are numbers of the form a + b sqrt d, where a and b are rationals, and d is an integer. In an earlier attempt, I tried data QF = QF Integer Rational Rational (see http://www.polyomino.f2s.com/david/haskell/hs/QuadraticField.hs.txt) The problem with this approach is that it's not really type-safe: I can attempt to add a + b sqrt 2 to c + d sqrt 3, whereas this should be a type error because 2 /= 3. So I thought I'd have a go at doing it with phantom types. In effect I'd be using phantom types to simulate dependent types. Here's the code: {-# OPTIONS_GHC -fglasgow-exts #-} import Data.Ratio class IntegerType a where value :: Integer data Two instance IntegerType Two where value = 2 data Three instance IntegerType Three where value = 3 data QF d = QF Rational Rational deriving (Eq) instance IntegerType d => Show (QF d) where show (QF a b) = show a ++ " + " ++ show b ++ " sqrt " ++ show value instance IntegerType d => Num (QF d) where QF a b + QF a' b' = QF (a+a') (b+b') negate (QF a b) = QF (-a) (-b) QF a b * QF c d = QF (a*c + b*d*value) (a*d + b*c) fromInteger n = QF (fromInteger n) 0 The problem is, this doesn't work. GHC complains: The class method `value' mentions none of the type variables of the class IntegerType a When checking the class method: value :: Integer In the class declaration for `IntegerType' Is what I'm trying to do reasonable? If no, what should I be doing instead? If yes, why doesn't GHC like it? Thanks, David

5 5

List comprehension desugaring
by Neil Mitchell 19 Aug '07

19 Aug '07

Hi, The Haskell desugaring for list comprehensions is given in: http://haskell.org/onlinereport/exps.html#list-comprehensions All the rules seem to be left to right rewrites, apart from the second one, which seems to be right to left. Is there some deep reason for this, or is this accidental. Thanks Neil

2 2

Using Apple Spotlight to search for .hs and .lhs files
by Dan Piponi 19 Aug '07

19 Aug '07

This isn't really a Haskell question but I'm guessing some Haskell hackers have a solution. MacOS X's Spotlight doesn't seem to be able to search for text in .lhs and .hs files. But it can find text in .txt files. Is there a way of getting Spotlight to treat .lhs and .hs files like .txt files so I can instantly search all of my Haskell source? -- Dan

2 1

Error building takusen with Cabal-1.1.6.2
by John Dell'Aquila 19 Aug '07

19 Aug '07

Setup.hs wants a module that Cabal hides. Am I doing something wrong (newbie :-) or should I try to fall back to Cabal-1.1.6.1? $ ghc --make -o setup Setup.hs Setup.hs:13:7: Could not find module `Distribution.Compat.FilePath': it is hidden (in package Cabal-1.1.6.2)

4 3

Sudoku Solver
by Adrian Neumann 19 Aug '07

19 Aug '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: RIPEMD160 Hi, I wrote a brute-force sudoku solver. It takes a [[Int]] and spits out the first solution it finds. Why is it, that [0,0,0,7,0,6,9,0,0] [9,0,0,0,0,4,7,0,0] [7,0,0,0,0,0,4,0,0] [0,2,7,0,3,5,8,0,0] [6,9,5,8,2,0,0,0,0] [0,8,0,0,0,0,5,0,0] [0,3,0,0,0,0,0,0,0] [8,7,0,9,0,0,0,0,0] [0,0,0,0,0,0,0,0,0] is solved instantly, but when I remove just one number my program takes forever? === import Array import List import System type Field = Array Int (Array Int Int) isEmpty :: Int -> Bool isEmpty = (==) 0 done :: Field -> Bool done a = isFull a && checkField a isFull::Field -> Bool isFull a = notElem (0) $ (concat.(map elems).elems) a readField :: [[Int]]->Field readField = (listArray (1,9).map (listArray (1,9))) checkField :: Field -> Bool checkField a = check (rows a) && check (columns a) && check (squares a) where check b = and $ ((map ((==1).length)).concat.(map group).clean) b clean = map (dropWhile isEmpty.sort) columns::Field -> [[Int]] columns a = [[a!i!j|i<-[1..9]]|j<-[1..9]] rows :: Field -> [[Int]] rows a = [[a!i!j|j<-[1..9]]|i<-[1..9]] squares :: Field -> [[Int]] squares a = [[a!(i+n)!(j+m)|i<-[1..3],j<-[1..3]] |n<-[0,3,6],m<-[0,3,6]] - -- creates a list of all allowed entries genMoves :: Field -> [Field] genMoves a = concat [update a i j|i<-[1..9],j<-[1..9],isEmpty (a!i!j)] where update b i j= [b //[(i,b!i //[(j,x)])]|x<-[1..9], checkField (b //[(i,b!i //[(j,x)])])] play :: [Field] -> Maybe Field play (f:a) | done f= Just f | isFull f= play a | otherwise = play ((genMoves f)++a) play [] = Nothing main :: IO () main = do x <- getArgs let field = read (x!!0) :: [[Int]] let erg=play [readField field] case erg of Just a -> putStrLn $ concat $ map ((++"\n").show) (rows a) Nothing -> putStrLn "Es gibt keine Loesung" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGtx0r11V8mqIQMRsRA7P5AJ9lG3mF3fHpUiyOqCeRVOOEGozp1wCeI80C RfD/U5y+yEgF0fe+kRwDbUI= =Ngss -----END PGP SIGNATURE-----

9 14

Chessboard-building in Haskell
by Andrew Wagner 19 Aug '07

19 Aug '07

I've started a blog series on writing a chess engine in Haskell. I just posted the second blog entry today: http://sequence.complete.org/node/361 I suspect there's more work to be done on that function, though. It seems like there should be a nice way to remove that flip in apply. Any thoughts?

2 1

Re: Hints for Euler Problem 11
by ronguida＠mindspring.com 18 Aug '07

18 Aug '07

Thank you for all the hints. Here's how I ended up solving it: > module Main > where > > import Data.List > import Data.Maybe > > gridText = "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08\n\ > \49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00\n\ > \81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65\n\ > \52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91\n\ > \22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80\n\ > \24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50\n\ > \32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70\n\ > \67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21\n\ > \24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72\n\ > \21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95\n\ > \78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92\n\ > \16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57\n\ > \86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58\n\ > \19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40\n\ > \04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66\n\ > \88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69\n\ > \04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36\n\ > \20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16\n\ > \20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54\n\ > \01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48" > type Grid a = [[a]] > readGrid :: (Read a) => String -> Grid a > readGrid = (map ((map read) . words)) . lines > grid :: Grid Integer > grid = readGrid gridText takeExactly - extract the first "n" elements of a list; there must be at least "n" elements, n > 0 > takeExactly :: (Monad m) => Int -> [a] -> m [a] > takeExactly n xs | n > 0 = > let ys = take n xs in > if length ys == n > then return ys > else fail "takeExactly: list is too short" > | otherwise = fail "takeExactly: empty list" > extractGroups :: Int -> [[a]] -> [[a]] > extractGroups n = catMaybes . map (takeExactly n) > gridExtractGroups :: Int -> Grid [a] -> Grid [a] > gridExtractGroups n = filter (not . null) . map (extractGroups n) gridTails - generate a list of grids with sequential starting places Parameters a and b determine the offset bewteen successive grids, as follows: (note that a, b may not be negative) Grid x_{0,1.., 0,1..} -> [ Grid x_{ 0, 1 .., 0, 1 ..} , Grid x_{ a, a+1 .., b, b+1 ..} , Grid x_{2*a, 2*a+1 .., 2*b, 2*b+1 ..} etc. ] > gridTails :: (Int, Int) -> Grid a -> [Grid a] > gridTails (a,b) xs = > if not $ null $ concat xs > then xs : gridTails (a,b) ((drop a) $ map (drop b) xs) > else [] transpose3d - converts x_{i,j,k} to x_{j,k,i} using sequence is x_{i,j,k} -> x_{j,i,k} -> x_{j,k,i} > transpose3d :: [Grid a] -> Grid [a] > transpose3d = map transpose . transpose reorient - transform a direction vector so that both components are non-negative > reorient :: (Int, Int) -> (Grid a -> Grid a, (Int, Int), Grid b -> Grid b) > reorient (a,b) > | a>=0 && b>=0 = (id , ( a, b), id) > | a< 0 && b>=0 = (reverse , (-a, b), reverse) > | a>=0 && b< 0 = (map reverse , ( a,-b), map reverse) > | a< 0 && b< 0 = (reverse . map reverse, (-a,-b), reverse . map reverse) makeEls - convert a grid to grid of Equidistant Letter Sequences > makeEls :: (Int, Int) -> Grid a -> Grid [a] > makeEls vec = let (reflect2, rvec, reflect1) = reorient vec in > reflect2 . transpose3d . gridTails rvec . reflect1 > getGroups :: Int -> (Int, Int) -> Grid a -> [[a]] > getGroups n (a,b) = concat . gridExtractGroups n . makeEls (a,b) > findMaxProduct :: (Ord a, Num a) => Int -> (Int, Int) -> Grid a -> a > findMaxProduct n (a,b) = maximum . map product . getGroups n (a,b) > main :: IO() > main = do > print $ findMaxProduct 4 (1,0) grid > print $ findMaxProduct 4 (0,1) grid > print $ findMaxProduct 4 (1,1) grid > print $ findMaxProduct 4 (1,-1) grid -- Ron

9 18

Bathroom reading
by Dougal Stanton 18 Aug '07

18 Aug '07

I'm looking for cool but mind-bending examples of functional brilliance. Let us say, hypothetically, you had a bathroom without any reading material. And having read all the Dilbert and Garfield you could seriously stomach, decide you should educate yourself while "on the job". :-) So you decide to print up some "one-liner" style programs into a little booklet. Something between credit-card and postcard sized, with a neat but mind-bending program on it. Don Stewart occasionally swoops in with some fixpoint malarkey to defuse heated discussions. I mean that kind of thing, but with a slightly wider scope than just fibs... Suggestions, please! D.

12 13

Re: [Haskell-cafe] monte carlo trouble
by Chad Scherrer 18 Aug '07

18 Aug '07

I'm using ListT now, trying to do this: type Sample a = ListT (State StdGen) a randomStreamR :: (RandomGen g, Random a) => (a,a) -> g -> ([a], g) randomStreamR bds g =(randomRs bds g1, g2) where (g1,g2) = split g sample :: [a] -> Sample a sample [] = ListT (State f) where f s = case next s of (_,s') -> ([],s') sample xs = do let bds = (1, length xs) xArr = listArray bds xs i <- ListT . State $ randomStreamR bds return $ (xArr ! i) -- Simple example, for testing main = mapM_ print . flip evalState (mkStdGen 1) . runListT $ do x <- sample [1..100] y <- sample [1..x] return (x,y) The abstraction seems much better now, but even the simple little example blows up in memory. Here's a snippet from profiling with -auto-all -caf-all (after I interrupted it): CAF:lvl4 Main 261 1 0.0 0.0 100.0 100.0 main Main 273 0 0.0 0.0 100.0 100.0 sample Main 274 1 100.0 100.0 100.0 100.0 randomStreamR Main 276 1 0.0 0.0 0.0 0.0 I'm wondering if the bind for ListT is still effectively building every possibility behind the scenes, and sampling from that. I could redo the Sample monad by hand, if that could be the problem, but I'm not sure what changes would need to be made. Or maybe it's building lots of different arrays and holding them for too long from the GC. Or maybe it's a strictness/laziness issue I'm missing. Still not so sure when I need case vs let/where. How would you guys going about tracking down the problem? Thanks, Chad On 8/15/07, Paul Johnson <paul(a)cogito.org.uk> wrote: > Chad Scherrer wrote: > > Thanks for your replies. > > > > I actually starting out returning a single element instead. But a > > given lookup might return [], and the only way I could think of to > > handle it in (State StdGen a) would be to fail in the monad. But > > that's not really the effect I want - I'd rather have it ignore that > > element. Another option was to wrap with Maybe, but then since I > > really want a sequence of them anyway, I decided to just wrap in a > > List instead. Is there a way Maybe would work out better? > Ahh. How about using ListT Gen then? That (if I've got it the right > way round) works like the list monad (giving you non-determinism), but > has your random number generator embedded as well. Take a look at "All > About Monads" at http://www.haskell.org/all_about_monads/html/. Each > action in the monad may use a random number and produce zero or more > results. > > Paul. > > >

2 1

defining last using foldr
by Alexteslin 18 Aug '07

18 Aug '07

Hi, I am trying to do the exercise which asks to define built-in functions 'last' and 'init' using 'foldr' function, such as last "Greggery Peccary" = 'y' the type for my function is: myLast :: [Char] -> Char I am not generalizing type so that make it less complicated. But what ever i am trying would not work. The only function type foldr takes as an argument is either (a->a->a) or (a->b->b) and none of the functions i found that would match this type from Char to Char. So in other words should be (Char->Char-Char). I can define the function without foldr but that misses the point of the exercise. Any hint will be appreciated, Thank you -- View this message in context: http://www.nabble.com/defining-last-using-foldr-tf4269357.html#a12151145 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

8 22

What to do about an absent package maintainer?
by Adam Langley 18 Aug '07

18 Aug '07

The HsOpenSSL package[1] is good work, but the author doesn't respond to email(*). I've a bunch of darcs patches to add tests, DSA support and fast Integer <-> BIGNUM functions. Can I use my Hackage account to upload version 0.2 if someone else uploaded 0.1? It is reasonable? I'm guessing that this issue will start to occur more often now that we have Hackage. It's probably good to have some policy. Cheers, (*) It's been > 1 month, with emails from several different addresses. [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsOpenSSL-0.1 -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org

1 0

Re: [Haskell-cafe] Remember the future
by Andrew Coppin 18 Aug '07

18 Aug '07

Gwern Branwen wrote: > On 0, Andrew Coppin <andrewcoppin(a)btinternet.com> scribbled: > >> I've seen comments in various places that monads allow you to "borrow >> things from the future". >> >> That sounds completely absurd to me... can anybody explain? >> > > Take a look at issue 6 of The Monad Reader; search for time travel. <http://www.haskell.org/haskellwiki/The_Monad.Reader> > Mmm... yes... I've read this before. It didn't make any sense then either. (The example is too big and complicated. It obscures the thing it's trying to illustrate...)

1 0

GHC linking problems
by SevenThunders 18 Aug '07

18 Aug '07

I have a large Haskell/C project that needs to be linked against an even larger set of C libraries and object files (OpNet) on a linux box (Fedora Core 7). So far I have been able to link my Haskell libraries to some C test code containing a main function without incident. However the link flags are very complex having been extracted from ghc. (e.g. it requires a host of -u flags and then a bunch of links to external C libraries including a long list of Haskell libraries.) Unfortunately control over the OpNet compilation and linking process is weak at best. Moreover it is not clear how the linking occurs and there is some suspicion that it ultimately creates a dynamic link library which it loads from some external process (the exact calls to gcc are hidden). During the OpNet build we get the following error /usr/lib/ghc-6.6.1/libHSrts.a(Main.o): In function `main': Main.c:(.text+0x22): undefined reference to `__stginit_ZCMain' Main.c:(.text+0x43): undefined reference to `ZCMain_main_closure' collect2: ld returned 1 exit status I do not get this error when statically linking my libraries to a standalone C function containing a main() function. What should I look for? Are there known work arounds to this problem? It's troubling that exporting Haskell to C should be this problematic. Unfortunately I can not use ghc to compile the OpNet code as you might imagine. -- View this message in context: http://www.nabble.com/GHC-linking-problems-tf4270650.html#a12155220 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 5

trouble building regex-base 0.91 on ghc 6.7
by Thomas Hartman 18 Aug '07

18 Aug '07

I'm trying to build the latest regex base, which is required for the other regex packages under ghc 6.7 It complains that it can't find Data.Sequence, because it's in a hidden module containers. I added containers to the cabal depends as can be seen in the grep below. And containers isn't hidden when I do ghc-pkg list. What gives? Still getting used to cabal... thomas. hartthoma@linuxpt:~/installs/regex-base-0.91>runghc Setup.hs build Preprocessing library regex-base-0.91... Building regex-base-0.91... Text/Regex/Base/RegexLike.hs:47:17: Could not find module `Data.Sequence': it is a member of package containers-0.1, which is hidden hartthoma@linuxpt:~/installs/regex-base-0.91>grep -i containers regex-base.cabal Build-Depends: base >= 2.0, mtl, containers hartthoma@linuxpt:~/installs/regex-base-0.91>ghc-pkg list /usr/local/lib/ghc-6.7.20070816/package.conf: Cabal-1.1.7, HUnit-1.1.1, QuickCheck-1.0.1, array-0.1, arrows-0.2.1, base-2.1, bytestring-0.1, cgi-3001.1.5, containers-0.1, directory-1.0, fgl-5.4.1, filepath-1.0, (ghc-6.7.20070816), haskell-src-1.0.1, haskell98-1.0, hpc-0.5, html-1.0.1, mtl-1.0.1, network-2.0.1, old-locale-1.0, old-time-1.0, packedstring-0.1, parallel-1.0, parsec-2.0, pretty-1.0, process-1.0, random-1.0, readline-1.0, regex-base-0.72, rts-1.0, stm-2.1, template-haskell-0.1, unix-2.0, xhtml-3000.0.2 --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

2 2

Basic question....
by rodrigo.bonifacio 18 Aug '07

18 Aug '07

Hi all. I want to create the following polymorphic type (EnvItem) that we can apply two functions (envKey and envValue). I tried the following: > type Key = String > data EnvItem a = EnvItem (Key, a) > envKey :: EnvItem (Key, a) -> String > envKey EnvItem (key, value) = key > envValue :: EnvValue(Key, a) -> a > envValue EnvItem (key, value) = value But this is resulting in the error: [Constructor "EnvItem" must have exactly 1 argument in pattern] I think this is a very basic problem, but I don't know what is wrong. Regards, Rodrigo.

4 3

Looking at program execution
by Ian Duncan 17 Aug '07

17 Aug '07

Is there any way to view the steps that a haskell program goes through step by step? I'm thinking something similar to what I've seen in things I've been reading. For example: foldl (+) 0 [1..10] => (0+1) => ((0+1)+2) => (((0+1)+2)+3) => etc. I've seen these sorts of line-by-line execution steps, but I was curious if there was any way to simply type in a function and its arguments and have ghci or hugs or something print out this sort of visual representation of what's going on. Anyone know of something like this?

3 3

Very fast searching of byte strings
by ChrisK 17 Aug '07

17 Aug '07

Post the the library mailing list at [1] is the Boyer-Moore algorithm implemented for strict and lazy bytestrings (and combinations thereof). It finds all the overlapping instances of the pattern inside the target. I have performance tuned it. But the performance for searching a strict bytestring is better then for a lazy bytestring (even if they only had a single strict chunk), which almost certainly means I was not clever enough to get GHC to produce the optimal code. There is much more description in the module's haddock header. Hopefully Don or other ByteString experts/maintainers can tweak this even further. Also at [1] is a Knuth-Morris-Pratt module which find non-overlapping instances and is slightly slower on my benchmarks. Happy Searching, Chris Kuklewicz [1] http://www.haskell.org/pipermail/libraries/2007-August/007987.html

1 0

Re: [Haskell-cafe] Basic question....
by Janis Voigtlaender 17 Aug '07

17 Aug '07

----- Ursprüngliche Nachricht ----- Von: "rodrigo.bonifacio" <rodrigo.bonifacio(a)uol.com.br> Datum: Freitag, August 17, 2007 3:11 pm Betreff: [Haskell-cafe] Basic question.... > Hi all. > > I want to create the following polymorphic type (EnvItem) that we > can apply two functions (envKey and envValue). I tried the following: > > > type Key = String > > > data EnvItem a = EnvItem (Key, a) > > > envKey :: EnvItem (Key, a) -> String > > envKey EnvItem (key, value) = key > > > envValue :: EnvValue(Key, a) -> a > > envValue EnvItem (key, value) = value > > But this is resulting in the error: [Constructor "EnvItem" must > have exactly 1 argument in pattern] > > I think this is a very basic problem, but I don't know what is wrong. You are simply missing some brackets: > envKey :: EnvItem (Key, a) -> String > envKey (EnvItem (key, value)) = key > envValue :: EnvValue(Key, a) -> a > envValue (EnvItem (key, value)) = value Ciao, Janis.

3 3

So far, so good! Until... (Haskell 98 Report questions)
by Ian Duncan 17 Aug '07

17 Aug '07

... I hit Chapter 3 and started reading about expressions. *If you are able to answer any of these questions, please send me an email. I am very lost and confused in this section, so even one answered question may help greatly.* I actually decided to sit down and figure out the Haskell 98 Report today. Everything was going well until I began Chapter 3. Here's the section that has me baffled: "In the syntax that follows, there are some families of nonterminals indexed by precedence levels (written as superscript). Similarly, the nonterminals op, varop, and conop may have a double index: a letter l, r, or n for left-, right- or non-associativity and a precedence level. A precedence-level variable i ranges from 0 to 9; an associativity variable, a, varies over {l,r,n}. For example, aexp -> ( exp^(i+1) qop^(a,i) ) actually stands for 30 productions, with 10 substitutions for i and 3 for a." *note that the "^" was used to indicate superscript. So here's my list of questions so far: 1. What are nonterminals? 2. What are productions and substitutions? I tried the dictionary and wikipedia, but neither were very helpful in defining those terms. Next, it says: exp -> exp^0 :: [context=>] type (expression type signature) | exp^0 exp^i -> exp^(i+1) [qop^(n,i) exp^(i+1)] | lexp^i | rexp^i ... From there it continues with more syntax stuff, but I'll stop there for the sake of not typing too much. So here are some questions about this section: 1. What the heck is going on? 2. How is an expression with a precedence of i considered to be an expression of i+1? 3. Within the ... section it mentions lexp and rexp. Do those stand for left-associative expressions and right-associative expressions respectively? I understand the concept of fixity when I see it mentioned in code, but I truly have no idea what 3.0 is talking about. Can anyone shed some light on any of this? I'm still in high school, so if anyone could please explain it to me in layman's terms primarily, or provide some resources explaining more complex terms, It would be greatly appreciated. Thanks a bunch! Ian Duncan

3 2

Hints for Euler Problem 11
by Ronald Guida 17 Aug '07

17 Aug '07

Hi, again. I started looking at the Euler problems [1]. I had no trouble with problems 1 through 10, but I'm stuck on problem 11. I am aware that the solutions are available ([2]), but I would rather not look just yet. In Problem 11, a 20x20 grid of numbers is given, and the problem is to find the largest product of four numbers along a straight line in the grid. The line can be horizontal, vertical, or diagonal. I figured out how to handle the horizontal and vertical products, but I'm stuck on how to approach the problem of extracting the diagonals. Here is what I have so far; it does the horizontal and vertical cases: > module Main > where > > import Data.List > > gridText = "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08\n\ > \49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00\n\ > \81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65\n\ > \52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91\n\ > \22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80\n\ > \24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50\n\ > \32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70\n\ > \67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21\n\ > \24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72\n\ > \21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95\n\ > \78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92\n\ > \16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57\n\ > \86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58\n\ > \19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40\n\ > \04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66\n\ > \88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69\n\ > \04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36\n\ > \20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16\n\ > \20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54\n\ > \01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48" > > readGrid :: (Read a) => String -> [[a]] > readGrid = (map ((map read) . words)) . lines > > grid :: [[Integer]] > grid = readGrid gridText > > makeGroups :: Int -> [a] -> [[a]] > makeGroups 0 _ = [] > makeGroups n xs = let ys = take n xs in > if n == length ys > then ys : (makeGroups n $ tail xs) > else [] > > maxHorizontal :: (Ord a, Num a) => Int -> [[a]] -> a > maxHorizontal length = maximum . map product . concat . map (makeGroups length) > > maxVertical :: (Ord a, Num a) => Int -> [[a]] -> a > maxVertical length = maxHorizontal length . transpose > > main :: IO() > main = do > print $ maxHorizontal 4 grid > print $ maxVertical 4 grid To handle the diagonals, my plan is to try to extract each diagonal as a list of elements and put all the diagonals into a list; then I can use maxHorizontal. I came up with this function to try to extract the main diagonal. > getDiag :: [[a]] -> [a] > getDiag = map (head . head) . iterate (tail . map tail) The problem is, this function doesn't work unless I have an infinite grid. Could anyone provide me with some hints to lead me in the right direction? Thank you -- Ron References: [1] http://projecteuler.net/index.php?section=view [2] http://www.haskell.org/haskellwiki/Euler_problems

7 7

Collada, FBX, XML schemas and Haskell?
by Peter Verswyvelen 17 Aug '07

17 Aug '07

I wanted to do some experiments with HOpenGL, and one of the things I tried is importing 3D models. So I searched for a library that could do that, but besides Frag, who uses the limited MD3 format, I did not find anything useful. Has any work been done on supporting that? Maybe just converting the 3D format into a series of HOpenGL calls (tools like that exists for C)? It would be nice to support Collada, one of the two standards for 3D models in the game industry. It is fully open and has exporters for most major 3D modeling packages. See http://en.wikipedia.org/wiki/COLLADA. The other standard is FBX, but that's a proprietary format from Autodesk. Collada comes with a full XML schema. Is it possible to convert an XML schema into Haskell data/code that automatically handles reading documents that follow the schema, as is commonly done when using Java or .NET? I looked at some XML libraries for Haskell, but could not figure out if they supported something like that. I can find the tools I need for C# or Java in an instance, but when it comes to Haskell I have difficulties understanding even the basic descriptions of some of the libraries ;-) Of course, that is because I'm a newbie in Haskell land and don't have a PhD degree, but it does make development in Haskell very frustrating! ;-) Thanks, Peter

2 1

trouble building 6.7 on ubuntu
by Thomas Hartman 17 Aug '07

17 Aug '07

I have gotten ghc 6.7 to compile, but not with extra libs. I'm on linux x86, ubuntu. Can someone suggest a download date that worked for them, or how to fix one or more of the problems described below? When I build from source (this is for august 2, unknown linux, as conal spoke well of it :) but I have tried with several other dates in august with the same result) I get Linux linuxpt 2.6.15-23-386 #1 PREEMPT Tue May 23 13:49:40 UTC 2006 i686 GNU/Linux The programs included with the Ubuntu system are free software; checking host system type... i686-pc-linux-gnu checking target system type... i686-pc-linux-gnu Which we'll further canonicalise into: i386-unknown-linux checking for path to top of build tree... ./configure: line 1433: cd: utils/pwd: No such file or directory ./configure: line 1438: -v0: command not found ./configure: line 1441: utils/pwd/pwd: No such file or directory configure: error: cannot determine current directory when I try unzip src and extralibs I get (aug 15... tail of a large out file after >1.5 hours of configure; make > out.txt) ...... make[1]: Leaving directory `/home/hartthoma/installs/ghc6.7/aug15/ghc-6.7.20070815/libraries' make -C libraries all make[1]: Entering directory `/home/hartthoma/installs/ghc6.7/aug15/ghc-6.7.20070815/libraries' rm -f -f stamp/configure.library.*.base base/unbuildable ( cd base && setup/Setup configure \ --enable-library-profiling --enable-split-objs \ --prefix='$topdir' \ --datadir='$prefix/share/ghc' \ --datasubdir='.' \ --libsubdir='$compiler/lib/$pkgid' \ --with-compiler=../../compiler/stage1/ghc-inplace\ --with-hc-pkg=../../utils/ghc-pkg/ghc-pkg-inplace\ --with-hsc2hs=../../utils/hsc2hs/hsc2hs-inplace \ --with-ld=/usr/bin/ld \ --haddock-args="--use-contents=../index.html \ --use-index=../doc-index.html" \ \ --configure-option=--with-cc=gcc ) \ && touch stamp/configure.library.build-profiling-splitting.base || touch base/unbuildable Setup: Warning: Unknown fields: nhc98-options (line 173) Fields allowed in this section: buildable, cc-options, ld-options, frameworks, c-sources, extensions, extra-libraries, extra-lib-dirs, includes, install-includes, include-dirs, hs-source-dirs, other-modules, ghc-prof-options, ghc-options, hugs-options, nhc-options, jhc-options, exposed-modules Setup: Warning: A package using section syntax should require "Cabal-Version: >= 1.2" or equivalent. HsColour 1.6 configure: Reading installed packages... Configuring base-2.1... configure: Flags chosen: isghc=True configure: Dependency rts-any: using rts-1.0 Setup: executing external program failed (exit 1) : /usr/local/bin/HsColour -version rm -f base/GNUmakefile cp Makefile.local base if ifBuildable/ifBuildable base; then \ cd base && setup/Setup makefile -f GNUmakefile; \ fi Setup: error reading dist/setup-config; run "setup configure" command? make[1]: *** [base/GNUmakefile] Error 1 make[1]: Leaving directory `/home/hartthoma/installs/ghc6.7/aug15/ghc-6.7.20070815/libraries' make: *** [stage1] Error 2 very weird error because when I execute /usr/local/bin/HsColour -version it doesn't fail but outputs 1.6. Can somebody suggest a download date known to compile (with extralibs) for ubuntu? Or suggest a fix? By the way I have also tried darcs get http://hackage.haskell.org/trac/ghc but this dies about halfway through (in the patch 10000s ) with libcurl error 18 or libcurl error 404 depending on the phase of the moon. I'm currently trying a build from darcs checkout with get --partial. If this is the preferred way of doing a checkout due to "crapping out halfway" issues maybe it should be said in the readme. (to be honest I forget what the difference between a partial darcs get and a complete get is.) Much obliged, thomas. "Conal Elliott" <conal(a)conal.net> Sent by: cvs-ghc-bounces(a)haskell.org 08/15/2007 06:38 PM To cvs-ghc(a)haskell.org cc Subject Fwd: how to get packages for HEAD snapshot ? oops -- meant to cc cvs-ghc btw, 20070802 does run and does have libs included. ---------- Forwarded message ---------- From: Conal Elliott < conal(a)conal.net> Date: Aug 15, 2007 2:56 PM Subject: Re: how to get packages for HEAD snapshot ? To: Stefan O'Rear <stefanor(a)cox.net> Thank Stefan, The latest mingw32 snapshot I see is 20070811. I installed it also, and when I run ghc, it just crashes (WinXP). Happens when I run it inside of emacs and also in a cygwin bash window. Is that a known problem? - Conal On 8/15/07, Stefan O'Rear < stefanor(a)cox.net> wrote: On Wed, Aug 15, 2007 at 02:20:53PM -0700, Conal Elliott wrote: > I installed ghc-6.7.20070810-i386-unknown-mingw32.exe. Though it runs, I > don't know how to get other packages installed, including Cabal. "ghc-pkg > list" tells me that I have only {ghc-6.7.20070810}, rts-1.0. > > Any suggestions? Upgrade GHC; Cabal should be installed but wasn't due to a bug in that snapshot. Stefan -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFGw3AnFBz7OZ2P+dIRArOwAKCMEEzbB/JwEYnA21wZb4gf1rCoDQCeL1DG u5ljAbmfp5j0sd5X51nf3wk= =hewc -----END PGP SIGNATURE----- _______________________________________________ Cvs-ghc mailing list Cvs-ghc(a)haskell.org http://www.haskell.org/mailman/listinfo/cvs-ghc --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: trouble building 6.7 on ubuntu
by Esa Ilari Vuokko 17 Aug '07

17 Aug '07

On 8/16/07, Thomas Hartman <thomas.hartman(a)db.com> wrote: > > > I repeated the install attempt described below using darcs head, including > the extra libs. > > I got the exact same error as before. > > Setup: Warning: Unknown fields: nhc98-options (line 173) .... and then a > cryptic error involving HsColour > > I think you run into Cabal bug - you need to remove (or upgrade?) HsColour in your path, if that's the case. Best regards, Esa

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Syntax for lambda case proposal could be "\of"
by Brian Hulley 17 Aug '07

17 Aug '07

Hi, On http://hackage.haskell.org/trac/haskell-prime/wiki/LambdaCase the proposed syntax for lambda case is: case of alts but this has a really bad downside for interactive editors: it doesn't allow one to distinguish between an incomplete construct and a completed construct thus removing one opportunity for an editor to alert the user (eg by highlighting) that there is something missing in the program text. Therefore I propose: \of alts which doesn't suffer this problem since the keyword "of" can never follow a '\' in the existing grammar. However I can't edit the H' wiki, so if anyone agrees that the above syntax is better please could they add the above to the page. Thanks, Brian.

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Pattern matching articles/explanations
by Ian Duncan 17 Aug '07

17 Aug '07

So what I noticed that "A Gentle Introduction to Haskell" mentioned that wild-cards are useful in constructors. For example: head (x:_) = x So, does that offer any performance benefits over: head (x:xs) = x Or is it primarily to indicate to the coder that xs is useless? I get the impression it has a very similar meaning to the irrefutable pattern in regards to not evaluating it when the function is called. Or am I way off?

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Re: hyperlinked haskell 98 grammar
by Peter Hercek 17 Aug '07

17 Aug '07

Hi Neil, For haskell-cafe members: This is what we are talking about: http://www.hck.sk/users/peter/ I'm glad somebody liked it. I do not understand what you mean by the extension request. Would you want to see the number of references of a production head even without opening the popup list box? If you meant that the pop-up list box should always display all backward links directly (without a scroll bar) then it would not work for a very big number of such links (in theory there can be hundreds there). If you do not like the default of 5 then change the nMaxRowCount "constant". Anyway, if it is complicated to describe, let it be. I do not expect to play with the java script code much. Only the most trivial things have a chance to be implemented by me. I have written the java script code few years ago and I do not really like java script ... Just wanted to give back something to the community - so I added the Haskell specific data. As for as putting it on the Haskell org I do not mind if somebody else does it. Although I'm not that sure if it would require license change. If I would believe giving it into public domain would make more grammars browsable like this one then I would do it. I do not really expect heavy loads. From my experience most language users never ever look at the grammar nor they care about it. And those who consult it a lot will take a local copy and tweak it anyway. Looking at the munin server loads, looks like you and Esa are the only ones who actually checked it out :-D Anyway, sending this to haskell cafe too (as recommended). I expect a discussion (if any) will continue there. Sorry about GHC user list abuse. Peter Hercek. Neil Mitchell wrote: > Hi > > In addition, perhaps this should be relocated to haskell.org, if your > server is not suitable for a large volume of users. I think it should > also be integrated somewhere (perhaps a link from the HTML report, and > certainly on the wiki) > > Thanks > > Neil > > On 8/16/07, Neil Mitchell <ndmitchell(a)gmail.com> wrote: >> Hi Peter, >> >> A nice application. One suggestion: I would have implemented it so >> that a maximum on one backward hyperlink menu could be visible at once >> - try clicking on two RHS's and you'll get too "menus" visible at the >> same time. >> >> You also might want to mail this out on haskell-cafe - which is more >> appropriate for "general haskell" things, ghc-users is more for direct >> issues with GHC (although haskell-cafe can also be used for those) >> >> Thanks >> >> Neil >> >> On 8/16/07, Peter Hercek <peter(a)syncad.com> wrote: >>> Hi, >>> >>> I was improving my Haskell knowledge lately and I created a small dhtml application which allows browsing of >>> Haskell 98 grammar. I contains both forward and backward hyperlinks. By backward hyperlink I mean that you can >>> click on an a production head and you get a popup list box where you can navigate to any production using the >>> nonterminal. If you like it please save and use your local copy since the server cannot handle much load. >>> >>> http://www.hck.sk/users/peter/ >>> >>> Peter. >>> >>> _______________________________________________ >>> Glasgow-haskell-users mailing list >>> Glasgow-haskell-users(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >>>

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Pattern matching articles/explanations
by Ian Duncan 17 Aug '07

17 Aug '07

Hello everyone, I've been working on improving my Haskell knowledge, and in doing so, I have read a little about as-patterns as well as some form of pattern that uses "~" that I don't really understand. I suspect there are even more lesser-known pattern matching expressions than just these, but I don't have the stamina to decode the Haskell 98 Report. Are there any articles anyone is aware of, or anything in the Haskell wiki that covers the built-in pattern matching facilities pretty comprehensively? I've looked in the Haskell wiki myself, but information on different pattern matching abilities seems rather scattered and I'm not entirely sure what I should be looking for. Thanks, Ian Duncan

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Re: [Haskell-cafe] monte carlo trouble
by Chad Scherrer 17 Aug '07

17 Aug '07

Yeah, I did have troubles with (StateT StdGen Maybe). If it hits a "Nothing", I'd like it to skip that one and try again with the next state. But instead, Nothing is treated as a failure condition that makes the whole thing fail. I just found MaybeT on the wiki, which looks like it could work. I'll take a look at that and the ListT thing. I'm starting to think the power of abstraction is a blessing and a curse. Haskell's abstraction mechanisms are so powerful that it's generally possible to come up with a way to solve a given problem elegantly and efficiently. On the other hand, if a problem isn't so well studied, the bulk of the work is in finding the right abstraction, which forces generalization beyond what would otherwise be needed (though it'll be easier the next time!). On 8/15/07, Paul Johnson <paul(a)cogito.org.uk> wrote: > Chad Scherrer wrote: > > Funny you should say that, I was just experimenting with generating > > one at a time using (StateT StdGen Maybe). If I get stuck (again) I'll > > check out ListT. Thanks! > > > > > You definitely want a list not a Maybe. List is for 0 or more results > whereas Maybe is for 0 or 1. Yes you can have a Maybe [a] with Nothing > instead of [], but its redundant. > > I'm fairly sure you need it the other way out too. All About Monads has > a demo of StateT s [a] where it is used to solve a logic problem. The > key point is that in the logic problem each possible result is > associated with a different state change. However here you want to have > a list of results and then a state change when you pick one (or a > subset, or whatever you want). So that would be ListT (StateT StdGen). > > Paul. > > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: trouble building 6.7 on ubuntu
by Stefan O'Rear 17 Aug '07

17 Aug '07

On Thu, Aug 16, 2007 at 01:01:12PM -0400, Thomas Hartman wrote: > I repeated the install attempt described below using darcs head, including > the extra libs. > > I got the exact same error as before. > > Setup: Warning: Unknown fields: nhc98-options (line 173) .... and then a > cryptic error involving HsColour I'm running a full get from 20070712 without problems. (admittedly not Ubuntu) Stefan

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Re: [Haskell-cafe] Explaining monads
by Gregory Propf 17 Aug '07

17 Aug '07

I made this mistake myself at first too. It seems that the Monad = "side effect machine" error is common to Haskell newbies. Probably to do with the fact that the first thing every programmer wants to do is write a hello world program and for that you need the IO Monad which requires some explanation of how a Monad can allow for side effects (at least the IO Monad). - Greg ----- Original Message ---- From: peterv <bf3(a)telenet.be> To: Kim-Ee Yeoh <a.biurvOir4(a)asuhan.com>; haskell-cafe(a)haskell.org Sent: Monday, August 13, 2007 10:31:48 AM Subject: RE: [Haskell-cafe] Explaining monads Ronald Guida wrote: > > Given the question "What is a Monad", I would have to say "A Monad is > a device for sequencing side-effects." > There are side-effects and there are side-effects. If the only monad you use is Maybe, the only side-effect you get is a slight warming of the CPU. .... "Side-effects" is a piece of linguistic cruft played fast-and-loose by too many people in this game. "Sequencing" suffers the same disease. ____________________________________________________________________________________ Sick sense of humor? Visit Yahoo! TV's Comedy with an Edge to see what's on, when. http://tv.yahoo.com/collections/222

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Erlang VM in Haskell
by Joel Reymont 16 Aug '07

16 Aug '07

Folks, I would like to write an Erlang VM in Haskell. I first thought of OCaml but Haskell has SMP and lazy evaluation may come in handy. Plus, I'll need help in this project like in no other and support from the Haskell community has always been outstanding. I'm doing this to learn more about the Erlang internals and to acquire some skills in the process. I'm also hoping to be at least as fast as the existing Erlang VM (written in C) and to be able to extend the VM in a functional language. With C you can implement a threaded VM using GCC labels or switch- style [1]. What is the most efficient approach in Haskell? I'm thinking using CPS and building up a chain of closures. Would this work well? Any other suggestions? The Haskell repo for the initial HEM (Haskell Erlang Machine) is at http://darcs.haskell.org/hem. A much improved HAW implementation will follow in the not so near future. Thanks, Joel [1] http://www.complang.tuwien.ac.at/forth/threaded-code.html -- http://wagerlabs.com

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Haskell for the Enterprise (topic for a MSc dissertation?)
by Lars Oppermann 16 Aug '07

16 Aug '07

Hi All, This is my first post here, so I'll start with a quick introduction... I live and work in Hamburg, Germany. My day job is as a software engineer at Sun Microsystems at the OpenOffice.org development team where I'm mostly doing XML (ODF) related things. Beside of that, I'm currently pursuing an M.Sc degree in software engineering in a part-time post graduate program where I am now nearing the phase where I have to develop a topic for my dissertation. Apart from a little dabbling with Miranda during my undergraduate CS days I haven't touched functional programming ever since and have been doing Java, C# and C++ most of the time. However, I have over the last few month "fallen in love" with the great expressiveness offered by Haskell and through that have gained some great new perspectives and discovered new ways to think when writing programs. Consequently, my idea to combine all of this into a topic for my dissertation is to write about the existing and potential benefits (and shortcomings) of a functional programming language for the development of typical enterprise applications (especially with regards to more and more concepts from functional programming being adopted in imperative languages). I would start off with an existing J2EE application, analyze the abstractions and patterns that it uses. I will then discuss similar and alternative abstractions and patterns available in functional programming which can be applied in order to solve the same problem in a functional setting, leveraging some existing infrastructure, such as HAppS. What would follow is a comparison of the two resulting applications from a software engineering standpoint: How maintainable are these implementations? How can they be tested? Can formal validation techniques be applied to parts of the functional implementation that can be used to establish its correctness. Maybe analysis of performance... One of my potential supervisors for the dissertation has already indicated some interest in the topic so it all looks quite well to me. The overview that I gave above obviously needs much more elaboration, but as I said, I have just started developing topics. I would love to hear your comments on this. Has this been done over and over already? Are there any source which you think of from the top of your head which I should check out? Books that you would want to recommend? As I wrote above, I'm quite new to Haskell. Thus if anyone wants to recommend some readings specifically on the aspect of modeling in a functional environment I'd be happy to hear about that too. All the best, Lars -- Lars Oppermann Hamburg, Germany

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monte carlo trouble
by Chad Scherrer 16 Aug '07

16 Aug '07

There's a problem I've been struggling with for a long time... I need to build a function buildSample :: [A] -> State StdGen [(A,B,C)] given lookup functions f :: A -> [B] g :: A -> [C] The idea is to first draw randomly form the [A], then apply each lookup function and draw randomly from the result of each. It's actually slightly more complicated than this, since for the real problem I start with type [[A]], and want to map buildSample over these, and sample from the results. There seem to be so many ways to deal with random numbers in Haskell. After some false starts, I ended up doing something like sample :: [a] -> State StdGen [a] sample [] = return [] sample xs = do g <- get let (g', g'') = split g bds = (1, length xs) xArr = listArray bds xs put g'' return . map (xArr !) $ randomRs bds g' buildSample xs = sample $ do x <- xs y <- f x z <- g x return (x,y,z) This is really bad, since it builds a huge array of all the possibilities and then draws from that. Memory is way leaky right now. I'd like to be able to just have it apply the lookup functions as needed. Also, I'm still using GHC 6.6, so I don't have Control.Monad.State.Strict. Not sure how much difference this makes, but I guess I could just copy the source for that module if I need to. Any help is greatly appreciated! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: [Haskell-cafe] Why monad tutorials don't work
by Dan Piponi 16 Aug '07

16 Aug '07

On 8/14/07, Dan Weston <westondan(a)imageworks.com> wrote: > Conor McBride and Ross Paterson said it best in the introduction to > their paper "Applicative programming with effects" [1]: As von Neumann said: "Young man, in mathematics you don't understand things, you just get used to them." Getting used to something is, practically by definition, something that you can't do just by reading the ultimate tutorial. You just have to write the code, see the pattern happen again and again, and abstract it. There's no short cut. (Well...sometimes...) On 8/14/07, Michael Vanier <mvanier(a)cs.caltech.edu> wrote: > I'm reminded of > a physics teacher who was having a similar problem explaining the concept of tensors, until he said > that "a tensor is something that transforms like a tensor does!". Grrr...must...hold...my...tongue... -- Dan

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Re: [Haskell-cafe] monte carlo trouble
by Chad Scherrer 16 Aug '07

16 Aug '07

Funny you should say that, I was just experimenting with generating one at a time using (StateT StdGen Maybe). If I get stuck (again) I'll check out ListT. Thanks! Chad On 8/15/07, Paul Johnson <paul(a)cogito.org.uk> wrote: > Chad Scherrer wrote: > > Thanks for your replies. > > > > I actually starting out returning a single element instead. But a > > given lookup might return [], and the only way I could think of to > > handle it in (State StdGen a) would be to fail in the monad. But > > that's not really the effect I want - I'd rather have it ignore that > > element. Another option was to wrap with Maybe, but then since I > > really want a sequence of them anyway, I decided to just wrap in a > > List instead. Is there a way Maybe would work out better? > Ahh. How about using ListT Gen then? That (if I've got it the right > way round) works like the list monad (giving you non-determinism), but > has your random number generator embedded as well. Take a look at "All > About Monads" at http://www.haskell.org/all_about_monads/html/. Each > action in the monad may use a random number and produce zero or more > results. > > Paul. -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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