- Haskell-Cafe - Haskell.org

nooby question on typing
by Han Joosten 14 Sep '08

14 Sep '08

Hi, I have a question about types in Haskell. I feel that I am overlooking some obvious solution, but I do not manage to get it right. Here is the plot: I have got 4 different types of 'rules', each with it's own constructor. So i defined: > type Rules = [Rule] > data Rule = RuRule > | SgRule > | GcRule > | FrRule > deriving (Eq,Show) > data RuRule > = Ru { rrsrt :: Char > , rrant :: Expression > , rrfps :: FilePos > } deriving (Eq,Show) > data SgRule > = Sg { srfps :: FilePos > , srsig :: Rule > , srxpl :: String > , srtyp :: (Concept,Concept) > } deriving (Eq,Show) > data GcRule > = Gc { grfps :: FilePos > , grspe :: Morphism > , grgen :: Expression > } deriving (Eq,Show) > data FrRule > = Fr { fraut :: AutType > , frdec :: Declaration > , frcmp :: Expression > , frpat :: String > } deriving (Eq,Show) Now I would like to be able to use these rules without knowing what kind they are, in lists, however i get errors like Couldn't match expected type `Rule' against inferred type `SgRule'. Is there any other (even trivial?) way to get this done? I know Haskell well enough to know that it is possible, but I don not know Haskell well enough to know how to do it :teeth: Any help is kindly appreciated! Han Joosten -- View this message in context: http://www.nabble.com/nooby-question-on-typing-tp19470727p19470727.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Fwd: Computer Language Benchmarks Game: [Which parallel problem would you add and why?]
by Don Stewart 14 Sep '08

14 Sep '08

The Computer Language Benchmarks Game has a quad core (as you probably know). But they're looking for new benchmarks. Ones that can actually exploit parallelism. If you've got suggestions on good parallelism benchmarks, and why they're good, add them to the alioth or reddit threads. ----- Forwarded message from Isaac Gouy ----- "Which parallel problem would you add and why?" http://alioth.debian.org/forum/forum.php?thread_id=14502&forum_id=2840 "Ask Proggit: Which parallel problem would you add now The Computer Language Benchmarks Game is measured on quad-core hardware, and why?" http://www.reddit.com/r/programming/comments/718qf/ask_proggit_which_parall… ----- End forwarded message -----

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Hackage needs a theme song!
by Jason Dagit 13 Sep '08

13 Sep '08

I realized tonight that Hackage needs a theme song. Here is my attempt at it, apologies to Jefferson Starship: We built this hackage, We built this hackage on lambda and types Say you don't know me, or the parameters I pass Say you don't care who instances this type class Knee deep in the thunk, value infinitely big Too many constraints eating up the type sig Main runs the Monad, Listen to the Reader Don't you remember? We built this hackage We built this hackage on lambda and types! We built this hackage, we built this hackage on lambda and types Built this hackage, we built this hackage on lambda and types Someone always playing package games Who cares they're always changing package names We just want to compile here, someone stole the type hack They call us lazy, evaluate us off the stack Main runs the Monad, Listen to the Reader Don't you remember? We built this hackage We built this hackage on lambda and types! We built this hackage, we built this hackage on lambda and types Built this hackage, we built this hackage on lambda and types It's just another version in our release style Cabal has got the build deps, and we upload the file Who counts the downloads underneath package node Who writes the type errors into our source code Don't tell us you need us, 'cause we're the db of tools Looking for pragmas, crawling through your RULES Don't you remember ... Main runs the Monad, Listen to the Reader - Don't you remember? We built this hackage, We built this hackage on lambda and types We built this hackage, we built this hackage on lambda and types Built this hackage, we built this hackage on lambda and types Built this hackage, we built this hackage on lambda and types Built this hackage, we built this hackage on lambda and types We built, we built this hackage yeah We built this hackage We built, we built this hackage (Repeat - fade)

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Windows details
by Andrew Coppin 13 Sep '08

13 Sep '08

I rather doubt anybody will know the answer to this, but I'll ask anyway... Under MS Windows, if you right-click on an executable file and select "properties", sometimes there's a "Version" tab that tells you the version number of the program. And sometimes there isn't. (Most especially, GHC-compiled programs do not have this tab.) Anybody know how to go about adding this? Also, anybody know how to give a GHC-compiled program a custom icon? If it's going to be really difficult then I won't bother, but it'd be interesting to know. (In other news, today I discovered that applying UPX to my 2.8 MB GHC-compiled executable shrinks it down to 800 KB and it still functions. And WinZip shrinks it down to just 300 KB. 2.8 MB for a program that just does a little text parsing does seem a touch excessive to me...)

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Erroneous package listings in hackage?
by Dougal Stanton 13 Sep '08

13 Sep '08

$ cabal update ... $ cabal list | less ... * cairo Latest version installed: 0.9.13 Homepage: http://haskell.org/gtk2hs/ License: BSD3 ... $ cabal install cairo cabal: There is no package named cairo I must admit I was surprised to see a gtk2hs module on Hackage, but not half as surprised when I found it wasn't there after all... D

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Windows console
by Andrew Coppin 13 Sep '08

13 Sep '08

When coding in a POSIX-compliant environment, you can usually write special escape codes to the console to change the text colour and so forth. However, this does not work on Windows. (Ignore all references you see to enabling ANSI.SYS in your config.sys file; this applies only to 16-bit MS-DOS programs, *not* to 32-bit Windows programs.) However - to my surprise - apparently the Win32 API provides a set of function calls that do in fact allow 32-bit applications to change the colours of the console on a character-by-character basis. However - wait for it - those *particular* functions aren't in Haskell's System.Win32.Console module. :-( Obviously, I know nothing about C. However, after much hunting around, it turns out that for some reason, GHC appears to ship with a complete set of C header files for the Win32 API. (In other words, the necessary header file for accessing the functions I want is there.) After staggering through the (very unhelpful) FFI language specification, I was able to make it so that I can apparently call these functions from Haskell. I was not, however, able to find any way at all to import the symbolic constants necessary, so I was forced to reading through the source code of the raw C header files to find out what the numeric values of these are (!!!) The long and short of it is, I now have a small Haskell library that enables me to print things in trippy colours on the screen from a normal Haskell CLI program. Yay for me! (Actually, you can also change the title on the window - so you can make your program say "My Fantastic Tool" instead of "C:\Documents and Settings\foo\Haskell\MyTool\MyTool.exe" in the titlebar. Again, this is for a normal Haskell console application with nothing done to it.) Would anybody else be interested in having this code? (Obviously, it's pretty tiny.) For that matter, would the maintainer of System.Win32.Console be interested in adding the necessary dozen lines of code to that module? Actually, now that I think about it, it would be kind of nice to have a magic package that writes out escape codes or calls the Win32 API depending on which platform your program is compiled on - in the style of System.FilePath. I don't know how to do that though... A nice idea, guys?

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ANN: Twidge
by John Goerzen 13 Sep '08

13 Sep '08

Hi folks, I've released Twidge, a command-line Twitter and Identi.ca client, written -- of course -- in Haskell. I'm pretty excited about it, but then hey, I'm biased. Anyhow, it's available from Hackage or from the homepage at: http://software.complete.org/twidge I've also got links to its (11-page!) manual as well as some HOWTOs and FAQs on the wiki at the above URL. Comments, patches welcome. Feel free to reply here or send them to @jgoerzen. -- John

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[Fwd: profiling in haskell]
by Vlad Skvortsov 13 Sep '08

13 Sep '08

Posting to cafe since I got just one reply on beginner@. I was suggested to include more SCC annotations, but that didn't help. The 'serialize' function is still reported to consume about 32% of running time, 29% inherited. However, functions called from it only account for about 3% of time. How do I get more insight into that? Thanks. -- Vlad Skvortsov, vss(a)73rus.com, http://vss.73rus.com

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uuid package on OS X
by Jason Dusek 13 Sep '08

13 Sep '08

The UUID package builds and runs fine on OS X without any need for `e2fsprog` -- just remove the `extra-libs` line from the Cabal file. (Apple put the UUID stuff in `libc`.) Has anyone tried to build it on Windows? -- _jsn |...UUID package...| http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uuid

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Tons of retainers when inserting 611, 756 elements into a Trie
by Denis Bueno 13 Sep '08

13 Sep '08

Dear haskell-cafe, I've got an anagram-finder ("puzzler") that uses a "dictionary" datatype, which in turn uses a trie. In src/hs/Main.hs, I create a new dictionary from a word list (a file containing one word per line) and perform a query on it in order to force it to actually load something from disk. By problem is that though my word list is just shy of 7MB, puzzler occupies nearly 1GB of resident memory as soon as the dictionary is loaded. I'm using GHC 6.8.3 and Gtk2hs 0.9.13 on OS X 10.4.11. To see for yourself, check out the code with git and run it: $ git clone git://github.com/dbueno/puzzler.git $ git checkout -b origin/bsbench $ cabal configure # needs gtk2hs $ cabal build $ ./dist/build/puzzler/puzzler # look at the resident memory Retainer profiling (attachment "puzzler-hr.pdf") shows that `insertWith' on my trie is responsible for most of the memory. As you will see, in `makeDictionary' I try to strictly apply the combining function to combat these retainers. But the problem doesn't go away. More strangely: puzzler has a second executable, puzzler-test, which runs unit tests, including loading the exact same anagram query. However, puzzler-test never uses more than 250MB or so. The dictionary (in module Puzzler.Anagram) is an array of bytestrings paired with a Trie mapping bytestrings to sets of indices in the array. > data Dictionary = Dictionary > { dictWords :: Words > , sortWords :: Trie IntSet } > type Words = Array Int ByteString The Trie is a custom bytestring trie datatype. Each level is mapped to by the character it represents. If that character is a terminal character for some word (Right (t,x)), then that word maps to x. Otherwise no word ends at that character, but there may be longer words, so the trie continues with (Left t). > newtype Trie a = Trie { unTrie :: IntMap (Either (Trie a) (Trie a, a)) } > deriving (Eq, Ord, Show) The following is where _I think_ the error lies (src/hs/Puzzler/Anagram.hs): > -- | Creates an anagram dictionary from a file of words, one per line. The > -- words may be compound, as long as there is one per line. > createDictionary :: FilePath -> IO Dictionary > createDictionary path = (makeDictionary . lines) `liftM` readFile path > > -- | Makes an anagram dictionary from a list of words. > makeDictionary :: [ByteString] -> Dictionary > makeDictionary ws = Dictionary > { dictWords = dw > , sortWords = go end Trie.empty } > where > dw = listArray (0, length ws - 1) ws ; (begin, end) = bounds dw > go i t | seq t $ False = undefined > | i < begin = t > | otherwise = go (i-1) > -- $ Trie.insertWith Set.union (BS.sort (dw!i)) (Set.singleton i) t > $ insertWith' Set.union (BS.sort (dw!i)) (Set.singleton i) t > > insertWith' f k v m | seq k $ False = undefined > insertWith' f k v m = case Trie.lookup k m of > Nothing -> Trie.insert k v m > Just s -> (Trie.insert k $! f v s) m I've been scouring haskell-cafe for similar issues, and all seem to have been solved by a function similar to my `insertWith''. Uncommenting `Trie.insertWith' and commenting out the rest doesn't appear to differ noticeably in memory consumption. The implementation of `Trie.insertWith' is: > insertWith :: (a -> a -> a) -> ByteString -> a -> Trie a -> Trie a > insertWith f bs x t@(Trie m) = case uncons bs of > Nothing -> t > Just (c, cs) -> Trie (Map.alter myAlter (ord c) m) > where > myAlter Nothing = Just $ cons (insertWith f cs x empty, x) > myAlter (Just (Left t)) = Just $ cons (insertWith f cs x t, x) > myAlter (Just (Right (t,x'))) = > Just $ Right (insertWith f cs x t, if isLast then f x x' else x') > > -- If `cs' is empty then `c' is the last char of the word to insert; > -- that is, we're done. > cons (t, x) = if isLast then Right (t, x) else Left t > isLast = BS.null cs Any ideas? Denis

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ANN: vxml (validating xml lib) - proof of concept - need some guidance - bad type level performance (ghc)
by Marc Weber 13 Sep '08

13 Sep '08

Hi @ll. This shouldn't have been an announce yet.. it's more crying for help to get to know how / wether to continue :-) I had an idea: a xml generating library validating the result against a given dtd (to be used in web frameworks etc .. ) After one week of coding I got the result git clone git://mawercer.de/vxml (1) (see README) git hash as of writing : 4dc53 A minimal example looks like this (taken from test.hs): import Text.XML.Validated.TH $( dtdToTypes "dtds/xhtml1-20020801/DTD/xhtml1-strict_onefile.dtd" (XmlIds (Just "-//W3C//DTD XHTML 1.0 Strict//EN") (Just "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd") ) ) main = putStrLn $ xml $ ((html << (head << (title <<< "vxml hello world"))) << (body << (div <<< "PCDATA text" )) ) Which takes about 7 secs to compile on my (fast) 2.5 Ghz machine. Before Oleg telling me about how to use the TypeEq implementation found in HList it took several hours (switch the cabal configure flag to see experience the incredible slow TypeToNat implementation) However type checking the simple small document (2) already takes 35 seconds, duplicating the tags within the body raises compilation time to 74 seconds. (Have a look at test/XmlToQ.hs to automatically create the code generating such a given xml file (its used in run-testcases.hs) The dtd representation used is very easy to understand: (3a) shows allowed subelements of the head tag Seq = sequence Star = zero or more ... (3b) shows the list of allowed attributes for a more commonly used tag such as div the element list (3a) is 324 lines long and there are approximately 15 attributes. Use a line such as putStrLn $ xml $ debugEl div to get this view. All the work is done by class Consume st el r | st el -> r -- result is on of C,CS,R,F class Retry elType st el st' | st el -> st' st = state as given in (3a) el = child to be added (Elem Html_T) or PCDATA C = element consumed, end CS a = element consumed, continue with state a R a = retry with given state (can happen after removing a Star = ()* on a no match ) F a = no match, show failure a instance Retry elType C el C instance Retry elType (CS st) el st instance ( -- retry Consume st el r , Retry elType r el st' ) => Retry elType (R st) el st' Maybe you knowing much more about ghc internals have some more ideas how to optimize? I only came up with a) implement xml HTrue $ xmldoc -- type checking variant xml HFalse $ xmldoc -- unchecked but faster variant so you can run xml HTrue once a day only while having lunch.. ( doubt Maybe you have to take lunch two or more times on intermediate web projects .. ) b) change (A attrType) to attrType only and (Elem e) to e which does not work for the same reason as class TypeEq a b c | a b -> c instance TypeEq a a HTrue instance TypeEq a b HFalse doesn't. Of course a a will match a b as well, but the result is totally different.. So would it be possible to either tell ghc to ignore this happily taking the result of the better matching instance? Or to explicitely tell ghc when an instance matches the way it does in current implementations make it a no match if some constraints can or cannot be satisfied such as this: class TypeEq a b c | a b -> c instance TypeEq a a HTrue instance [ NoMatchOn a b -- if this can be satisfied ignore this instance ] => TypeEq a b HFalse class NoMatchOn a b instance NoMatchOn a a c) Having a reduction rule such as this: If the left side of (Or a b) returnns C (consumed) tell ghc to not evaluate the result of (Consume b element result) which must be done to see wether there is closer instance match (Is this correct ?) Of course not everything is implemented yet.. but except speed I consider it beeing quite usable. He, thanks for reading till the end. Any feedback and suggestions are appreciated. If you have any trouble contact me on irc. I've been using $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 Sincerly Marc Weber (1) If you are interested and either don't have git or are not familiar using it I can send you a tarball. (2) <!DOCTYPE html SYSTEM "/tmp/dtd.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en-US" xml:lang="en-US"> <head> <title>Example 6 - XHTML 1.0 Strict as application/xhtml+xml</title> <meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" /> <link rel="stylesheet" type="text/css" href="style.css" /> </head> <body> <h1>Example 6 - XHTML 1.0 Strict as application/xhtml+xml</h1> <p> This document is valid XHTML 1.0 Strict served as <code>application/xhtml+xml</code>. </p> <p> This document references CSS rules contained in an external stylesheet via <code>link</code>. </p> <p> Note how the CSS rules for the background are applied in Netscape 7.x, Mozilla, Opera 7 but that Internet Explorer can not display the page at all. </p> <p> <a href="http://validator.w3.org/check/referer"><img src="http://www.w3.org/Icons/valid-xhtml10" alt="Valid XHTML 1.0!" height="31" width="88" /></a> </p> </body> </html> (3a) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Or (Seq (Elem Title_T) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Query (Seq (Elem Base_T) (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))))))) (Seq (Elem Base_T) (Seq (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T)))))) (Seq (Elem Title_T) (Star (Or (Elem Script_T) (Or (Elem Style_T) (Or (Elem Meta_T) (Or (Elem Link_T) (Elem Object_T))))))))))) (3b) (HCons (A Lang_A) (HCons (A Xml:lang_A) (HCons (A Dir_A) (HCons (A Id_A) (HCons (A Profile_A) HNil)))))

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Text.JSON idiomatic use
by ntupel 13 Sep '08

13 Sep '08

As a follow up to my previous JSON serialization post I came up with a first draft of some simple record type serialization/deserialization. What I would like to know is, whether this is the right approach or what better ways there are to make a custom data type an instance of class JSON. Any chance to reduce the amount of boilerplate required to do this? I would be grateful for any feedback (also general style comments are much appreciated). Many thanks! module Test where import Text.JSON data Message = Error { event :: String, channel :: String, id :: String, cause :: String, message :: String} | Join { event :: String, channel :: String, id :: String, name :: String} | Leave { event :: String, channel :: String, id :: String, really :: Bool} deriving (Eq, Show, Read) asJSString :: String -> JSValue asJSString = JSString . toJSString asString :: JSValue -> String asString (JSString s) = fromJSString s asBool :: JSValue -> Bool asBool (JSBool b) = b showErrorJSON, showJoinJSON, showLeaveJSON :: Message -> JSValue showErrorJSON (Test.Error evt cha id cau msg) = showJSON $ toJSObject [("event", evt), ("channel", cha), ("id", id), ("cause", cau), ("message", msg)] showJoinJSON (Join evt cha id nme) = showJSON $ toJSObject [("event", evt), ("channel", cha), ("id", id), ("name", nme)] showLeaveJSON (Leave evt cha id rly) = showJSON $ toJSObject [("event", asJSString evt), ("channel", asJSString cha), ("id", asJSString id), ("really", JSBool rly)] createMessage, readErrorJSON, readJoinJSON, readLeaveJSON :: [(String, JSValue)] -> Maybe Message readErrorJSON xs = do evt <- lookup "event" xs cha <- lookup "channel" xs id <- lookup "id" xs cau <- lookup "cause" xs msg <- lookup "message" xs Just (Test.Error (asString evt) (asString cha) (asString id) (asString cau) (asString msg)) readJoinJSON xs = do evt <- lookup "event" xs cha <- lookup "channel" xs id <- lookup "id" xs nme <- lookup "name" xs Just (Join (asString evt) (asString cha) (asString id) (asString nme)) readLeaveJSON xs = do evt <- lookup "event" xs cha <- lookup "channel" xs id <- lookup "id" xs rly <- lookup "really" xs Just (Leave (asString evt) (asString cha) (asString id) (asBool rly)) createMessage obj = do evt <- lookup "event" obj case asString evt of "/error" -> readErrorJSON obj "/me/add" -> readJoinJSON obj "/me/remove" -> readLeaveJSON obj _ -> Nothing instance JSON Message where showJSON x@(Test.Join _ _ _ _) = showJoinJSON x showJSON x@(Test.Leave _ _ _ _) = showLeaveJSON x showJSON x@(Test.Error _ _ _ _ _) = showErrorJSON x readJSON (JSObject o) = case createMessage . fromJSObject $ o of Just m -> Ok m Nothing -> Text.JSON.Error "Parsing failed." readJSON _ = Text.JSON.Error "Records must be JSObjects"

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template haskell -- include a file?
by Jason Dusek 13 Sep '08

13 Sep '08

I'd like to use template Haskell to include as a string in a Haskell file. How do I do it? Is there any lengthy documentation with examples for Template Haskell? The pages linked to from `haskell.org` are a little sparse. -- _jsn

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Haskell and Java
by Maurício 13 Sep '08

13 Sep '08

Hi, I use Haskell, and my friends at work use Java. Do you think it could be a good idea to use Haskell with Java, so I could understand and cooperate with them? Is there a a Haskell to Java compiler that's already ready to use? Thanks, Maurício

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JSON serialization/deserialization
by ntupel＠googlemail.com 13 Sep '08

13 Sep '08

Dear list members, I try to use Text.JSON (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/json) to serialize and deserialize record types. As I understand it, the data types need to be instances of class JSON. However I have difficulties to come up with a nice implementation of showJSON and readJSON. For example: module Test where import Text.JSON data Record = One { field1 :: String, field2 :: String } | Two { field :: String } deriving (Eq, Ord, Show, Read) showJSON (One x y) = toJSObject [("field1", x), ("field2", y)] showJSON (Two x) = toJSObject [("field", x)] readJSON x = -- ??? This lacks the instance declaration itself but my problem is more fundamental. How can I write the readJSON method? In general how would one approach the problem of serialization/deserialization? I am puzzled that the type classes Show and Read can be derived automatically. I can not think of any way to do this. Thanks for your help!

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Can you do everything without shared-memory concurrency?
by Bruce Eckel 13 Sep '08

13 Sep '08

As some of you on this list may know, I have struggled to understand concurrency, on and off for many years, but primarily in the C++ and Java domains. As time has passed and experience has stacked up, I have become more convinced that while the world runs in parallel, we think sequentially and so shared-memory concurrency is impossible for programmers to get right -- not only are we unable to think in such a way to solve the problem, the unnatural domain-cutting that happens in shared-memory concurrency always trips you up, especially when the scale increases. I think that the inclusion of threads and locks in Java was just a knee-jerk response to solving the concurrency problem. Indeed, there were subtle threading bugs in the system until Java 5. I personally find the Actor model to be most attractive when talking about threading and objects, but I don't yet know where the limitations of Actors are. However, I keep running across comments where people claim they "must" have shared memory concurrency. It's very hard for me to tell whether this is just because the person knows threads or if there is truth to it. The only semi-specific comment I've heard refers to data parallelism, which I assumed was something like matrix inversion, but when I checked this with an expert, he replied that matrix inversion decomposes very nicely to separate processes without shared memory, so now I'm not clear on what the "data parallelism requires threads" issue refers to. I know that both Haskell and Erlang only allow separated memory spaces with message passing between processes, and they seem to be able to solve a large range of problems -- but are there problems that they cannot solve? I recently listened to an interview with Simon Peyton-Jones where he seemed to suggest that this newsgroup might be a helpful place to answer such questions. Thanks for any insights -- it would be especially useful if I can point to some kind of proof one way or another. -- Bruce Eckel

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mailing list choices?
by Conal Elliott 12 Sep '08

12 Sep '08

I want to set up some kind of mailing list for reactive (which I plan to release soon). The most obvious thing is to set up a mailman-based list on haskell.org, but I wonder -- do people really want to keep using mailman technology? Or something more modern like Yahoo or Google groups. I use my email reader also with Yahoo & Google groups, so my day-to-day experience is the same as with mailman-based lists. For less regular operations like searching archives, I prefer the experience on Yahoo & Google group. And I like some of the other modern features, like personal profiles and polls. I've been wondering if haskellers really like mailman or are just used to it. Maybe people make new mailman lists simply because others did it before them. Any thoughts/preferences? Thanks, - Conal

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Workshop on Generic Programming: Call for Participation (co-located w/ ICFP08)
by Matthew Fluet (ICFP Publicity Chair) 12 Sep '08

12 Sep '08

Dear all, the Workshop on Generic Programming is only a few days away: 20th September 2008 (http://www.regmaster.com/conf/icfp2008.html) ==> Invited talk: The Generic Paradigm ==> Lambert Meertens (Utrecht University) ==> We have reserved 20 minutes for *lightning talks*. If you plan to ==> attend and if you would like to give a short talk (about half-baked, ==> exciting, new stuff) please drop me a short note. Slots will be ==> reserved on a first-come-first-serve basis. Looking forward to seeing you in Victoria, Ralf Hinze ============================================================================ CALL FOR PARTICIPATION Workshop on Generic Programming 2008 Victoria, Canada, 20th September 2008 http://www.comlab.ox.ac.uk/ralf.hinze/wgp2008/cfp.{html,pdf,ps,txt} The Workshop on Generic Programming is sponsored by ACM SIGPLAN and forms part of ICFP 2008. Previous Workshops on Generic Programming have been held in Marstrand (affiliated with MPC), Ponte de Lima (affiliated with MPC), Nottingham (informal workshop), Dagstuhl (IFIP WG2.1 Working Conference), Oxford (informal workshop), Utrecht (informal workshop), and Portland (affiliated with ICFP). ============================================================================ Preliminary program ------------------- 9:00 - 10:00, Session Chair: Ralf Hinze (University of Oxford) Welcome Invited talk: The Generic Paradigm Lambert Meertens (Utrecht University) 10:30 - 12:00, Session Chair: Jeremy Gibbons (University of Oxford) A Functional Model-View-Controller Software Architecture for Command-oriented Programs Alley Stoughton (Kansas State University) A Lightweight Approach to Datatype-Generic Rewriting Thomas van Noort (Radboud University Nijmegen), Alexey Rodriguez, Stefan Holdermans (Utrecht University), Johan Jeuring (Utrecht University and Open University of the Netherlands), Bastiaan Heeren (Open University of the Netherlands) Lightning talks 13:30 - 15:00, Session Chair: Ralf Hinze (University of Oxford) Report from the program chair Ralf Hinze (University of Oxford) Scala for Generic Programmers Bruno C. d. S. Oliveira, Jeremy Gibbons (University of Oxford) A Comparison of C++ Concepts and Haskell Type Classes Jean-Philippe Bernardy, Patrik Jansson, Marcin Zalewski, Sibylle Schupp, Andreas Priesnitz (Chalmers University of Technology and University of Gothenburg) Lightning talks 15:30 - 17:30, Session Chair: Patrik Jansson (Chalmers University of Technology and University of Gothenburg) Polytypic Programming in Coq Wendy Verbruggen, Edsko de Vries, Arthur Hughes (Trinity College Dublin) Bialgebra Views: A Way for Polytypic Programming to Cohabit with Data Abstraction Pablo Nogueira, Juan Jose Moreno-Navarro (Universidad Politecnica de Madrid) Discussion ============================================================================

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darcs hacking sprint, venues confirmed! (25-26 October)
by Eric Y. Kow 12 Sep '08

12 Sep '08

Dear darcs users and Haskell hackers, Earlier I wrote an announcement for the darcs hacking sprint on 25-26 October. Tonight, I am delighted to announce that we have two venues confirmed for the sprint and one serious offer. Venues ------ We plan to host the sprint across three sites: * CONFIRMED: Brighton, UK (University of Brighton) * CONFIRMED: Portland, USA (Galois Connections) * likely: Paris, France (Université Paris Diderot) So if you were waiting to book tickets, this is the time! For more details, please see http://wiki.darcs.net/index.html/Sprints Agenda ------ During this first sprint, we shall be focusing our attention on the day to day performance issues that darcs users commonly face. This is what we are reaching for: 1. Fast network operations. We want to make it very pleasant for users to darcs get a repository and pull some patches to it over http and ssh. Git does this very well, and we plan to learn from them. 2. Cutting memory consumption. We want to profile the heck out of operations like darcs record, darcs convert and darcs whatsnew. What's eating up all the memory? And how can we can cut it down to size? 3. Responsiveness. Sometimes basic darcs commands can take long enough for programmers to lose their train of thought. We want to track down these lost seconds and kill that dreaded context switch. Of course, if you are interested in other areas, then you can work on those instead. Note that if you are new to the darcs code or to Haskell, there will also be a lot interesting jobs for you to get started with. Everyone will have something to hack on, so come join us! Thanks very much to the University of Brighton, Galois connections and University of Paris VII for their generous offers. Hope to see you there, everyone! :-) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

2 1

Connecting to wireless network
by Marco Túlio Gontijo e Silva 12 Sep '08

12 Sep '08

Hello, I've made a small program[0] to connect to a wireless network. Comments are welcome. Greetings. 0: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/n-m -- Marco Túlio Gontijo e Silva Página: http://marcotmarcot.iaaeee.org/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

1 0

typeclass question
by Tim Docker 12 Sep '08

12 Sep '08

I have a typeclass related question that I have been puzzling over. In a library I am working on, I have a series of functions for converting values to Renderables: | labelToRenderable :: Label -> Renderable | legendToRenderable :: Legend -> Renderable | axisToRenderable :: Axis v -> Renderable | layoutToRenderable :: Layout x y -> Renderable These names are overloaded for convenience via a typeclass: | class ToRenderable a where | toRenderable :: a -> Renderable | | instance ToRenderable Label where | toRenderable = labelToRenderable | ... But some recent changes mean that Renderable needs to become a type constructor, and the functions now product different types: | labelToRenderable :: Label -> Renderable () | legendToRenderable :: Legend -> Renderable String | axisToRenderable :: Axis v -> Renderable () | layoutToRenderable :: Layout x y a -> Renderable a Is there a nice way to overload a "toRenderable" function for these? Something like this is possible: | class ToRenderable a b where | toRenderable :: a -> Renderable b But the above is, I think, too general for my needs. I don't want to be able to generate Renderables of different type b for a single input type a. Also, MPTC take me out of the world of haskell 98, which I was trying to avoid. Am I missing something simple? Any pointers would be much appreciated. Tim

5 9

Haskell as a scripting language.
by David F. Place 11 Sep '08

11 Sep '08

Hi, All. I needed to make a batch of edits to some input files after a big change in my program. Normally, one would choose some scripting language, but I can't bear to program in them. The nasty thing about using Haskell is that giving regexes as string constants sometime requires two levels of quoting. For instance. (mkRegex "\\\\\\\\") matches \\. To get around that, I put the regexes in the head of a literate program and let the program gobble itself up. Works great! I think I'll always turn to Haskell for my scripting needs now. I put the file in the Haskell Pastebin, if you would like to see it. http://hpaste.org/10249 Cheers, David

5 5

Field names
by Daryoush Mehrtash 11 Sep '08

11 Sep '08

In type section of the Gentle Introduction to Haskell http://www.cs.sfu.ca/CC/SW/Haskell/hugs/tutorial-1.4-html/moretypes.htmlthe… is this example: > data Point = Pt {pointx, pointy :: Float} > abs (Pt {pointx = x, pointy = y}) = sqrt (x*x + y+y) Why is it pointx=x and not x=pointx? -- Daryoush Weblog: http://perlustration.blogspot.com/

7 10

Re: [Haskell-cafe] Re: Field names
by Richard A. O'Keefe 11 Sep '08

11 Sep '08

On 11 Sep 2008, at 3:54 am, Brandon S. Allbery KF8NH wrote: > I think that only counts as the origin of the idea; isn't :-prefixed > infix constructors a ghc-ism? Haskell 98 report, page 10: "An operator symbol starting with a colon is a constructor". (I seem to have four copies of the report on my Mac...)

1 0

STM and FFI
by Mauricio 11 Sep '08

11 Sep '08

Hi, Is it possible to use foreign function interface with STMs? If so, where can I find examples? Thanks, Maurício

7 9

Re: [Haskell-cafe] packages and QuickCheck
by Sean Leather 11 Sep '08

11 Sep '08

> How do folks like to package up QuickCheck tests for their libraries? In > the main library? As a separate repo & package? Same repo & separate > package? Keeping tests with the tested code allows testing of non-exported > functionality, but can add quite a lot of clutter. > I have QuickCheck properties plus HUnit tests, but I think the question is the same. For me, it's in the same repository and shipped with the package source. I think that if you ship source (even via Hackage), you should also ship tests. So, if somebody wants to modify the source, they can run the tests. And making it convenient to test is very important, so I have "cabal test" (or "runhaskell Setup.hs test" without cabal-install) configured to run the tests. I don't think tests should (in general) be part of the user-visible API, so I have them external to the module hierarchy. Testing non-exported functionality without exporting the test interface seems difficult in general. Is there a way to hide part of a module interface with Cabal? Then, you could have a 'test' function exported from each module for testing but hidden for release. My current leaning is to split a package "foo" into packages "foo" and > "foo-test" > What benefit does this provide? Thanks, Sean

9 18

Haskell stacktrace
by Pieter Laeremans 11 Sep '08

11 Sep '08

Hello, I've written a cgi script in haskell, it crashes sometimes with the error message Prelude . tail : empty list In Java we would use this approach to log the erro try { } catch (Exception e) { } -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

11 12

User defined annotations for Haskell (Was: [Haskell-cafe] packages and QuickCheck)
by Max Bolingbroke 10 Sep '08

10 Sep '08

2008/9/10 Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de>: > >>> Has there ever been a discussion of typed, user-definable, >>> user-processable source code annotations for Haskell? >> >> afair it was on haskell-prime list > > http://hackage.haskell.org/trac/haskell-prime/ticket/88 > > if you can call that a discussion :-) I had to implement a kind of annotation system for GHC as part of my Summer of Code project for compiler plugins. You can see a discussion at http://hackage.haskell.org/trac/ghc/wiki/Plugins/Annotations, though I was not aware of that Haskell' ticket. The system as I have implemented is quite basic: the absolute minimum I needed for plugins. The syntax is essentially: {-# ANN foo "I am an annotation!" #-} Where "I am an annotation" may be replaced with any Typeable value, and is subject to staging requirements analgous to Template Haskell (e.g. you may not refer to things in the module being compiled other than by Template Haskell Name). As well as values you can annotate types and modules, and annotations are accessible via the GHC API. See that wiki page for full details. This system has not yet been merged into GHC (or even code reviewed by GHC HQ), so if/when it becomes available to end users the design may have changed drastically. Cheres, Max

1 0

trying to understand monad transformers....
by Daryoush Mehrtash 10 Sep '08

10 Sep '08

5 5

Functional references
by Tim Newsham 10 Sep '08

10 Sep '08

I'm playing with functional references and looking for some feedback on a small FRec library I put together: http://www.thenewsh.com/~newsham/FRef.hs Novel (I think) is that the library is applied to some data accesses that are not normally covered by functional references -- ie. extracting words or replacing words in a string or values in an association list. I'm hoping it will provides a useful framework for editing complex values such as data embedded in Base64 cookies in an HTTP header. Tim Newsham http://www.thenewsh.com/~newsham/

8 11

Unicode and Haskell
by Mattias Bengtsson 10 Sep '08

10 Sep '08

Today i wrote some sample code after a Logic lecture at my university. The idea is to represent the AST of propositional logic as an ADT with some convenience functions (like read-/show instances) and then later perhaps try to make some automatic transformations on the AST. After construction of the Show instances i found the output a bit boring and thought that some Unicode math symbols would spice things up. What happens can be seen in the attached picture (only 3k, that's ok right?). My terminal supports UTF-8 (when i do cat Logic.hs i can see the unicode symbols properly). What might be the problem? regards Mattias

2 3

haskell core definition
by Vlad Skvortsov 10 Sep '08

10 Sep '08

Hi, I'm trying to profile my haskell code (see thread "profiling in haskell") and to get more insight I made GHC produce the haskell Core code. However, I'm not quite sure how to interpret it, is there a definition (quick search didn't reveal one)? Also, how do I demangle the names? It seems that, for example, 'base:GHC.Base.ZC' is a (:) function on strings, but where how am I supposed to figure that out? Thanks! -- Vlad Skvortsov, vss(a)73rus.com, http://vss.73rus.com

5 4

Hackage -> MacPorts?
by John MacFarlane 09 Sep '08

09 Sep '08

It would be great if there were an automated or semi-automated way of generating a MacPorts Portfile from a HackageDB package, along the lines of dons' cabal2arch. Has anyone been working on such a thing? And, are any haskell-cafe readers MacPorts committers? John

8 14

packages and QuickCheck
by Conal Elliott 09 Sep '08

09 Sep '08

How do folks like to package up QuickCheck tests for their libraries? In the main library? As a separate repo & package? Same repo & separate package? Keeping tests with the tested code allows testing of non-exported functionality, but can add quite a lot of clutter. My current leaning is to split a package "foo" into packages "foo" and "foo-test", but first I'd like to hear about others' experiences, insights, and preferences. Thanks, - Conal

2 2

haskell job offer.
by Lionel Barret De Nazaris 09 Sep '08

09 Sep '08

We (Gamr7, see at the bottom) are looking from a senior dev/Technical director. We don't really care about the title but we want someone good (who doesn't ?). We need someone able to model *and* code well (No architect who never codes). The ability to communicate well with a team is also a big plus. You don't need to speak French (we are in France) but a reasonably good English is mandatory. If You : • like coding 4+ hours straight. • like to solve a coding problem elegantly (and are bothered if can't) • like and read *real* CS books (SICP, EGB, TAOCP, etc...) • code in Haskell, python and c++. • are a gamer (this one is optional) • are interested in computer graphics (optional too) You fit the bill. We would like to talk to you. Contact us (contact at gamr7 dot com). What we offer: • interesting problems and creative freedom • quality of life (no overtime, sunny countryside, French food and low rent) • a pay in Euros • comfortable workplace, etc... • coding in Haskell, python. The boss *really * codes . He likes Haskell, Python, Reddit, and wants the team to be there for the long run (i.e happy). He also modestly wrote and posted this jobs offer. About : Gamr7 is a startup focused on procedural city generation for the game and simulation market. We are located in France, near Lyon. -- Best Regards, lionel Barret de Nazaris, Gamr7 Founder & CTO ========================= Gamr7 : Cities for Games http://www.gamr7.com

2 3

Re: [Haskell-cafe] Can you do everything without shared-memory concurrency?
by Richard A. O'Keefe 09 Sep '08

09 Sep '08

On 9 Sep 2008, at 8:15 am, Kyle Consalus wrote: > Anyway, for the time being I believe there are operations that can be > done with shared memory > that can't be done with message passing if we make "good performance" > a requirement. One of our people here has been working on Distributed Shared Memory for some years. It's a programming model where you write AS IF you had shared memory, but you really don't. He actually uses plain C with a library, but I've got a student project to wrap some syntax around that. typedef struct Foo { ... variables ... } Foo; typedef struct Bar { ... variables ... } Bar; shared Foo foo; shared Bar bar; shared (const *f = &foo) { ... here we have read access to f->variables ... } shared (*b = &bar) { ... here we have read-write access to b->variables ... } Underneath, it's message passing. When you get a view on a shared region, a copy is fetched from the cheapest source that has a current copy. When you release a write view, you become the holder of the only current copy. Compressed differences are sent around the local net. Zhiyi Huang's library is called VODCA. There's a plug-compatible version developed by his colleagues in China called Maotai, so the _same_ code can be run on a multicore system or on a network of workstations. The trick is to choose the chunk size of your problem so that computation and communication costs are in the right balance. This certainly seems to be adequate for numerical software, raytracing, game playing, ... One issue is that real shared memory comes at a price that most people don't know they are paying. We wouldn't need MOESI protocols or the related bus traffic if there were known to be no sharing, and one of the things that gets in the way of massive multicore is keeping caches coherent. No shared memory => no coherence problem => no extra bus traffic => faster. -- If stupidity were a crime, who'd 'scape hanging?

1 0

Re: [Haskell-cafe] Can you do everything without shared-memory concurrency?
by Richard A. O'Keefe 09 Sep '08

09 Sep '08

I think the demonstration is in Hoare's book on co-operating sequential processes, but if you have pure processes and message passing, you can simulate conventional variables. Here's an Erlang version: variable_loop(State) -> receive {ask,Sender} -> Sender!{self(),State}, variable_loop(State) ; {tell,New_State} -> variable_loop(New_State) end. new_variable(Initial_State) -> spawn(fun () -> variable_loop(Initial_State) end). fetch(Variable) -> Variable!{ask,self()}, receive {Variable,State} -> State end. store(Variable, New_State) -> Variable!{tell,New_State}. new_array(Size, Initial_State) -> list_to_tuple([new_variable(Initial_State) || Dummy <- lists:seq(1, Size)]). fetch(Array, Index) -> fetch(element(Index, Array)). store(Array, Index, New_State) -> store(element(Index, Array), New_State). The simulation of (shared) mutable variables and arrays in pure CSP is very similar. This pretty much _has_ to be possible in any language that has concurrent processes that can communicate in some fashion. There are also quite elementary ways to simulate locking. So we can solve any concurrent problem that, say, C++ with Intel's TBB library, or Java with java.util.concurrent, or any other such language can solve. We can even get ourselves just as far into extremely serious trouble as we can using those languages, it's just that we don't _have_ to. -- If stupidity were a crime, who'd 'scape hanging?

1 0

Re: building unix package on windows
by David Hotham 09 Sep '08

09 Sep '08

But the unix package isn't actually required to build HAppS on Windows. If you remove the dependency from the .cabal file (and also remove the -DUNIX build option) then you'll be fine.

1 0

monadic map on a Data.IntMap
by minh thu 09 Sep '08

09 Sep '08

Hi, Is there something like a fmapM_ ? In particular, I'd like to mapM_ a Data.IntMap instead of a List Thank you, Thu

4 4

Re: [Haskell-cafe] HXT from schema to data model
by Uwe Schmidt 08 Sep '08

08 Sep '08

Hello Pierre-Edouard, > Is there a way to generate a data model and a set of picklers from an XML (or > RelaxNG) Schema using the HXT tool box? the generation of a DTD out of the picklers is in an experimental stage. The generation of a RelaxNG Schema is technically possible, but it's an open project. It's on our TODO list for HXT. Validation with RelaxNG within HXT is complete. What is missinig is a complete datatype library for XML Schema Datatypes. The still missing part of the W3C XML Schema datatype library are the datatypes for date and time, all others are implemented. Cheers, Uwe -- Uwe Schmidt FH Wedel http://www.fh-wedel.de/~si/

1 0

Re: A problem with nested regions and higher-order?functions
by Chung-chieh Shan 08 Sep '08

08 Sep '08

Mario Bla??evi?? <mblazevic(a)stilo.com> wrote in article <23051.1216949255%241220747147(a)news.gmane.org> in gmane.comp.lang.haskell.cafe: > I'm trying to apply the nested regions (as in Lightweight Monadic > Regions by Oleg Kiselyov and Chung-chieh Shan) design pattern, if > that's the proper term, in hope to gain a bit more type safety in > this little library I'm working on (Streaming Component Combinators, > available on Hackage). I guess the problem is that I'm getting too > much type safety now, because I can't get the thing to compile. ... Hi! Thanks for your interest. It sounds like a promising application of region checking. I actually side with the type checker on this problem: > > type SingleHandler x y = forall r1s rs. Ancestor r1s rs => > > Handle r1s x -> Region rs y > > type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d rd) => > > Handle r1d x -> Handle r2d y -> Region rd z Here a SingleHandler is defined as an operation, in an arbitrary region on an arbitrary handle, that is valid as long as the region of the operation descends from the region of the handle. For example, reading a string from an arbitrary open file is a "SingleHandler File String". However, copying a string from an arbitrary open file to another fixed file is not a SingleHandler, because such an operation is valid only in a region that descends from the destination file handle's region! That's what GHC complained about as it checks your mapD: > > mapD :: (SingleHandler x z -> SingleHandler y z) > > -> DoubleHandler x w z -> DoubleHandler y w z > > mapD f d = \y w-> f (\x-> d x w) y So we really need to change the type of mapD if we want it to be accepted by a sound type system. The simplest way is to not require that mapD return a DoubleHandler (a binary operation that works in an arbitrary region): mapD :: ((Handle r1 x -> Region r z) -> (Handle r1 x -> Region r z)) -> ((Handle r1 x -> Handle r2 w -> Region r z) -> (Handle r1 x -> Handle r2 w -> Region r z)) mapD f d = \y w-> f (\x-> d x w) y This type is a special case of the type automatically inferred for mapD by Haskell if you omit the type signature. Haskell doesn't have any trouble type-checking higher-order functions, such as this mapD, but higher-rank types can call for manual annotations, as illustrated below. Another way to get mapD to type-check is to pass it (as its first argument) not a SingleHandler transformer but a transformer of operations that may work only in descendents of a given region r2. We want to write type H r2 x y = forall r1 r. (Ancestor r1 r, Ancestor r2 r) => Handle r1 x -> Region r y mapD :: (forall r2. H r2 x z -> H r2 y z) -> DoubleHandler x w z -> DoubleHandler y w z mapD f d = \y w-> f (\x-> d x w) y ("forall r2" takes scope over both "H r2 x z" and "H r2 y z" above) but GHC doesn't see that we intend to unify the "r2d" in the expansion of "DoubleHandler y w z" with the "r2" in this type signature of "mapD". So, we need to put an explicit type annotation on the use of "f" in the body of "mapD". To do so, we add {-# LANGUAGE ScopedTypeVariables #-} and expand the type synonym "DoubleHandler y w z" in the type signature of "mapD": mapD :: forall x y z w r1d r2d rd. (Ancestor r1d rd, Ancestor r2d rd) => (forall r2. H r2 x z -> H r2 y z) -> DoubleHandler x w z -> Handle r1d y -> Handle r2d w -> Region rd z mapD f d = \y w-> (f :: H r2d x z -> H r2d y z) (\x-> d x w) y Not so pretty anymore, but it does pass the type checker (GHC 6.8.2). -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig ----- Ken Shan

1 0

experimental static blog engine in Haskell (file based, markdown syntax)
by jinjing 08 Sep '08

08 Sep '08

Hia, It's called Panda. It's pretty young, no theme, no tags, no comments, around 360 lines of code and uses Kibro to bootstrap. hosted on GitHub: http://github.com/nfjinjing/panda/ a quick demo at: http://jinjing.blog.easymic.com/ cheers, - jinjing

2 2

building unix package on windows
by jean-christophe mincke 08 Sep '08

08 Sep '08

Hello, I have installed GHC and cygwin on windows XP and I am trying to build the unix package (required to install HApps) ** When I use .../ghc/gcc as c compiler I receive the following error during the configure* D:\temp\haskell\unix-2.3.0.0>runhaskell Setup configure --ghc --prefix=D:\apps\h-lib Configuring unix-2.3.0.0... checking for gcc... gcc checking for C compiler default output file name... *configure: error: C compiler cannot create executables* See `config.log' for more details. and config.log says that : * ld: crt2.o: No such file: No such file or directory* ** When I use the gcc in cygwin : /usr/bin/gcc, the configure phase is ok but the build stop on the following error:* D:\temp\haskell\unix-2.3.0.0>runhaskell Setup build Preprocessing library unix-2.3.0.0... *In file included from System\Posix\DynamicLinker\Module.hsc:57:0: include/HsUnix.h:32:23: sys/times.h: No such file or directory* include/HsUnix.h:38:26: sys/resource.h: No such file or directory include/HsUnix.h:41:22: sys/wait.h: No such file or directory include/HsUnix.h:62:21: termios.h: No such file or directory include/HsUnix.h:65:25: sys/utsname.h: No such file or directory include/HsUnix.h:68:17: pwd.h: No such file or directory include/HsUnix.h:71:17: grp.h: No such file or directory include/HsUnix.h:81:17: pty.h: No such file or directory include/HsUnix.h:84:18: utmp.h: No such file or directory include/HsUnix.h:87:19: dlfcn.h: No such file or directory In file included from System\Posix\DynamicLinker\Module.hsc:57:0: include/HsUnix.h: In function `__hsunix_wifexited': include/HsUnix.h:103:0: warning: implicit declaration of function `WIFEXITED' include/HsUnix.h: In function `__hsunix_wexitstatus': include/HsUnix.h:104:0: warning: implicit declaration of function `WEXITSTATUS' include/HsUnix.h: In function `__hsunix_wifsignaled': include/HsUnix.h:105:0: warning: implicit declaration of function `WIFSIGNALED' include/HsUnix.h: In function `__hsunix_wtermsig': include/HsUnix.h:106:0: warning: implicit declaration of function `WTERMSIG' include/HsUnix.h: In function `__hsunix_wifstopped': include/HsUnix.h:107:0: warning: implicit declaration of function `WIFSTOPPED' include/HsUnix.h: In function `__hsunix_wstopsig': include/HsUnix.h:108:0: warning: implicit declaration of function `WSTOPSIG' include/HsUnix.h: In function `__hsunix_rtldDefault': include/HsUnix.h:115:0: error: `RTLD_DEFAULT' undeclared (first use in this function) include/HsUnix.h:115:0: error: (Each undeclared identifier is reported only once include/HsUnix.h:115:0: error: for each function it appears in.) include/HsUnix.h: In function `__hsunix_lstat': include/HsUnix.h:134:0: warning: implicit declaration of function `lstat' include/HsUnix.h: In function `__hsunix_mknod': include/HsUnix.h:140:0: warning: implicit declaration of function `mknod' compiling dist\build\System\Posix\DynamicLinker\Module_hsc_make.c failed *command was: d:\apps\ghc\bin\ghc.exe -c -package base-3.0.2.0 -package directory-1.0.0.1 -Iinclude dist\build\System\Pos ix\DynamicLinker\Module_hsc_make.c -o dist\build\System\Posix\DynamicLinker\Module_hsc_make.o* Could someone help me. Thank you J-C

2 1

Re: [Haskell] Top Level <-
by Adrian Hey 08 Sep '08

08 Sep '08

I have a feeling this is going to be a very long thread so I'm trying to go to Haskell cafe again (without mucking it up again). Derek Elkins wrote: > Haskell should be moving -toward- a capability-like model, not away from > it. Could you show how to implement Data.Random or Data.Unique using such a model, or any (preferably all) of the use cases identified can be implemented? Like what about implementing the socket API starting with nothing but primitives to peek/poke ethernet mac and dma controller registers? Why should Haskell should be moving -toward- a capability-like model and why does top level <- declarations take us away from it? Regards -- Adrian Hey

21 178

language proposal: ad-hoc overloading
by Ryan Ingram 08 Sep '08

08 Sep '08

The point of having a strongly typed language is so the compiler can do more work for you. But right now I do a lot of typing (pun not intended) to appease the compiler. Let me give you an example: module Prob where import qualified Data.Map as M ... newtype Prob p a = Prob { runProb :: [(a,p)] } combine :: (Num p, Ord a) => Prob p a -> Prob p a combine m = Prob $ M.assocs $ foldl' (flip $ uncurry $ M.insertWith (+)) M.empty $ runProb m Do you see it? All those "M." just seem dirty to me, especially because the compiler should be able to deduce them from the types of the arguments. My proposal is to allow "ad-hoc" overloading of names; if a name is ambiguous in a scope, attempt to type-check the expression against each name. It is only an error if type-checking against all names fails. If type-checking succeeds for more than one then the expression is ambiguous and this is also an error. Pros: shorter code, less busywork to please the compiler Cons: potentially exponential compile time? Any thoughts? -- ryan

16 25

HXT from schema to data model
by Pierre-Edouard Portier 08 Sep '08

08 Sep '08

Hi! Is there a way to generate a data model and a set of picklers from an XML (or RelaxNG) Schema using the HXT tool box? Thank You, Pierre-Edouard

2 1

Online Real World Haskell, problem with Sqlite3 chapters
by david48 07 Sep '08

07 Sep '08

In the online version of Real world Haskell, there's a problem with examples in ghci when the module Database.HDBC.Sqlite3 is imported. It goes on like this for all of chapter 21 and 22. Example : ( note: this is not me trying to reproduce the examples, it's an actual copy & paste from the site url http://book.realworldhaskell.org/read/using-databases.html ) ghci> :module Database.HDBC Database.HDBC.Sqlite3 Could not find module `Database.HDBC.Sqlite3': Use -v to see a list of the files searched for. ghci> conn <- connectSqlite3 "test1.db" <interactive>:1:8: Not in scope: `connectSqlite3' ghci> :type conn <interactive>:1:0: Not in scope: `conn' ghci> disconnect conn <interactive>:1:0: Not in scope: `disconnect' <interactive>:1:11: Not in scope: `conn'

11 14

A problem with nested regions and higher-order functions
by Mario Blažević 07 Sep '08

07 Sep '08

I forgot the subject, sorry for reposting... I'm trying to apply the nested regions (as in Lightweight Monadic Regions by Oleg Kiselyov and Chung-chieh Shan) design pattern, if that's the proper term, in hope to gain a bit more type safety in this little library I'm working on (Streaming Component Combinators, available on Hackage). I guess the problem is that I'm getting too much type safety now, because I can't get the thing to compile. Most of the existing code works, the only exceptions seem to be various higher-order functions. I've reduced the problem to several lines of Literate Haskell code below, can anybody find a solution or confirm there isn't one? > {-# LANGUAGE EmptyDataDecls, Rank2Types #-} > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, IncoherentInstances #-} > module Main where > main = undefined I'll call the main type, originally a monad transformer, simply Region. I'm leaving out the Monad and MonadTransformer instances, because they don't contribute to the problem. The parameter r is the phantom region type. > newtype Region r a = Region a The Ancestor class is used to denote relationship between two regions where one is nested in another. > data Child r > class Ancestor r r' > instance Ancestor r (Child r) > instance Ancestor r1 r2 => Ancestor r1 (Child r2) Handle is a simple wrapper around a value. It carries information about the region that originates the value. > data Handle r x = Handle x A typical calculation in the Region monad will take a bunch of Handles inherited from an Ancestor region and do something with them. The Ancestor constraint is there to ensure that the handles are not fake but genuinely inherited. > type SingleHandler x y = forall r1s rs. Ancestor r1s rs => > Handle r1s x -> Region rs y > type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d rd) => > Handle r1d x -> Handle r2d y -> Region rd z And now I get to the problem. The following higher-order function doesn't type-check: > mapD :: (SingleHandler x z -> SingleHandler y z) > -> DoubleHandler x w z -> DoubleHandler y w z > mapD f d = \y w-> f (\x-> d x w) y I get the same error from GHC 6.8.2 and 6.8.2: Test.lhs:36:28: Could not deduce (Ancestor r2d rs) from the context (Ancestor r1s rs) arising from a use of `d' at Test.lhs:36:28-32 Possible fix: add (Ancestor r2d rs) to the context of the polymorphic type `forall r1s rs. (Ancestor r1s rs) => Handle r1s x -> Region rs z' In the expression: d x w In the first argument of `f', namely `(\ x -> d x w)' In the expression: f (\ x -> d x w) y The same code compiles just fine if all the Ancestor constraints are removed. I don't see any place to add the extra (Ancestor r2d rs) constraint, as GHC recommends. I think it ought to be able to figure things out based on the exisisting constraints, but I may be wrong: perhaps higher-order functions pose a fundamental problem for type-level programming. Can anybody shed any light on this?

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(no subject)
by Mario Blažević 07 Sep '08

07 Sep '08

Hello. I'm trying to apply the nested regions (as in Lightweight Monadic Regions by Oleg Kiselyov and Chung-chieh Shan) design pattern, if that's the proper term. I was hoping to gain a bit more type safety in this little library I'm working on -- Streaming Component Combinators, available on Hackage. I guess the problem is that I'm getting too much type safety now, because I can't get the thing to compile. Most of the existing code works, the only exceptions seem to be various higher-order functions. I've reduced the problem to several lines of Literate Haskell code below, can anybody think of a solution or a reason there can't be one? > {-# LANGUAGE MultiParamTypeClasses, EmptyDataDecls, Rank2Types #-} > {-# LANGUAGE FunctionalDependencies, FlexibleInstances, IncoherentInstances #-} > module Main where > main = undefined I'll call the main type, originally a monad transformer, simply Region. I'm leaving out the Monad and MonadTransformer instances, because they don't contribute to the problem. The parameter r is the phantom region type. > newtype Region r a = Region a The Ancestor class is used to denote relationship between two regions where one is nested in another. > data Child r > class Ancestor r r' > instance Ancestor r (Child r) > instance Ancestor r1 r2 => Ancestor r1 (Child r2) Handle is a simple wrapper around a value. It carries information about the region that originates the value. > data Handle r x = Handle x A typical calculation in the Region monad will take a bunch of Handles inherited from an Ancestor region and do something with them. The Ancestor constraint is there to ensure that the handles are not fake but genuinely inherited. > type SingleHandler x y = forall r1s rs. Ancestor r1s rs => > Handle r1s x -> Region rs y > type DoubleHandler x y z = forall r1d r2d rd. (Ancestor r1d rd, Ancestor r2d rd) => > Handle r1d x -> Handle r2d y -> Region rd z And now I'm getting to the problem. The following higher-order function doesn't type-check: > mapD :: (SingleHandler x z -> SingleHandler y z) > -> DoubleHandler x w z -> DoubleHandler y w z > mapD f d = \y w-> f (\x-> d x w) y I get the same error from GHC 6.8.2 and 6.8.2: Test.lhs:36:28: Could not deduce (Ancestor r2d rs) from the context (Ancestor r1s rs) arising from a use of `d' at Test.lhs:36:28-32 Possible fix: add (Ancestor r2d rs) to the context of the polymorphic type `forall r1s rs. (Ancestor r1s rs) => Handle r1s x -> Region rs z' In the expression: d x w In the first argument of `f', namely `(\ x -> d x w)' In the expression: f (\ x -> d x w) y The same code compiles just fine if all the Ancestor constraints are removed. I don't see any place to add the extra (Ancestor r2d rs) constraint, as GHC recommends. I think it ought to be able to figure things out based on the exisisting constraints, but I may be wrong: perhaps higher-order functions pose an insurmountable problem for type-level programming in Haskell. Can anybody shed any light on this?

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Re: [xmonad] ANNOUNCE: xmonad 0.8 released!
by Don Stewart 07 Sep '08

07 Sep '08

nomeata: > Hi, > > Am Freitag, den 05.09.2008, 18:38 -0700 schrieb Don Stewart: > > xmonad packages are available in the package systems of at least: > > > > Debian, Gentoo, Arch, Ubuntu, OpenBSD, > > NetBSD, FreeBSD, Gobo, NixOS, Source Mage, Slackware > > > > and 0.8 packages will appear in coming days (some are already available). > > Debian xmonad and xmonad-contrib package uploaded (although > xmonad-contrib is in the two-day delayed queue, to give autobuilders a > chance to build xmonad first). Great work, Joachim! Thanks for being so responsive on this. Go Debian! -- Don

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