- Haskell-Cafe - Haskell.org

tail recursion ?
by H. 12 Jun '07

12 Jun '07

Hello @ all, Sometimes one has an imperative algorithm, and wants to write a program in Haskell which do has the same effect... So the question is - how a construct as the following can efficiently be written? -- Pseudo code: n[1..10000] = false for (1..10000) |i| for (i,2*i..10000) |j| n[j] = not n[j] -- Certainly it is in this special case equivalent to (True where the index is): map (^2) [1..100] But I mean the destructive updates in the imperative code in Haskell without filling the (many times more than in imperative languages) memory with recursively called functions... The idea, I thought, is tail recursion, so perhaps I just have a big bug in my code, caused by the fact, that it needs even for 5000 approximately 100MB memory: -- import Data.Array main :: IO () main = putStr $! unlines $! map show $! filter snd $! zip [1..] $! elems $! calc $! la 5000 where la x = array (1,x) [(y,False)|y<-[1..x]] calc :: Array Int Bool -> Array Int Bool calc x = f 1 x where k :: Int k = snd $ bounds x f :: Int -> Array Int Bool -> Array Int Bool f !a !x | a < k = f (a+1) $! g a x | otherwise = g a x g !a !x = x//[(j,not (x!j))|j<-[a,a*2..k]] -- -- Thanks for you answers in advance H.

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killThread on a terminated thread.
by Bit Connor 12 Jun '07

12 Jun '07

Hello, Is it safe to use killThread to terminate a thread that has already terminated(it's IO action has run to completion)? Or are ThreadIds reused, potentially causing an unwanted thread to be terminated? I'm using ghc 6.6 Thanks, bit

2 1

Thought for today
by Andrew Coppin 12 Jun '07

12 Jun '07

"Data in Haskell is like Schrodinger's famous undead cat - it doesn't 'exist' until you 'obverse' it." I just thought I'd share this useful (?) metaphore for describing to people what lazy evaluation is all about. ;-)

2 1

Head and tail matching question
by Olivier Boudry 12 Jun '07

12 Jun '07

Hi all, I'm trying to write a untab function that would split a string on tabs and return a list. Code is here. import Data.List (break, unfoldr) import Data.Char (String) untab :: String -> [String] untab s = unfoldr untab' s untab' :: String -> Maybe (String, String) untab' s | s == "" = Nothing | otherwise = Just (h, ts) where (h, t:ts) = break (== '\t') s This code raises an exception when handling the last portion of the string. Break returns a ("something", "") and t:ts cannot match on "". I was wondering if there way a clean way of handling this last case without adding tons of code. Some kind of "idiomatic expression" ;-) Thanks, Olivier.

3 2

More on the random idea
by Andrew Coppin 12 Jun '07

12 Jun '07

Since the online lambdabot still doesn't seem to want to talk to me, I've been thinking about how I might rectify the situation... Apparently GHC has a flag that makes it execute a Haskell expression directly. For example, C:\> ghc -e "map (2*) [1,2,3]" [2,4,6] Now, if I could just figure out how to make a web server call GHC... I'm laughing! Oh, but there is the *minor* detail that I am literally allowing unauthenticated users to perform arbitrary code execution. For example, C:\> ghc -e "writeFile \"Test.txt\" \"Hi mum!\"" (Generates a file on my harddrive "Test.txt" containing the text "Hi mum!".) AFAIK, Lambdabot dissalows any expression that performs IO. In Haskell, this is beautifully easy: reject any expression having an IO type. And it seems that GHC accepts not just an *expression*, but a *command*. In particular, this works: C:\> ghc -e ":t writeFile \"Test.txt\" \"Hi mum!\"" writeFile "Test.txt" "Hi mum!" :: IO () However, when you consider that the result type could be "IO ()" or "IO String" or "IO [Either (Maybe Int, (String, Bool)) [Either (Int -> String) (Complex Integer)]]", and the expression itself may well contain the "::" sequence... you see we have a nontrivial parsing task here! (Get the parsing wrong and somebody might be able to do Evil Things to the box.) The other possibility is to somehow run GHC under a user context that doesn't *have* write access to anything on the filesystem. That way there is no margin for error. This leaves only the problem of how to make a web server call GHC. I can think of a number of possibilities. - Write my own HTTP server from scratch. (Not keen...) - Configure Apache to do it. (Is that physically possible?) - Use Apache and some bizzare Perl scripting to do the actual call. (Assuming *that* is possible.) - Use Apache and some Perl scripts to write the data to a text file, and write a small Haskell program to poll the filesystem waiting for request files to appear, run then though GHC, and put the result back into a file. Write another Perl script to slurp up the result and send it back to the caller. - Doesn't Java have a free HTTP server implementation? Maybe I could use that... - ...others? (I'm not sure why this should be, but it seems that Don has made several replies to my emails that didn't show up in my inbox, and only show up on the list archives. Oh well, anyway...) I lurk on the POV-Ray NNTP server, and we recently had quite a discussion about Haskell. I'd *love* to be able to say to people "hey, you don't even need to bother working out how to install GHC, just CLICK THIS LINK and you can play with Haskell instantly!" But at the moment I'm not entirely sure how to set this up. Ideas?

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Examples of OpenAL and ALUT?
by Dan Piponi 11 Jun '07

11 Jun '07

Does anyone have any sample Haskell code they'd like to share for doing things like creating a waveform from a list of samples or a mathematical function and playing them using these libraries (or indeed any easy to install on MacOS X Haskell library)? -- Dan

2 1

LaTeX
by Andrew Coppin 11 Jun '07

11 Jun '07

Does anybody know what the magical LaTeX command is to turn (say) "++" into two overprinted pluses? (As seems to be fashionable...)

5 5

OSCON 2007, who's going?
by Christopher Milton 11 Jun '07

11 Jun '07

Are a lot of Haskellers going to be at OSCON, or just Simon Peyton Jones and myself? http://conferences.oreillynet.com/os2007/ I've never been to Portland, Oregon, before. Chris Milton AIM: cmiltonperl

2 2

Which regular syntax does Text.Regex use?
by L.Guo 11 Jun '07

11 Jun '07

Hi All: I wrote this func : match = matchRegex . mkRegex And when using it, I found that I have not even know the syntax of Regex. Eager for your hint. Regards -------------- L.Guo 2007-06-11

2 1

Shim missing Distribution.Verbosity
by Johan Grönqvist 11 Jun '07

11 Jun '07

Hi, I am trying to install shim (http://shim.haskellco.de/trac/wiki) on windows XP (GHC 6.6.1). When I run "./Setup.lhs build", ghc tells me that the module Distribution.Verbosity can not be found. The module is imported by Shim.GhcCompat. ghcii also tells me that Distribution.Verbosity can not be found. When I look at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html, I do not see Distribution.Verbosity there either. I have no idea how to proceed. Thanks for any help, Johan

2 1

Switch optimization
by Tom Pledger 11 Jun '07

11 Jun '07

Stefan O'Rear wrote: | On Mon, Jun 11, 2007 at 09:43:17AM +1000, Thomas Conway wrote: : | > codeLen 127 = 0 | > codeLen 128 = 1 | > ... | > codeLen 255 = 8 | > | > Now, compiling with ghc 6.6.1 and -O3, I see that it generates a long | > chain of conditional branches. : That's deeply tied in with Num being a subclass of Eq, isn't it? Pattern matching for /non-numeric/ data constructors should be more direct, at any optimisation level, thanks to the taglessness of the STG machine. | I'd suggest learning how to hack on GHC, I get a chain of branches even | at the maximum optimization setting. Hackers are always appreciated! Such a GHC hack may have to be limited to some known, standard numeric types. User-defined numeric types may not provide anything that you can use to look up a jump table. How would an array go, as a user-level optimisation? Regards, Tom

1 0

Re: HList questions
by oleg＠pobox.com 11 Jun '07

11 Jun '07

Marc Weber wrote: > Do you know what a type indexed coproduct is ? > (TIC.hs from HList) > What is the purpose of this module? In a regular Haskell record, we can retrieve the value of one of its components given the label. A type-indexed Product (TIP, or TIR) is a similar collection of values -- indexed not by their label but by their type. Given a TIP of an Int and a Char, we can retrieve either value just by specifying what return type we want (Int or Char). A type-indexed co-product, TIC, is a type-indexed sum. A TIC of Int and Char has the type of the union of Int and Char (think of Either Int Char). We can likewise ask for an Int element -- and receive either (Just val) or Nothing. Unlike (Either Int Char), we do not have to know whether Int is the first alternative of Either or the second one. We just ask for an Int. TIP and TIC have been introduced in M. Shields and E. Meijer. Type-indexed rows. (ref 31 of the HList paper) We wanted to show that HList can implement both. An implementation of TIC different from that of App C of the HList paper is used in the OOHaskell paper. That new implementation is based on nested Either datatypes rather than on Dynamics. The application is placing objects of two different types in the same list. There are two approaches: find the intersection of the object types and coerce objects to that intersection -- or find the union and inject objects into that union. The OOHaskell paper shows both approaches, with meet and joins of object types computed automatically. > Why the Proxy type has been introduced? Just to emphasize that we use the type as the index; to clarify the role of that argument. One can get be without them, their purpose is mere stylistic.

2 1

Switch optimization
by Thomas Conway 11 Jun '07

11 Jun '07

Hi All, I'm writing some variable byte codec routines, which are used in inner^N loops, and which I want to make really efficient. There are several spots where the code uses lookup tables. The best example is the table, which given the first byte, returns the number of additional bytes. It is a precomputed version of the following function: > codeLength :: Word8 -> Int > codeLength w > | w .&. 0x80 == 0 = 0 > | otherwise = 1 + (codeLength $ w `shiftL` 1) from which we construct a 256 entry table: codeLen 0 = 0 codeLen 1 = 0 codeLen 2 = 0 ... codeLen 127 = 0 codeLen 128 = 1 ... codeLen 255 = 8 Now, compiling with ghc 6.6.1 and -O3, I see that it generates a long chain of conditional branches. Now, even taking laziness into account, this seems like terribly inefficient code. I wrote this thinking it would be more efficient than constructing a CAF array: codeLens = listArray (0,255) (map codeLength [0..255]) but I'm guessing the CAF version would probably work out much more efficient in the long run. However, I think ghc (and any other compiler), should detect the following properties: 1. all the clauses are mutually exclusive, so the sequencing is irrelevant to the semantics 2. Given an explicit type declaration Word8 -> ...., the 256 cases cover all the possible constructors of the type, so there are no missing clauses. I would have expected the generated code to have the form: codeLen: x <- pick up arg return closure (codeLen' x) codeLen': x' <- force x update precomputed-table[x'] Even if you leave out property 2 and include bounds checks, this seems like an important kind of function to optimize well. So what have I missed, or is it time to learn how to hack on ghc? T. -- 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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How to dynamically attach readers to streams?
by Marc Weber 11 Jun '07

11 Jun '07

Example: main = readFile "/dev/input/mouse" >>= mapM print . streamToEvents That's no easy. Now I want to listen to those events remotely using a network conncetion Data.Network does provide everything I need. Is this done by substituting the print function with something like sent event = do writers <- getWritersFromMVar mapM (sent event) writers and using another lightweight thread to add writers/connections to the MVar? Would you implement this in another way? Marc

1 0

Data polymophism
by Phlex 10 Jun '07

10 Jun '07

Hello all, It seems this message was lost somehow, so i'm trying to send it again ....sorry if it comes up twice on the list ! I have this class which gives a common interface to (UniqueIndex a k) and (MultiIndex a k) : class (Show a, Key_ k) => Index_ i a k | i -> k, k -> a where buildKey :: (a -> k) insertIndex :: Id -> a -> i -> Maybe i deleteIndex :: Id -> a -> i -> i updateIndex :: Id -> a -> a -> i -> Maybe i updateIndex id oldItem newItem index = insertIndex id newItem $ deleteIndex id oldItem index Now i need to have these indexes in a list, so i declared that type : data DbIndex = forall a k i. (Show a, Key_ k, Index_ i a k) => DbIndex i Up to this point everything is fine, I can create concrete Indexes and put these different index types in the same list, after wrapping them in a DbIndex. But i don't seem to find a way to get out of this DbIndex type to actually work on the enclosed index. for instance, this doesn't work: liftDbIndex (DbIndex index) fun = DbIndex (fun index) this doesn't work either : dbIndexBuildKey (DbIndex index) value = (buildKey index) value How can i access the enclosed concrete index, via its Index_ class ? Maybe am I totally on the wrong track ? Thanks in advance, Sacha

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Re: [Haskell-cafe] LaTeX
by Andrew Coppin 10 Jun '07

10 Jun '07

Brian Sniffen wrote: > lhs2TeX uses > \newcommand{\plus}{\mathbin{+\!\!\!+}} Ah, OK. That looks good... Thanks.

1 0

haskell version of fractal benchmark
by brad clawsie 09 Jun '07

09 Jun '07

i recently saw a (yet-another) benchark comparing various languages: http://www.timestretch.com/FractalBenchmark.html while no haskell example was listed, i thought i would try a naive implementation myself for comparison. it is available here: http://www.b7j0c.org/dev/haskell/misc/time.hs my timing of the compiled code was slightly under three seconds over a few tests, landing sort of where i would expect a naive haskell implementation to place. i am sure people here could greatly improve my attempt. it may be that my solution is not even correct.

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Fast number parsing with strict bytestrings [Was: Re: Seemingly subtle change causes large performance variation]
by dons＠cse.unsw.edu.au 09 Jun '07

09 Jun '07

mdanish: > Hello, > > I've been playing with the INTEST problem on SPOJ which demonstrates > the ability to write a program which processes large quantities of > input data. http://www.spoj.pl/problems/INTEST/ > But when I make a slight modification, the program chews up a ton more memory > and takes more time: > > import Control.Monad > import Data.Maybe > import qualified Data.ByteString.Char8 as B > > divisibleBy :: Int -> Int -> Bool > a `divisibleBy` n = a `rem` n == 0 > > main :: IO () > main = do > [n,k] <- (map int . B.split ' ') `fmap` B.getLine :: IO [Int] > > let > doLine :: Int -> Int -> IO Int > doLine r _ = B.getLine >>= return . testDiv r > -- 'return' moved here ^^ What follows is a solution to the original question, and then a dramatic rewrite, showing the fastest way (that I know of) to parse \n separated lists of numbers in Haskell. The results should outperform C fairly well. *** Solution 1: don't be so lazy in the fold. First, look at that lazy fold. A simple fix, try being explict about forcing your accumulator: doLine :: Int -> Int -> IO Int doLine r _ = B.getLine >>= \s -> return $! testDiv r s And some timing data: Original: $ time ./A < in 29359 ./A < in 1.52s user 0.06s system 93% cpu 1.679 total Too lazy: $ time ./B < in 29359 ./B < in 3.84s user 0.26s system 82% cpu 4.957 total Hand back some strictness hints: $ time ./D < in 29359 ./D < in 1.52s user 0.03s system 94% cpu 1.637 total *** Solution 2: use lazy bytestrings to avoid gunky IO Now, however, I'd give up on that explict getLine stuff, and use a lazy bytestring. Something like this: import Data.Maybe import Data.List import qualified Data.ByteString.Lazy.Char8 as L main :: IO () main = do (l:ls) <- L.lines `fmap` L.getContents -- done with IO now. let [n,k] = map int (L.split ' ' l) print . foldl' (test k) 0 . map int . take n $ ls test :: Int -> Int -> Int -> Int test k acc n | n `divisibleBy` k = acc+1 | otherwise = acc int :: L.ByteString -> Int int = fst . fromJust . L.readInt divisibleBy :: Int -> Int -> Bool a `divisibleBy` n = a `rem` n == 0 The general rule for bytestring loops is to avoid IO, and to use lazy bytestrings if you need 'lines'. Also, program in a high level, using combinators, rather than your own loops, so that fusion will kick in (we get some list fusion here). And running it: $ time ./C < in 29359 ./C < in 1.22s user 0.04s system 94% cpu 1.335 total Ok, faster, and cleaner. Avoid mixing IO into your code! *** Solution 3: 4x faster by processing strict cache chunks Now the fun part. The following code is the fastest way I know to process lists of numbers (in any language). Its' based on similar code I wrote for the language shootout. The key trick is to use lazy bytestrings *only* as a method for filling the cache with newline-aligned chunks of numbers. Once you've got that perfectly-sized chunk, walk its lines, and process them. This is all done in Haskell, and relies on an understanding of the low level details of bytestring optimisations. The general framework could be reused for any code that needs to process a list of numbers in a file, where you care about speed. It performs as follows: $ time ./F < in 29359 ./F < in 0.24s user 0.01s system 76% cpu 0.327 total Pretty fast.. Previous experience[1] indicates it is pretty hard to write a C line parsing program[2] that that run this fast. And the code, with comments: 1. http://shootout.alioth.debian.org/gp4/benchmark.php?test=sumcol&lang=ghc&id… 2. http://shootout.alioth.debian.org/gp4/benchmark.php?test=sumcol&lang=gcc&id… {-# OPTIONS -fbang-patterns #-} import Data.Char import Data.Maybe import Data.ByteString.Base import qualified Data.ByteString.Char8 as S import qualified Data.ByteString.Lazy.Char8 as L main = do ss <- L.getContents -- done with IO now. let (l,ls) = L.break (=='\n') ss -- don't need count, we're allocating lazily k = fst . fromJust . L.readInt . last . L.split ' ' $ l file = L.toChunks (L.tail ls) -- a lazy list of strict cache chunks print $ process k 0 file divisibleBy :: Int -> Int -> Bool a `divisibleBy` n = a `rem` n == 0 -- --------------------------------------------------------------------- -- -- Optimised parsing of strict bytestrings representing \n separated numbers -- -- -- we have the file as a list of cache chunks -- align them on \n boundaries, and process each chunk separately -- when the next chunk is demanded, it will be read in. -- process :: Int -> Int -> [S.ByteString] -> Int process k i [] = i process k !i (s:t:ts) | S.last s /= '\n' = process k (add k i s') ts' where (s',r) = S.breakEnd (=='\n') s ts' = (S.append r t) : ts -- join chunks on line boundaries process k i (s: ss) = process k (add k i s) ss -- -- process a single cache-sized chunk of numbers, \n aligned -- add :: Int -> Int -> S.ByteString -> Int add k i s | S.null s = i | otherwise = test k i (parse x) xs where (x,xs) = uncons s -- -- process a single line, until \n -- test :: Int -> Int -> Int -> ByteString -> Int test k i !n t | y == '\n' = -- done reading the line, process it: if n `divisibleBy` k then add k (i+1) ys else add k i ys | otherwise = test k i n' ys where (y,ys) = uncons t n' = parse y + 10 * n parse c = ord c - ord '0' -- fastest way to take the head of a strict bytestring uncons s = (w2c (unsafeHead s), unsafeTail s) Cheers, Don

3 5

class MonoidBreak?
by Alex Jacobson 09 Jun '07

09 Jun '07

Is there a standard class that looks something like this: class (Monoid m) => MonoidBreak m where mbreak::a->m a->(m a,m a) and it should follow some law like: m == uncurry mappend $ mbreak x m -Alex-

3 3

Memoization
by Mark Engelberg 09 Jun '07

09 Jun '07

I'd like to write a memoization utility. Ideally, it would look something like this: memoize :: (a->b) -> (a->b) memoize f gives you back a function that maintains a cache of previously computed values, so that subsequent calls with the same input will be faster. I've searched the web for memoization examples in Haskell, and all the examples use the trick of storing cached values in a lazy list. This only works for certain types of functions, and I'm looking for a more general solution. In other languages, one would maintain the cache in some sort of mutable map. Even better, in many languages you can "rebind" the name of the function to the memoized version, so recursive functions can be memoized without altering the body of the function. I don't see any elegant way to do this in Haskell, and I'm doubting its possible. Can someone prove me wrong? --Mark

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type versus data declarations
by egallego＠babel.ls.fi.upm.es 09 Jun '07

09 Jun '07

Hello, I'm wondering why you can write > data FSet a = Show a => M (a -> Double) > > a :: FSet Double > a = M $ \x -> 0 and it works, but > type FSet a = Show a => (a -> Double) > > a :: FSet Double > a _ = 0 fails with > All of the type variables in the constraint `Show a' are already in scope > (at least one must be universally quantified here) > In the type synonym declaration for `FSet' If I understood the GHC manual, this should work, as FSet a should be replaced in a, so a type signature would come: > a :: Show a => (a -> Double) which is OK. Sorry if this is FAQ, but I did some searches and nothing came out. Thanks in advance, Emilio

3 6

Problem when class function doesn't mention all type parameters or...
by Daniil Elovkov 09 Jun '07

09 Jun '07

Hi again So, I keep trying to implement the dsl, I mentioned yesterday. I ran into a problem which can be illustrated by the following small example. > class Cl s1 a1 s2 a2 where > clF :: a1 -> a2 clF doesn't mention s1 and s2, they're used only to restrict types below > data T s a where > C :: (Cl s1 a1 s2 a2) => T s1 a1 -> T s2 a2 > f :: T s a -> a > f (C t) = let v = f t in clF v This doesn't typecheck with the error Could not deduce (Cl s11 a1 s2 a) from the context (Cl s1 a1 s a) arising from use of `clF' Here T depends on s, while the result of f doesn't. What happens is s gets lost. I'm not sure if the problem is that stated in the subject, really. But, I wonder how at all ghc should guess about s parameters. Maybe if f were T s a -> (s,a) where s were never evaluated... ...but no, this > f :: T s a -> (s,a) > f (C t) = let (s,v) = f t in (s,clF v) also fails to unify those s types. Can someone sched some light, please?

1 0

Re: I just don't get it (data structures and OO)
by Christopher Lane Hinson 08 Jun '07

08 Jun '07

> Let's say i have a deep nested data structure. > Universe containing galaxies, containing solar systems, containing > planets, containing countries, containing inhabitants, containing > ...whatever. Oh. I had /exactly/ this problem. If you use separate types (i.e. a newtyped integer, acting kindof like a pointer) to represent (1) the identity and (2) the state of the object, you can use a separate data structure that remembers which object is inside which other object. http://www.downstairspeople.org/darcs/unstable/roguestar-engine/src/Insiden… Where "InsidenessMap a b c" represents a relationship where b's are inside a's, and b's have a state of c. Then, you need to declare a separate InsidenessMap for each possible relationship, but this ensures that you'll never put a galaxy inside a solar system. Or you can make 'a' be a reference to any type of object; there are options. But, you just update this structure once, with no thought of recursively updating a heterogenous tree of data. It may not be the best solution, but if I knew of something better I would be using the better thing instead. --Lane

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Parallelism and expensive calculations that may not be needed
by Felipe Almeida Lessa 08 Jun '07

08 Jun '07

Hello everybody! (Sorry if this is a newbie question, couldn't find the answer anywhere) Suppose I have an expensive function (such that even to be reduced to WHNF it takes a long processing time) > expensive :: Foo -> Maybe Bar and I want to calculate it on multiple processors, like in > calculate = parMap rnf expensive Well, fine. But suppose that then I give the result to a function like > final xs = reverse $ final' xs [] where > final' [] r = r > final' (x:xs) r = case x of Nothing -> [] > Just x -> final' xs (x:r) Given list :: [Foo], a sequential calculation as in > resultS = final $ map expensive list would not call expensive after finding a Nothing. But > resultP = final $ calculate list will keep calculating all the other expensive's because they were sparked? Also, suppose that in list there are thousands of elements, whereas the last one is the only x such as "expensive x == Nothing". More importantly, calculating "expensive x" is very fast for some reason. In both versions above, all the initial elements will have to be calculated in order to find that the last one (that probably was already computed in the parallel version) was Nothing and that all the work must have to be thrown away. Are the observations above valid? Is there a way to improve this style of computation? Any pointers are appreciated =). I think that to solve the second problem, I needed some sort of fold that is aware of parallel nature of the list and that folds on the order in that the thunks get evaluated, and not on order of the list. But I guess it's not possible to write something like this, right? Cheers, Felipe. -- Felipe.

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Re: [Haskell] ANN: Catch (first ever release)
by Neil Mitchell 08 Jun '07

08 Jun '07

Following up on haskell-cafe: Hi Chris, > > For the last few years I've been working on a pattern-match checker > > for Haskell, named Catch. I'm now happy to make a release: > I would love to use this with regex-tdfa (and the other regex-* modules). > > At the moment regex-tdfa is uses a few extensions such as recursive "mdo" > notation and parts hook up to the MPTC + fundeps used in regex-base. The > MPTC+fundeps are not part of the workhorse modules, they are just "type class > syntactic sugar" that makes it easier for the library consumer. But "mdo" _is_ > used in two of the important internal functions. You have several options: * Wait until GHC Core is working, when Catch will work with all these things automatically. * Rewrite your code to eliminate mdo, if you wish to port regex to Yhc you will need to do this anyway. * Find an mdo preprocessor (I think one exists?) and preprocess the code before checking. Obviously the GHC Core one is least work for you, but won't be available for a while. Thanks Neil

1 0

HopenGL
by Ruben Zilibowitz 08 Jun '07

08 Jun '07

Hi, If anyone knows how to get HOpenGL to draw coloured lines, I'd like to know. Currently I can draw coloured objects like planes cylinders or spheres, but not lines. Lines always seem to appear black. Regards, Ruben

4 5

haskell wiki indexing
by Vincent Kraeutler 08 Jun '07

08 Jun '07

as was pointed out on the programming reddit [1], crawling of the haskell wiki is forbidden, since http://www.haskell.org/robots.txt contains User-agent: * Disallow: /haskellwiki/ and indeed, a google search gives the old wiki http://www.google.ch/search?q=haskell+wiki i.e. http://haskell.org/hawiki/FrontPage rather than http://haskell.org/haskellwiki/Haskell in other words, it seems like the most relevant search engines do not index the haskell wiki. i have reported this on #haskell about a week ago[2], and this setting was acknowledged to be a measure deliberately taken to curb excessive load from spiders crawling the wiki. i still believe this a highly harmful stance, and just to make sure that whoever may feel concerned is at least aware of the problem, i am now cross-posting to this list as well. kind regards, v. [1] http://programming.reddit.com/info/1qzjx/comments/c1r3ok [2] http://tuukka.iki.fi/tmp/haskell-2007-05-16.html

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ghc wiki registration failure ( was: Failure in GHC compilation)
by Daniil Elovkov 08 Jun '07

08 Jun '07

Hi I wanted to add a couple of words that another solution would be to add an option to xargs in target.mk xargs -n NNN where NNN is less than the OS limit. (That helped me to build LambdaVM on windows, there are quite a lot of class files there, and no SPLITOBJS will obviously help) Editing target.mk isn't a good thing, probably. Maybe some $XARGSOPTS could be introduced... BUT trying to register on the wiki resulted in an error with this python traceback Traceback (most recent call last): File "/usr/lib/python2.3/site-packages/trac/web/cgi_frontend.py", line 131, in run dispatch_request(req.path_info, req, env) File "/usr/lib/python2.3/site-packages/trac/web/main.py", line 139, in dispatch_request dispatcher.dispatch(req) File "/usr/lib/python2.3/site-packages/trac/web/main.py", line 107, in dispatch resp = chosen_handler.process_request(req) File "build/bdist.linux-i686/egg/acct_mgr/web_ui.py", line 117, in process_request File "build/bdist.linux-i686/egg/acct_mgr/web_ui.py", line 138, in _do_create File "build/bdist.linux-i686/egg/acct_mgr/api.py", line 88, in set_password File "build/bdist.linux-i686/egg/acct_mgr/htfile.py", line 50, in set_password File "build/bdist.linux-i686/egg/acct_mgr/htfile.py", line 85, in _update_file IOError: [Errno 13] Permission denied: '/srv/trac/ghc/trac.htpasswd' 2007/6/4, Simon Peyton-Jones <simonpj(a)microsoft.com>: > I've extended the "what to look for if your build fails" section in > http://hackage.haskell.org/trac/ghc/wiki/Building/Windows > > Please do keep adding to this page! Every time you trip over something, think about whether your experience could be used to help someone else. > > Simon > > | -----Original Message----- > | Wrom: KEDOTWFAOBUZXUWLSZLKBRNVWWCUFPEGAUTFJMVRESKPNKMBIPBARHDMNNSKVFVWRKJVZCMHVIBGDADRZFSQHYUCDDJ > | Ilari Vuokko > | Sent: 02 June 2007 21:09 > | To: Monique Monteiro > | Cc: haskell-cafe(a)haskell.org > | Subject: Re: [Haskell-cafe] Failure in GHC compilation > | > | Hi, > | > | On 6/2/07, Monique Monteiro <monique.louise(a)gmail.com> wrote: > | > has anyone any idea about what may cause the following error in GHC > | > compilation? > | > | It's exactly what it says...too long argument list (it's limited in Windows.) > | This is usually caused by having split-objs on, which causes *a lot* of object > | files being generated, all of them passed to ar or ld. > | > | > c:\ghc\ghc-6.6.1\bin\ar.exe: creating libHSbase.a > | > xargs: c:/ghc/ghc-6.6.1/bin/ar: Argument list too long > | > make[2]: *** [libHSbase.a] Error 126 > | > make[2]: *** Deleting file `libHSbase.a' > | > Failed making all in base: 1 > | > make[1]: *** [all] Error 1 > | > make[1]: Leaving directory `/cygdrive/c/GHC6.6.1/ghc-6.6.1/libraries' > | > make: *** [stage1] Error 2 > | > | Untested: setting SplitObjs=NO in build.mk might help (as there will be less > | object files to pass to ar. > | > | If ghc head is already using cabal, this problem is solved there. > | > | It might also be possible to extend ghc build machinery to pass parameters > | in file for ar and ld - but that requires new enough mingw toolchain. > | > | HTH, > | Esa > | _______________________________________________ > | Haskell-Cafe mailing list > | Haskell-Cafe(a)haskell.org > | http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 1

OK, so this VIM thing -- how do I make it actually work?
by Michael T. Richter 08 Jun '07

08 Jun '07

I've given up on getting a decent text editor for editing Haskell (specifically literate Haskell -- plain Haskell works fine in GEDIT). Instead I fire up VIM and get ... a total mess. At first I think maybe I've screwed up a whole bunch of settings or something, so I nuke everything in my home directory that begins with .vim and then, for added measure, head over to /usr/share and nuke the entire ./vim directory tree. I then reinstall vim (from the Ubuntu Edgy archives) to get a brand new set of config files unsullied by my hands. I still get a dog's breakfast. A screen shot of what I'm seeing with a representative example of a .lhs file to show what I mean can be found at http://img357.imageshack.us/img357/5798/gvimexamplezv4.png. (I've pared it down to the minimum I could find that shows the behaviour clearly.) The problems I'm seeing are the ugly white-on-red for underlines, the lack of any kind of differentiation for keywords/operators/etc. vs. identifiers (although some punctuation is recognized, specifically curly braces), comments not being noted, etc. Basically it looks like the Haskell is simply not being recognized at all (and, if the @saBinds@ thing is what I think it is, it looks like some latex isn't being recognized fully either). Can anybody vim-centric please take a look at this and give me a few educated guesses as to what is happening here? For good measure, here's the beginning of the lhaskell.vim file that comes with my vim distribution (7.0 in the Ubuntu archives). If it's desired I can attach the whole file. (It isn't actually all that large.) " Vim syntax file " Language: Haskell with literate comments, Bird style, " TeX style and plain text surrounding " \begin{code} \end{code} blocks " Maintainer: Haskell Cafe mailinglist <haskell-cafe(a)haskell.org> " Original Author: Arthur van Leeuwen <arthurvl(a)cs.uu.nl> " Last Change: 2004 Aug 31 " Version: 1.01 " " Thanks to Ian Lynagh for thoughtful comments on initial versions and " for the inspiration for writing this in the first place. " " This style guesses as to the type of markup used in a literate haskell " file and will highlight (La)TeX markup if it finds any " This behaviour can be overridden, both glabally and locally using " the lhs_markup variable or b:lhs_markup variable respectively. " " lhs_markup must be set to either tex or none to indicate that " you always want (La)TeX highlighting or no highlighting " must not be set to let the highlighting be guessed " b:lhs_markup must be set to eiterh tex or none to indicate that " you want (La)TeX highlighting or no highlighting for " this particular buffer " must not be set to let the highlighting be guessed " " " 2004 February 18: New version, based on Ian Lynagh's TeX guessing " lhaskell.vim, cweb.vim, tex.vim, sh.vim and fortran.vim " 2004 February 20: Cleaned up the guessing and overriding a bit " 2004 February 23: Cleaned up syntax highlighting for \begin{code} and " \end{code}, added some clarification to the attributions " -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) It's OK to figure out murder mysteries, but you shouldn't need to figure out code. You should be able to read it. (Steve McConnell)

12 17

Scope of type variables in associated types
by Matthew Sackman 08 Jun '07

08 Jun '07

The following doesn't seem to work. Is this a limitation of the current implementation or will it never work? Are there any work arounds without introducing extra type params into the data type E? > class G a b | a -> b where > data E a :: * > wrap :: b -> E a > unwrap :: E a -> b > instance G a b where > data E a = EC b -- this line - the b is not in scope. > wrap = EC > unwrap (EC b) = b I get "Not in scope: type variable `b'". I was rather hoping it would be in scope. I've tried: > instance forall a b . G a b where > ... but that doesn't seem to extend the scope as it does with functions. I realise this is bleeding edge stuff. But I can't seem to work out from the various wiki pages on this whether this is going to be eventually supported. This is all with today's GHC HEAD. Cheers, Matthew -- Matthew Sackman http://www.wellquite.org/

7 14

Re: Good article on why implementing a functional
by Monique Monteiro 08 Jun '07

08 Jun '07

I think that using an existing back end of an optimizing compiler such as GHC is more promising than targetting Java, C# or other imperative/OO language. GHC already does a lot of work to achieve a good performance. So, if you generate Java code, you'd have to do a lot of work to get good performance in the final code, 'cause of the huge semantic gap between Haskell and Java. Also, you don't have any control on the JVM internals, such as GC and other things. However, if you want portability or easy access to object-oriented APIs, generating Java code (or .NET code, for interoperability with other languages) should be your choice. Cheers, Monique > Gentlefolk: > > Help. I need support for a technical argument: why going to an intermediate > form for an existing functional back end like Haskell really, truly is > better for implementing a functional language than is going to an > intermediate form like the Java intermediate form and re-doing all the > various specialized mechanism needed to support a true lazy functional > language. In other words, I need a pithy paper or book chapter that will > convince someone unfamiliar with functional languages that Functional Back > Ends Really Are Different --- no, Really Truly Different. Something on the > order of Why Functional Programming Matters, but for the implementor of the > language, not the programmer. > > Any pointers to a good article or book chapter that might help? I keep > saying that this bookkeeping is a BIG task, but they keep saying they know > they can "handle" it. Pithy comments? Staff year estimates? Horror > stories? > > Thanks, anyone. > > Dave Barton > -- __________________________________________________________ Monique Monteiro, MSc MCP .NET Framework 2.0 / SCJP / IBM OOAD Project Manager Recife Microsoft Innovation Center +55 81 34198137 http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br MSN: monique_louise(a)msn.com

1 0

faster factorial function via FFI?
by Dan Drake 07 Jun '07

07 Jun '07

Hello everyone, I have some code in which the bottleneck is the factorial function. I began by using a naive fac n = product [1..n] but it looks like there are faster ways to do it. I could try to look up those faster algorithms and implement them, but I'm guessing that using libgmp's factorial function is the best way to do it. I'm a poor programmer, so I don't trust myself to implement those algorithms properly. Hence I need to use FFI. ...but ghc already uses libgmp for Integers, so shouldn't I be able to somehow call libgmp's factorial function? Using regular FFI stuff, it looks like this might get complicated, since the function doesn't return one of the usual foreign types, so I'd need to mess around to get the result into an Integer, which is what I need. Is there an easy way to do this? It might also be faster to use a lookup table, since most of the time I'll be taking factorials of relatively small numbers. Thanks, Dan -- Ceci n'est pas une .signature.

9 16

Finding points contained within a convex hull.
by Daniel McAllansmith 07 Jun '07

07 Jun '07

Hello. I've got a system of linear inequalities representing half-spaces. The half-spaces may or may not form a convex hull. I need to find the integral coordinates that are contained within the convex hull, if there is one. For example, given 0 <= x <= 4 0 <= y <= 3 0 <= 2x - y 0 <= 1.2y - x I want the following (x,y) coordinates [(0,0),(1,1),(1,2),(2,2),(2,3),(3,3)] Anybody have any suggestions, or libraries, for solving this in many dimensions and equations? Thanks Daniel

7 8

Compiling packages for GHCi
by Grzegorz 07 Jun '07

07 Jun '07

Hi, I'm having problems using a package which links in foreign libraries from GHCi. I use a .cabal file to build the package and have the following option there: extra-libraries: stdc++ maxent z m gfortran m gcc_s After installation, it works fine when I compile code using that package. However, when I try it from GHCi I get: Loading package binary-0.3 ... linking ... done. Loading package filepath-1.0 ... linking ... done. Loading package haskell98 ... linking ... done. Loading package mtl-1.0.1 ... linking ... done. Loading package parsec-2.0 ... linking ... done. Loading package gramlab-lib-0.1 ... linking ... done. Loading package maxent-0.0 ... can't load .so/.DLL for: maxent (libmaxent.so: cannot open shared object file: No such file or directory) I don't have a libmaxent.so, the maxent library in at /usr/local/lib/libmaxent.a Can I somhow use my package with GHCi? Thanks!, Grzegorz

4 4

Newbie Q: Monad 'fail' and 'error'
by Dmitri O.Kondratiev 07 Jun '07

07 Jun '07

Monad class contains declaration *fail* :: String -> m a and provides default implementation for 'fail' as: fail s = error s On the other hand Prelude defines: * error* :: String -> a which stops execution and displays an error message. Questions: 1) What value and type 'error' actually returns in: error "some message" ? 2) How declaration String -> m a matches with String -> a ? 3) In Maybe monad: fail = Nothing When and how 'fail' is used in Maybe monad? Thanks! -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

5 5

Parsec problem
by Neil Mitchell 07 Jun '07

07 Jun '07

Hi, I'm using parsec to parse something which is either a "name" or a "type". The particular test string I'm using is a type, but isn't a name. I want things to default to "name" before "type". Some examples of the parsec function, and the result when applied to a test string: > parsecQuery = do spaces ; types Right Query {scope = [], names = [], typeSig = Just [a], items = [], flags = []} > parsecQuery = do spaces ; names Left (line 1, column 1): unexpected "[" expecting white space, "--", "/", "(", letter, "::" or end of input > parsecQuery = do spaces ; try names <|> types Left (line 1, column 1): unexpected "[" expecting white space, "--", "/", "(", letter, "::" or end of input I would have expected try names <|> types to return a Right (since types matches), but it doesn't. I assumed this would be a property of Parsec, but it doesn't appear to hold. Can anyone shed any light? Thanks Neil

5 7

FP v. OOP
by PR Stanley 06 Jun '07

06 Jun '07

Hi BAE Systems which specialises in military technology is looking for programmers who have experience in C, C++ and Java and UML. Where soundness and completeness but especially soundness is of the essence the OOP framework is far from ideal. The FP paradigm, on the other hand, offers the perfect alternative. Yet, its application is for the most part limited to the realms of academic research. Discuss. Regards, Paul

5 4

Use of Data.Generics.everywhere'
by Neil Mitchell 06 Jun '07

06 Jun '07

Hi, I'm looking into SYB, and I'm trying to find examples where people have used everywhere' (note: not everywhere, but with a \prime) - unfortunately I can't find any. I've tried Google Code Search, and it comes up with one rather trivial example which could equally have been everywhere. The original SYB paper doesn't give an example of everywhere' in use. So is everywhere' useful for anything? Has anyone used everywhere' in practice? Thanks Neil

1 0

HList questions
by Marc Weber 06 Jun '07

06 Jun '07

Do you know what a type indexed coproduct is ? (TIC.hs from HList) What is the purpose of this module? Why the Proxy type has been introduced? Can you think of a short application? Marc

1 0

standard function
by Steffen Mazanek 06 Jun '07

06 Jun '07

Hello, is there a function f::[a->b]->a->[b] in the libraries? Couldn't find one using hoogle although this seems to be quite a common thing... Steffen

7 7

I saw this... and thought of you
by Andrew Coppin 06 Jun '07

06 Jun '07

http://dis.4chan.org/read/prog/1180896798/ (It's been a while since I touched Java, and I must confess I can't even comprehend this code...)

2 1

(pre)compiled Haskell compiler for True64/Alpha
by Lutz Donnerhacke 06 Jun '07

06 Jun '07

Does anyone have a haskell compiler for True64 (formerly known as OSF/1) on Alpha hardware? I'm currently unable to compile the first bootstrap compiler. Any hint which compiler should I start with? I'll send detailed error reports only if I do not succeed in the next hours.

2 2

compiling shared library with GHC on x86-64
by Alexander Vodomerov 06 Jun '07

06 Jun '07

Hello! I'm trying to build shared library from Haskell source to call it from external C program. Everything works fine on usual x86 machine. However, any attempt to compile code as position independent on x86-64 result in fatal error. This is simple example: $ cat adder.hs module Adder where f :: Int -> Int -> Int f x y = x + y $ ghc -fPIC -c adder.hs ghc-6.6.1: panic! (the 'impossible' happened) (GHC version 6.6.1 for x86_64-unknown-linux): initializePicBase Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug $ I've read source file compiler/nativeGen/PositionIndependentCode.hs and found that function initializePicBase is not implemented for x86-64 platform. Are there any plans to implement such a functionality? Are there any global difficulties with such an implementation or nobody just cared about it? What functions are needed to be implemented for building PIC code on x86-64? If it is not very difficult, may be I will try myself. What is best source of GHC code generator documentation? Any other ideas? With best regards, Alexander.

2 1

What puts False before True?
by PR Stanley 06 Jun '07

06 Jun '07

Hi What is the basic philosophy for Bool being a member of Ord? What justifies False < True? many Thanks in advance, Paul

18 28

Hardware
by Andrew Coppin 06 Jun '07

06 Jun '07

OK, so... If you were going to forget everything we humans know about digital computer design - the von Neuman architecture, the fetch/decode/execute loop, the whole shooting match - and design a computer *explicitly* for the purpose of executing Haskell programs... what would it look like? What design decisions would you make? Why?

9 11

Failure in GHC compilation
by Monique Monteiro 06 Jun '07

06 Jun '07

Hi, has anyone any idea about what may cause the following error in GHC compilation? c:\ghc\ghc-6.6.1\bin\ar.exe: creating libHSbase.a xargs: c:/ghc/ghc-6.6.1/bin/ar: Argument list too long make[2]: *** [libHSbase.a] Error 126 make[2]: *** Deleting file `libHSbase.a' Failed making all in base: 1 make[1]: *** [all] Error 1 make[1]: Leaving directory `/cygdrive/c/GHC6.6.1/ghc-6.6.1/libraries' make: *** [stage1] Error 2 Thanks in advance, -- __________________________________________________________ Monique Monteiro, MSc MCP .NET Framework 2.0 / SCJP / IBM OOAD Project Manager Recife Microsoft Innovation Center +55 81 34198137 http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br MSN: monique_louise(a)msn.com

7 8

Re: [Haskell-cafe] Idiomatically using lists
by kevin birch 06 Jun '07

06 Jun '07

On 火, 2007-6月-05, at 02:54, Greg Fitzgerald wrote: > > rotating the fourth element 2 positions would result in: [1, 2, > 4, 3, 5] > Seems odd. Should that be [4,1,2,3,5]? > Yes, I meant to use the 5 element in my second example. Sorry for the confusion. > > Is there an idomatic way to handle both of these cases in a > function? > Generally people like to see your attempt at a solution before > giving the idomatic one so that they are sure it's not a homework > question. What do you have so far? > Yeah, I only wish I had gone to a school that would be forward thinking enough to each FP. ;-) Here is my version: rotate :: Array Integer Card -> Integer -> Integer -> Array Integer Card rotate a i n | i <= u - n = a // [(i, a ! (i + 1)), (i + 1, a ! (i + 2)), (i + 2, a ! i)] | otherwise = a // zip [l..u] (h ++ [a ! i] ++ filter (not . (== (a ! i))) t) where (l, u) = bounds a (h, t) = splitAt (fromInteger ((i - u) + n)) $ elems a This function is part of my implementation of the Solitaire encryption algorithm, so that is why I have the reference to a Card data type. This does what I want, and seems basically idiomatic, but perhaps it could be better. Thanks, Kevin

3 2

FOL
by Tony Morris 06 Jun '07

06 Jun '07

I would like to know if all 16 possible functions that accept two boolean arguments have names in First-Order Logic. I know they have Haskell function names (e.g. \p -> \_ -> id p, \_ -> \_ -> const True), but I'm hoping there is a more general name that is recognised by anyone with a feel for logic, but not necessarily Haskell. I have listed all sixteen of these functions below using Haskell (named a to p) along with the name of those that I am aware of having a name as a comment. Thanks for any tips. {- p q | a b c d e f g h i j k l m n o p 0 0 | 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 1 | 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 1 0 | 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 1 1 | 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 -} a :: Bool -> Bool -> Bool a _ _ = False b :: Bool -> Bool -> Bool -- conjunction AND b False _ = False b _ q = q c :: Bool -> Bool -> Bool c False _ = False c _ q = not q d :: Bool -> Bool -> Bool d p _ = p e :: Bool -> Bool -> Bool e False q = q e _ _ = False f :: Bool -> Bool -> Bool f _ q = q g :: Bool -> Bool -> Bool -- exclusive disjunction XOR g False q = q g _ q = not q h :: Bool -> Bool -> Bool -- disjunction OR h False False = False h _ _ = True i :: Bool -> Bool -> Bool -- negation of disjunction NOR i False False = True i _ _ = False j :: Bool -> Bool -> Bool -- biconditional j False q = not q j _ q = q k :: Bool -> Bool -> Bool k _ q = not q l :: Bool -> Bool -> Bool l False q = not q l _ _ = True m :: Bool -> Bool -> Bool m p _ = not p n :: Bool -> Bool -> Bool -- implication n False _ = True n _ q = q o :: Bool -> Bool -> Bool -- negation of conjunction NAND o False _ = True o _ q = not q p :: Bool -> Bool -> Bool p _ _ = True -- Tony Morris http://tmorris.net/

5 4

Why does the class called "Real" support only rationals, and not all reals?
by bretm 05 Jun '07

05 Jun '07

I just got started learning Haskell a few days ago. I've implemented a numeric data type that can represent some irrational numbers exactly, and I'd like to instantiate the RealFrac class so I can do truncate, round, etc., in the most natural way in the language. Implementing properFraction (which is in RealFrac) would not a problem at all, but implementing RealFrac requires implementing Real, which has the function toRational, which Haskell 98 says "returns the rational equivalent of its real argument with full precision" which isn't possible for this number type (which includes irrationals). I'd also like to implement approxRational since this can be done in a straightforward manner with my data type, but this appears to be just a library function in the Ratio library and not a function defined in a class, and is dependent on the toRational function which doesn't make sense for a data type that can represent irrational values exactly. So my questions are: 1) Am I understanding the Haskell numeric classes correctly by thinking that to define properFraction I also have to define (incorrectly in this case) the toRational function? 2) Is a literal reading of Haskell 98 necessary for toRational or can it just be approximate? (Seems like not, since there's approxRational, and no other way to indicate the desired accuracy for toRational.) 3) If I'm right about toRational requiring "full precision", why was implementing this made a requirement for implementing properFraction? 4) Why on earth is there a class called "Real" that requires the ability to convert to Rational with full precision, which can't be done with all real numbers? As an example, consider a data type that can represent a quadratic surd, e.g. (1, 5, 2) might mean (1 + (sqrt 5)) / 2. With this data type it's straightforward to implement the Num and Fractional classes and get full arithmetic operations +, -, *, /, etc. The properFraction function also makes sense: there's an integer portion equal to 1 in this case, and a fractional portion equal to (-1, 5, 2), exactly. So truncate, round, ceiling, and floor make sense (in RealFrac). But toRational can't be implemented with "full precision". With this example you can also see how approxRational is a perfectly good idea, and would be something likely to be needed, but this function isn't a class function designed to be implemented by custom data types. It seems to me (with my meager few days of Haskell experience) that approxRational should have been part of the Real class, and that toRational should be relegated the Ratio library instead. Or else, that the functions that are in RealFrac should be in the Fractional class instead. So I'm guessing there are some good reasons for having RealFrac in addition to Fractional that I'm not yet aware of. -- View this message in context: http://www.nabble.com/Why-does-the-class-called-%22Real%22-support-only-rat… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 7

compiler implementation details
by José Miguel Vilaça 05 Jun '07

05 Jun '07

Hi, Im curious about how the Haskell compilers nowadays take care of a simple function definition like append append [] l = l append (h:t) l = h: (append t l) Do they take a rewriting way like append [1,2] [3,4] ==> 1: (append [2] [3,4]) ==> 1: (2 : (append [] [3,4]) ) ==> 1 : (2: [3,4]) ==> [1,2,3,4] Or do they converte the append definintion to some lambda-calculus flavour language append = Y (\f -> \l1 l2 -> case l1 of [] -> l2 (h:t) -> h: (f t l2)) And then apply this converted definition to the arguments? Best Miguel

1 0