- Haskell-Cafe - Haskell.org

Control.Exception.evaluate - 'correct definition' not so correct
by Bryan Donlan 19 May '08

19 May '08

Hi all, After some discussion on #haskell I decided to bring this issue to haskell-cafe. GHC's documentation for Control.Exception.evaluate states: evaluate :: a -> IO a Forces its argument to be evaluated when the resultant IO action is executed. It can be used to order evaluation with respect to other IO operations; its semantics are given by evaluate x `seq` y ==> y evaluate x `catch` f ==> (return $! x) `catch` f evaluate x >>= f ==> (return $! x) >>= f Note: the first equation implies that (evaluate x) is not the same as (return $! x). A correct definition is evaluate x = (return $! x) >>= return However, if >>= is strict on its first argument, then this definition is no better than (return $! x). One might next consider: evaluate x = (return x) >>= (return $!) However, according to the monad laws, this is equivalent to: evaluate x = return $! x and we're back to where we started. Although GHC's implementation of IO is somewhat more relaxed about this, there is no guarentee that this will be the case in all IO implementations, or future versions of GHC, or different optimization flags, or... The best I've come up with so far is: evaluate x = newIORef (return $! x) >>= join . readIORef In any case, if >>= is to be guarenteed lazy, this ought to be documented somewhere (or is it?). Otherwise Control.Exception's docs should be updated to provide a more correct example and/or the caveat that >>= must not be left-strict for the example implementation to be correct. Thanks, Bryan Donlan

10 13

Two-iteration optimisation (was: GHC Predictability)
by Paul Johnson 19 May '08

19 May '08

We've had a big debate over > mean xs = foldl' (+) 0 xs / fromIntegral (length xs) For anyone who didn't follow it, the problem is that "mean" needs to traverse its argument twice, so the entire list has to be held in memory. So if "xs = [1..1000000000]" then "mean xs" uses all your memory, but "foldl' (+) 0 xs" and "length xs" by themselves do not. The solution is for the programmer to rewrite "mean" to accumulate a pair containing the running total and count together, then do the division. This makes me wonder: could there be a compiler optimisation rule for this, collapsing two iterations over a list into one. I've never tried writing GHC rules, but something like: > f (foldl' g x) (foldl' h x) = (uncurry f) (foldl' (\i (gt,ht) -> (g i gt, h i ht))) The first problem that occurs to me is the number of permutations of fold and map functions. Paul.

8 10

Question on declaring an instance
by john lask 19 May '08

19 May '08

Consider the data type: Foo i o a = ... we want to declare an instance of Monad and Arrow for Foo without using a newtype (repackaging). The effect we want is to be able to construct an expression like: ( f>> g )>>> ( h>> i ) where f, g, h, i :: Foo i o a, for some i o a it is easy to declare instance Monad (Foo i o) where ... but how do we declare instance Arrow (Foo ... ), this instance takes a kind * -> * -> * it is easy to declare instance Arrow (Bar a) where ... where Bar a i o is a flipping of the order of the arguments to Foo i.e. Bar = ((flip . (flip .)) Foo Can Foo be declared an instance of both Arrow and Monad ? _________________________________________________________________ It's simple! Sell your car for just $30 at CarPoint.com.au http://a.ninemsn.com.au/b.aspx?URL=http%3A%2F%2Fsecure%2Dau%2Eimrworldwide%…

2 1

Parsec3 performance issues (in relation to v2)
by Neal Alexander 19 May '08

19 May '08

Just a heads up - i only have a month or so experience with Haskell, so alot of these issues may be my own fault. Anyway, the log file that i'm parsing uses English grammar, and the performance really dropped just by upgrading to Parsec3. I was hoping to use the ByteString support to boost the speed of already slow code, but had no such luck. It basicly went from "Ugh, this is kinda slow" to "Uhhhh i'm gonna go grab a burger and let this melt my CPU" haha. If anything, its probably all the look-ahead the rules have to do to get the context specific stuff right. Some of the code is here: http://hpaste.org/7578 ------------------------------------------------------------- #1 . Parsec 2: total time = 46.44 secs (2322 ticks @ 20 ms) total alloc = 16,376,179,008 bytes (excl. profiling overheads) Parse taking 51.3% time and 65.3% alloc. ------------------------------------------------------------- ------------------------------------------------------------- #2 . Parsec3 (4 times slower, no code changes): total time = 181.08 secs (9054 ticks @ 20 ms) total alloc = 46,002,859,656 bytes (excl. profiling overheads) Text.Parsec.Prim Taking 84.7% time and 86.0% alloc. ------------------------------------------------------------- ------------------------------------------------------------- #3 . Parsec3 but with the whole project converted to ByteString: (8 times slower): total time = 378.22 secs (18911 ticks @ 20 ms) total alloc = 100,051,417,672 bytes (excl. overheads) ------------------------------------------------------------- The third parse probably isn't a great indicator, since i reverted some rule-set optimizations that were causing errors. Plus i ended up packing the parsec String results to ByteStrings to fit in with everything else. I can post the full profiling info if anyone really cares.

6 11

Re: [Haskell-cafe] Re: ghc on Ubuntu Linux?
by Galchin, Vasili 19 May '08

19 May '08

Hi Josef, What generates dist/setup-config? When I run "runhaskell Setup.hs configure", nothing including dist/setup.config gets generated. ?? Regards, Vasili On Sat, May 17, 2008 at 11:02 AM, Josef Svenningsson < josef.svenningsson(a)gmail.com> wrote: > On Sat, May 17, 2008 at 1:00 PM, Galchin, Vasili <vigalchin(a)gmail.com> > wrote: > > Josef, > > > > E.g. > > vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs > configure > > Configuring unix-2.3.0.0... > > > > Normally copious output ... ??? > > > That's what it should look like! It just means you have a recent > version of cabal which doesn't spit out tons of information when > configuring. Happily all is well. > > /Josef >

3 2

Richer (than ascii) notation for haskell source?
by Patrick Surry 19 May '08

19 May '08

Probably a silly question, but for me one of the nice things about Haskell is that it's a lot like just writing math(s). But in contrast to math you lose a lot of notational flexibility being limited to the ascii character set in your source code. It would be nice to be able to use a richer set of symbols in your source code for operators and functions (e.g. integral, sum, dot and cross-product, ...), as well as variables (the standard upper and lower-case greek for example, along with things like super- and sub-scripting, bold/italic and what-not). You could imagine ending up with source code that reads just like a mathematical paper. Don't know how I'd actually then write/maintain the source-code - some WYSIWYG editor or effectively writing it in '(la)tex'? Maybe that's what Knuth is on about with his 'literate programming' weave/tangle stuff which I don't know much about - does that translate to Haskell? Patrick Patrick.Surry(a)portraitsoftware.com <mailto:Patrick.Surry@portraitsoftware.com> , VP Technology Tel: (617) 457 5230 Mob: (857) 919 1700 Fax: (617) 457 5299 Map <http://maps.google.com/maps?f=q&hl=en&q=125+Summer+St,+Boston,+MA+02110 &ie=UTF8&z=15&ll=42.353153,-71.057296&spn=0.022644,0.054245&om=1&iwloc=A > Portrait Software introduces Portrait Campaign Manager <http://www.portraitsoftware.com/Products/portrait_campaign_manager> : Easy, integrated campaign management for automated and highly targeted outbound marketing http://supportcentre.portraitsoftware.com/Vmail/emailfooter/balloon.gif <http://www.portraitsoftware.com/> http://supportcentre.portraitsoftware.com/Vmail/emailfooter/portrait_sof tware_logo.gif <http://www.portraitsoftware.com/> www.portraitsoftware.com <http://www.portraitsoftware.com/> ________________________________________________________________________ DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.

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Write Haskell as fast as C. [Was: Re: GHC predictability]
by Don Stewart 19 May '08

19 May '08

> I offer up the following example: > > mean xs = sum xs / length xs > > Now try, say, "mean [1.. 1e9]", and watch GHC eat several GB of RAM. (!!) > > If we now rearrange this to > > mean = (\(s,n) -> s / n) . foldr (\x (s,n) -> let s' = s+x; n' = n+1 > in s' `seq` n' `seq` (s', n')) (0,0) > > and run the same example, and watch it run in constant space. > > Of course, the first version is clearly readable, while the second one > is almost utterly incomprehensible, especially to a beginner. (It's even > more fun that you need all those seq calls in there to make it work > properly.) > > The sad fact is that if you just write something in Haskell in a nice, > declarative style, then roughly 20% of the time you get good > performance, and 80% of the time you get laughably poor performance. I've written an extended post on how to understand and reliably optimise code like this, looking at it all the way down to the assembly. The result are some simple rules to follow for generated code as good as gcc -O2. Enjoy, http://cgi.cse.unsw.edu.au/~dons/blog/2008/05/16#fast -- Don

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ByteString comparison question (was: another Newbie performance question)
by Philip Müller 19 May '08

19 May '08

Hi, I'm changing my CSV program to use ByteStrings, but I have problems with this: readCSVLine :: String -- line as String -> [String] -- line broken down into the value Strings readCSVLine = unfoldr builder where builder [] = Nothing builder xs = Just $ readField xs readField [] = ([],[]) readField (',':xs) = ([],xs) readField ('"':xs) = (field,rest) where (field,'"':rest) = break (== '"') xs So far, I have something like this - doesn't look too good to me, and doesn't compile: import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as C8 type CSV = [[B.ByteString]] readCSVLine :: B.ByteString -- line as String -> [B.ByteString] -- line broken down into the value Strings readCSVLine = unfoldr builder where builder xs | xs == B.empty = Nothing | otherwise = Just $ readField xs readField xs | xs == B.empty = ([],[]) | B.head xs == ',' = ([], B.tail xs) | B.head xs == '"' = (field, B.tail rest) where field,rest) = B.break (== '"') xs I do not know how to compare a Word8 to a Char. Or maybe I don't need to? Regards Philip

3 4

Type inference; always
by Peter Verswyvelen 19 May '08

19 May '08

In Haskell, I sometimes have to annotate code with type info because the type-inferer otherwise fails (even with |-XNoMonomorphismRestriction)|. Surely, most of the time this is because I was writing buggy code, but sometimes, type annotation just seems needed to get a successful compilation (luckily not as often as in C# 3.0) It seems some new papers appeared regarding this, e.g. http://research.microsoft.com/~simonpj/papers/boxy Is this work being incorporated into GHC? Unfortunately I'm not able to read those papers, so I'm not really sure what it means anyway and what implications (example code?) it would have ;-) Cheers, Peter

5 5

STM applications
by Nehir Sonmez 19 May '08

19 May '08

Good day all, We are Haskell STM researchers, looking for Haskell applications written using STM for profiling their STM behaviour. The benchmark currently consists of the ones in: www.cs.rochester.edu/meetings/TRANSACT07/papers/perfumo.pdf (3rd page) + binary trees + parallel sudoku and additional programs would be greatly greatly appreciated... Cheers, Nehir Sonmez Barcelona Supercomputing Center PS: We also know of Conjure and Happs.

1 0

hmp3 compilation problem
by Levi Stephen 19 May '08

19 May '08

Hi, I am trying to compile hmp3. I have the version from darcs at http://code.haskell.org/~dons/code/hmp3 I am getting the following errors: Tree.hs:190:14: No instance for (Binary FilePathP) arising from a use of `get' at Tree.hs:190:14-16 Possible fix: add an instance declaration for (Binary FilePathP) In a 'do' expression: nm <- get In the expression: do nm <- get i <- get return (File nm i) In the definition of `get': get = do nm <- get i <- get return (File nm i) Tree.hs:236:31: Couldn't match expected type `L.ByteString' against inferred type `bytestring-0.9.0.1:Data.ByteString.Lazy.Internal.ByteString' In the second argument of `L.writeFile', namely `(compress (encode s))' In the expression: L.writeFile f (compress (encode s)) In the definition of `writeTree': writeTree f s = L.writeFile f (compress (encode s)) Tree.hs:240:46: Couldn't match expected type `bytestring-0.9.0.1:Data.ByteString.Lazy.Internal.ByteString' against inferred type `L.ByteString' In the first argument of `decompress', namely `s' In the first argument of `decode', namely `(decompress s)' In the first argument of `return', namely `(decode (decompress s))' Any help is much appreciated. Thanks, Levi

2 1

Starting Haskell with a web application
by Jonathan Gardner 19 May '08

19 May '08

I am sure I am missing something obvious, but since the cafe is the place to ask stupid questions... I am getting more and more excited about Haskell as I learn more and more about it. Professionally, and as a hobby, I love working on web applications. Where do I get started in writing a web app with Haskell? I am looking more for a framework like Pylons and less like Apache, if that helps. I am sure someone has already done the homework on this, so a link will be fine. -- Jonathan Gardner jgardner(a)jonathangardner.net

11 15

Another optimization question
by Jeroen 19 May '08

19 May '08

Hi, I know there's been quite some performance/optimization post lately, so I hope there still room for one more. While solving a ProjectEuler problem (27), I saw a performance issue I cannot explain. I narrowed it down to the following code (never mind that 'primes' is just [1..], the problem is the same or worse with real primes): primes :: [Int] primes = [1..] isPrime :: Int -> Bool isPrime x = isPrime' x primes where isPrime' x (p:ps) | x == p = True | x > p = isPrime' x ps | otherwise = False main = print $ length (filter (== True) (map isPrime [1..5000])) $ time ./experiment1 5000 real 0m4.037s user 0m3.378s sys 0m0.060s All good, but if I change isPrime to the simpeler isPrime x = elem x (takeWhile (<= x) primes) it takes twice as long: time ./experiment2 5000 real 0m7.837s user 0m6.532s sys 0m0.141s With real primes, it even takes 10 times as long. I tried looking at the output of ghc -ddump-simpl, as suggested in a previous post, but it's a bit over my head (newby here...). Any suggestions? Thanks a lot, Jeroen

7 14

Std lib equivalent for liftM concat . sequence
by Alistair Bayley 18 May '08

18 May '08

A couple of days ago I had need for: > concatM :: Monad m => [m [a]] -> m [a] > concatM = liftM concat . sequence but found no such thing in the std libs, except perhaps for msum (I don't want to add instances for MonadPlus. Should I have to?). Have I missed something trivial? Alistair

4 5

Random numbers and splitting
by Lauri Oksanen 18 May '08

18 May '08

Hi, Does there exists any random number generator in Haskell that is suitable for doing heavy simulations and that can be splitted? At least there exists some c implementations of such generators, see http://www.iro.umontreal.ca/~lecuyer/myftp/papers/streams00.pdf Also some new theory for Mersenne Twister and its relatives exists, see http://www.iro.umontreal.ca/~lecuyer/myftp/papers/jumpf2.pdf http://www.iro.umontreal.ca/~lecuyer/myftp/papers/jumpmt.pdf I'm trying to implement Metropolis algorithm with lazy mutations and a state space that is infinite dimensional in theory (although, of course, not in practice). Without good splitting I'm forced to write as ugly code as with non-lazy language. Thanks in advance, Lauri

2 1

Multi threaded garbage collector
by Peter Verswyvelen 18 May '08

18 May '08

Hi all, I did some experiments with concurrent Haskell, and unfortunately I couldn't get my code run faster when using more cores, actually it ran a bit slower. Now I noticed after profiling that about 70% of the time my program was performing garbage collection (I had lists of 50000 tiny objects that got recreated 100 times per second). If I understood it correctly, the current garbage collector of GHC is single threaded, which really hinders SMP Any plans on improving this or workarounds? Thanks, Peter

4 3

Short circuiting and the Maybe monad
by John Hamilton 18 May '08

18 May '08

I'm trying to understand how short circuiting works with the Maybe monad. Take the expression n >>= f >>= g >>= h, which can be written as (((n >>= f) >>= g) >>= h) because >>= is left associative. If n is Nothing, this implies that (n >>= f) is Nothing, and so on, each nested sub-expression easily evaluating to Nothing, but without there being a quick way to short circuit at the beginning. Now take the example do x <- xs y <- ys z <- zs return (x, y, z) which I believe desugars like xs >>= (\x -> ys >>= (\y -> zs >>= (\z -> return (x, y, z)))) Here the associativity of >>= no longer matters, and if xs is Nothing the whole expression can quickly be determined to be Nothing, because Nothing >>= _ = Nothing. Am I looking at this correctly? - John

10 14

another Newbie performance question
by Philip Müller 18 May '08

18 May '08

Hi everybody, I was doing an assignment in Java for my university concerning a program that reads, modifies and writes CSV files, when I suddenly had the idea of implementing parts of this in Haskell for fun. When I finished the Haskell programm, I was disappointed by the performance: To parse a 200k lines CSV file, insert a line (yes I know i could insert a line without parsing the file, that's just an example) at pos. 199999 and write the file again, the Java program takes 1.1 seconds while the Haskell program takes 12.5 seconds. I have read Don's blog post but am unsure how to implement his tips into my program, as I am still kind of a Haskell beginner. The source code (40 lines incl. comments and empty lines) and the 200k CSV file I used for testing and a smaller CSV file demonstrating the special easy-to-parse CSV syntax are available on my ftp server, ftp://baah.servegame.org/public/haskell The call syntax is <program> <csv file> <line to insert> e.g. main lang.csv "test","this","line" I haven't posted the source code here directly because I thought it might be too long. If someone here finds the time to look at my code and give me some hints, that would really be nice. Regards Philip

6 5

Mapping Haskell Concepts to C# 3
by Galchin, Vasili 18 May '08

18 May '08

Kaveh, Maybe I am misguessing why you are asking your question, but wouldn't it be better to ask how to map these Haskell concepts to CLR? If so, check out work on Mondrian. Vasili

1 0

more runhaskell question
by Galchin, Vasili 18 May '08

18 May '08

vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs configure --prefix=$HOME Configuring mlock-1.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0/tests/mlock$ runhaskell Setup.lhs build Setup.lhs: error reading dist/setup-config; run "setup configure" command? No dist/setup-config is being generated! what is the "setup configure" command?? Regards, Vasili

1 0

Calling haddock in a portable way
by Misha Aizatulin 18 May '08

18 May '08

hello, the new version of haddock (2.0.0) needs a new option -B that tells it the GHC lib directory. How do I find out the correct value for this option in a makefile, so that the makefile stays portable? Cheers, Misha

2 2

Mapping Haskell Concepts To C# 3
by Kaveh Shahbazian 18 May '08

18 May '08

I have question on mapping some Haskell concepts to C# 3 ones. Maybe there are not any strict equivalents; yet it helps: 1 - What is the equivalent of "Type Constructor" in C#? 2 - What is the equivalent of "Data Constructor" in C#? 3 - What is the logical implementation of pattern matching in C#? (For example using structures with indicator fields or using interfaces and inheritance and dynamically dispatch in calling overloaded methods. Also this question contain a hidden one...GADTs!) Best Regards -- Kaveh Shahbazian email: kaveh.shahbazian(a)gmail.com

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[ANN] Next Bay FP Meeting: Bryan O'Sullivan on Concurrent and multicore programming in Haskell
by Keith Fahlgren 18 May '08

18 May '08

Hi, Our next BayFP meeting will be this Thursday, May 8th, 2008 at 7:30pm. We'll feature Bryan O'Sullivan on Concurrent and multicore programming in Haskell. Bryan is a co-author of the upcoming O'Reilly book Real World Haskell [http://book.realworldhaskell.org/] (among all sorts of other snazzy endeavors) Talking about multicore is all the rage, so I expect folks to bring a lot of people! :-) Details here: http://www.bayfp.org/blog/2008/05/02/may-8th-meeting-bryan-osullivan-on-con… Many thanks to Alex Payne at Twitter for hosting this month's meeting. They've been very supportive of BayFP and we appreciate their continued hosting. Twitter's address is: 164 South Park St San Francisco, CA 94107 Alex says: It's a building with a dark green door. People can just come on in and walk to their right to a large conference room. We'll start at 7:30pm. As always, this is a free event. If you want pizza, please select which type here (and bring a few $$s): Pizza Selection Form [http://bayfp.wufoo.com/forms/bryan-osullivan-bay-fp/] Keith

2 3

Haskell-Cafe Info Page
by D. Gregor 18 May '08

18 May '08

Hello, Common Lisp is a multiparadigm, general purpose programming language that supports imperative, functional, and object-oriented programming paradigms. Haskell is purely functional. Is this a reason why there is not macro feature in Haskell? I feel the object-oriented paradigm of CL and Scheme is the reason for the macro feature in these two languages. If it's not, then what does the macro feature provide, and why isn't it in Haskell? Douglas

4 3

A point in favour of -XOverlappingInstances (and -XTypeSynonymInstances)
by Achim Schneider 18 May '08

18 May '08

Token.hs:103:15: Overlapping instances for Show (SourcePos, Tok) arising from a use of `anyToken' at Token.hs:103:15-22 Matching instances: instance (Show a, Show b) => Show (a, b) -- Defined in GHC.Show instance [overlap ok] Show Token -- Defined at Token.hs:(39,0)-(40,23) I was just trying _not_ to show the a of (a,b) -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

2 1

ghc on Ubuntu Linux?
by Galchin, Vasili 17 May '08

17 May '08

Hello, I am running ghc and tools on Ubuntu. I seem to be running into very strange problems between ghc-pkg and the Ubuntu package manager. Suddenly "runhaskell Setup.hs configure" just stops working ... Are any other ghc/Ubuntu users running into problems? If so, please tell me what problems. This is killing my Haskell work! Regards, Vasili

3 5

Couple of formal questions
by Creighton Hogg 17 May '08

17 May '08

Hello Haskell, So there's two questions that have been bothering me lately & while they are, as usual, a little off topic I figured this might be a good forum: Where could I find a good treatment on data vs. codata & the difference between well-founded recursion & well-founded(?) corecursion? Where could I find a proof that the initial algebras & final coalgebras of CPO coincide? I saw this referenced in the "Bananas.." paper as a fact, but am not sure where this comes from. Thanks, Creighton Hogg

6 8

alex + happy parser problem
by Sean McLaughlin 17 May '08

17 May '08

Hi, To learn alex and happy, I'm trying to write a parser for a simple expression language. When I wrote my own lexer and just used happy, it was fine. When I used the basic wrapper of alex it was also fine. However, when I use the posn wrapper to get position information, I get a strange exception when the parse error occurs at the end of the input. For example, parsing "1 + " yields "Internal Happy error" rather than something like "Parse error at line 1, column 5" The lexer and parser are attached. Can anyone see what I'm doing wrong? calling parse "1+" yields a "Internal Happy error" instead of a parse error as I would expect. Thanks, Sean ------------------ -- Lexer ------------------ { module ExprLexer ( Token(..), AlexPosn(..), alexScanTokens, tokenPosn ) where } %wrapper "posn" $digit = 0-9 tokens :- $digit+ { (\p s -> Int p (read s)) } [\+] { (\p s -> Sym p (head s)) } { data Token = Sym AlexPosn Char | Int AlexPosn Int deriving (Eq, Show) tokenPosn (Sym p _) = p tokenPosn (Int p _) = p } -------------------------- --- Parser -------------------------- { module ExprParser where import ExprLexer (Token(..), alexScanTokens, tokenPosn, AlexPosn(..)) } %name parseExp %tokentype { Token } %token int { Int _ $$ } '+' { Sym _ '+' } %right '+' %% Exp : Exp '+' Exp { Add $1 $3 } | int { Const $1 } { data Expr = Const Int | Add Expr Expr deriving Show parse :: String -> Expr parse = parseExp . alexScanTokens happyError :: [Token] -> a happyError tks = error ("Parse error at " ++ lcn ++ "\n") where lcn = case tks of [] -> "end of file" tk:_ -> "line " ++ show l ++ ", column " ++ show c where AlexPn _ l c = tokenPosn tk }

1 0

Proving my point
by Achim Schneider 17 May '08

17 May '08

"test.l" (line 7, column 1): unexpected end of input expecting "(", Lambda abstraction, Let binding, Atom, end of input or Function application I obviously don't know anything about Parsec's inner workings. I'm going to investigate as soon as I stopped despairing. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

4 14

How to use arrays efficiently?
by Lauri Oksanen 17 May '08

17 May '08

Hi, I have a list of index-value pairs and I want to get a list which is sorted by the index and which contains index-wise sums of all the values. Here is a function that does this (and a bit more). --- pixelHistogram samples = let index s t = let x = round $ (fromIntegral imageWidth) * (1-s) y = round $ (fromIntegral imageHeight) * (1-t) in y*imageWidth + x indexedSamples = [(index s t, color) | (s, t, color) <- samples] pixelArray :: Array Int Color pixelArray = accumArray (\+) black (0, imageWidth*imageHeight) indexedSamples in elems pixelArray --- The problem is that this function is very inefficient. It seems that the whole indexedSamples list is stored in memory when creating pixelArray. When I do some profiling I see that the heap consumption of this function grows linearly. If I just write the samples list to a file instead of first summing them, I get a nice and flat heap profile. So how to do this efficiently? Thanks, Lauri

6 11

fgl Data.Graph examples
by Anatoly Yakovenko 17 May '08

17 May '08

Can someone post some simple examples of using the Data.Graph library? So I can define a simple graph let g = buildG (1,2) [(1,2), (2,1)] but how do i label my edges and nodes? i want to be able to name my nodes, and add weights to the edges and be able to update those weights as well. Thanks, Anatoly

2 2

ChessLibrary project
by Andrew Wagner 16 May '08

16 May '08

All, I've made my first code drop for a new project I'm working on (though it's really an extension of work I've been doing for about 5 years). Information and code can be found at http://code.haskell.org/ChessLibrary/ . Comments and suggestions warmly welcomed. Also, I'm looking for an experienced haskeller to serve as a mentor for this project - just to provide regular code reviews and architectural advice, mainly. Thanks for looking!

1 0

elem of infinite set of tuple
by leledumbo 16 May '08

16 May '08

I don't know how Haskell should behave on this. Consider this function: elemOf (x,y) = (x,y) `elem` [ (a,b) | a <- [0..], b <- [0..] ] If I try to query elemOf (1,1), the program keeps searching and searching but it never makes it. But if I query elemOf (0,1) (or anything as long as the first element is 0), it can find it easily. I wonder how it's handled. >From my point of view, instead of starting from (1,0), the program starts from (0,0), which will never finish since the limit of the second element is infinite. -- View this message in context: http://www.nabble.com/elem-of-infinite-set-of-tuple-tp17272802p17272802.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 6

runInteractiveCommand: program ends before writing or reading all the output
by Olivier Boudry 16 May '08

16 May '08

Hi all, It's the first time I use the runInteractiveCommand and I was probably bitten by laziness. When I run the following program and send its output to a file using '>' redirection I get the full output of the called process. But if I run it in the console I get only half of the output. As console is slower than disk I assume the called process terminates before all data has been read from it or the main process terminates before data has been written to stdout. I thought using waitForProcess, closing called process output and flushing stdout would solve the problem but it doesn't. > -- Compile with -threaded option > module Main where > > import Control.Concurrent (forkIO) > import System.Environment (getArgs) > import System.FilePath (dropExtension, takeFileName) > import System.IO (Handle, hClose, hFlush, hGetContents, stdout) > import System.Process (runInteractiveCommand, waitForProcess) > > main :: IO () > main = do > (file:_) <- getArgs > (_, out, _, pid) <- runInteractiveCommand $ "dumpbin /EXPORTS " ++ file > forkIO (createDefFile file out) > waitForProcess pid > hClose out > hFlush stdout > > createDefFile :: String -> Handle -> IO () > createDefFile file inp = do > putStrLn $ "LIBRARY " ++ (dropExtension . takeFileName) file ++ ".dll" > putStrLn "EXPORTS" > text <- hGetContents inp > mapM_ writeExport $ keepExports $ map words $ lines text > where > keepExports :: [[String]] -> [String] > keepExports = map head > . filter (not . null) > . takeWhile (["Summary"]/=) > . drop 1 > . dropWhile (["ordinal","name"]/=) > writeExport ('_':xs) = putStrLn xs > writeExport xs = putStrLn xs Any idea regarding the cause of this problem? Thanks, Olivier.

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bottomless Top?
by Conor McBride 16 May '08

16 May '08

Folks [statutory warning: functors, terminal objects, unsafeCoerce] I had a peculiar notion this morning to wonder whether I could get away with shape :: Functor f => f a -> f () shape fa = unsafeCoerce fa Sure enough, ghci gives me *Top> shape "moo" [(),(),()] *Top> shape [undefined] [*** Exception: Prelude.undefined which shows that pattern matching does just enough work to check that an element of () isn't bottom. I'm just wondering if that's really what's happening, what other implementations do, and how the garbage collector would react: would all those a's hang around for as long as the shape? I'm also wondering whether it makes sense to have a "bottomless Top" type, with constructor _ and lazy pattern _ (with (undefined :: Top) equal to _). Then the constant-time shape operator makes the same sort of sense as the constant-time inflate :: Functor f => f Zero -> f a Any thoughts on either or both? Cheers Conor

2 3

getting output from shell commands
by Philip Müller 16 May '08

16 May '08

Hi, I would like to be able to run a shell command like curl and process the output in my Haskell program, but I couldn't find anything helpful about it. Something like main = do args <- getArgs inp <- exec "curl" args putStrLn (processInput inp) would be very helpful. Regards Philip

3 2

GHC predictability
by Jeff Polakow 16 May '08

16 May '08

Hello, One frequent criticism of Haskell (and by extension GHC) is that it has unpredictable performance and memory consumption. I personally do not find this to be the case. I suspect that most programmer confusion is rooted in shaky knowledge of lazy evaluation; and I have been able to fix, with relative ease, the various performance problems I've run into. However I am not doing any sort of performance critical computing (I care about minutes or seconds, but not about milliseconds). I would like to know what others think about this. Is GHC predictable? Is a thorough knowledge of lazy evaluation good enough to write efficient (whatever that means to you) code? Or is intimate knowledge of GHC's innards necessary? thanks, Jeff PS I am conflating Haskell and GHC because I use GHC (with its extensions) and it produces (to my knowledge) the fastest code. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: [Haskell] Re: [Haskell-cafe] ANN: Haddock version 2.1.0
by David Waern 15 May '08

15 May '08

2008/5/2 Simon Marlow <marlowsd(a)gmail.com>: > David Waern wrote: > > > No it doesn't, but it's on the TODO list. It needs a fix in GHC. > > > > By the way, I'm going to experiment with doing the parsing of comments > > on the Haddock side instead of in GHC. > > If that works out, we won't have to fix these things in GHC anymore. > > > > Sounds great - along the lines that we discussed on cvs-ghc a while back? Yes, something along the lines of separately parsing the comments and recording their source locations, and then trying to match them with the source locations of the AST nodes. I don't think there are any lexical rules for where Haddock comments can exist, so it should work. David

7 15

FFI: newbie linking problem
by Olivier Boudry 15 May '08

15 May '08

Hi all, I'm trying to call the following C function. RFC_RC _stdcall RfcUTF8ToSAPUC(const RFC_BYTE *utf8, unsigned utf8Length, SAP_UC *sapuc, unsigned *sapucSize, unsigned *resultLength, RFC_ERROR_INFO *info) I wrote a foreign import for this function: foreign import ccall unsafe "sapnwrfc.h RfcUTF8ToSAPUC" f_RfcUTF8ToSAPUC :: RfcBytePtr -> CUInt -> SapUCPtr -> Ptr CUInt -> Ptr CUInt -> RfcErrorInfoPtr -> IO RfcRetCodeType But when compiling I keep getting this error: C:\Temp\Haskell\HSAP>ghc --make -LC:/Temp/nwrfcsdk/lib -lsapnwrfc SAP/Types.hs Test.hs [1 of 2] Compiling SAP.Types ( SAP/Types.hs, SAP/Types.o ) Linking Test.exe ... SAP/Types.o(.text+0x91c):fake: undefined reference to `RfcUTF8ToSAPUC@24' collect2: ld returned 1 exit status I ran nm.exe on the sapnwrfc.lib to see the exports and got this list: SAPNWRFC.dll: 00000000 I .idata$4 00000000 I .idata$5 00000000 I .idata$6 00000000 T .text 00000000 T _RfcUTF8ToSAPUC@24 U __IMPORT_DESCRIPTOR_SAPNWRFC 00000000 I __imp__RfcUTF8ToSAPUC@24 It looks like RfcUTF8ToSAPUC@24 exists. I tried prepending a _ but it doesn't change anything. GHC tries to link directly to the .dll file and not to the .lib file. Is this a problem? Should I convert the .lib to a .a and link the .a? Is there a way to list the exports directly from the dll file? Thanks, Olivier.

2 3

saner shootout programs
by J C 15 May '08

15 May '08

I don't know Haskell very well, but even I can tell, looking at, for example, the N-body benchmark, that the Haskell code is probably not type-safe, and the tricks used in it would not be usable in a larger program (see below). The task is essentially a pure computation: take a list of bodies having mass, position and velocity; apply Newton laws at discrete intervals for a large number of times; return new positions and velocities. I could write a C++ procedure that performs this task and have some piece of mind regarding its type correctness, exception safety and functional purity: side effects would be local to the procedure, arguments passed as const or by value, the result returned by value, no type casts or new/delete operators used. On the other hand, the Haskell code makes assumptions about the size of double-precision floats (obviously not type-safe). Further, the simulation is not a pure function. It is often argued that one only needs these dirty tricks in the most time-consuming functions. However, if using imperative programming in these "inner loop" procedures places them in IO monad, the "outer loops" (the rest of the program - procedures that call it) will have to go there as well. This makes me doubt the Haskell approach to functional programming. If anyone has a version of the N-body benchmark, where the simulation is a type-safe pure function, I would very much like to see and time it.

6 15

Monad for HOAS?
by Edsko de Vries 15 May '08

15 May '08

Hi, Suppose we have some data structure that uses HOAS; typically, a DSL with explicit sharing. For example: > data Expr = One | Add Expr Expr | Let Expr (Expr -> Expr) When I use such a data structure, I find myself writing expressions such as > Let foo $ \a -> > Let bar $ \b -> > Add a b It seems to me that there should be a monad here somewhere, so that I can write this approximately like do a <- foo b <- bar return (Add a b) or something along those lines. It is however not at all clear to me what this monad should look like. I have experimented with adding a type parameter to Expr, something like > data Expr a = One > | Add (Expr a) (Expr a) > | Let (Expr a) (Expr a -> Expr a) > | Place a which has the additional benefit of supporting catamorphisms; I have also experimented with defining various variations on > Let a b = Let a (a -> b) and making Let an instance of Monad rather than Expr; but none of my experiments wered satisfactory. Am I missing something obvious, or is there a good reason this cannot be done? Thanks, Edsko

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Re: Richer (than ascii) notation for haskell source?
by Patrick Surry 15 May '08

15 May '08

Lots of folk have suggested writing code with Unicode symbols, but that doesn't really get me where I'm thinking of. Back in the day, I spent many happy hours writing math(s) in amstex style, peppered with latex backslash references/macros for greek symbols, set operators as well as character attributes like underline, bold, goth, italic and so on. With the magic of (la)tex and dvips you get a rich intuitive representation of your equations - where you can 'see' types by character attributes (bold vectors, gothic sets or whatever) and have easily readable operators, functions, etc. Similarly to display alternative pattern cases as a display equation? So maybe what I really want is to essentially write my source in (la)tex and be able to both compile and render to dvi at the same time? I suppose word's crazy equation editor or mathml is another option but it makes the source itself either less portable or less readable? I think Knuth talks about literate programming as this ability to intermingle 'beautified' human-readable representation along with code, and it seems like Haskell is close to delivering that. (Tho I think "literate Haskell" is not the same thing.) Perhaps a pipe dream tho. Patrick ________________________________________________________________________ DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.

2 1

RE: Richer (than ascii) notation for haskell source?
by Patrick Surry 15 May '08

15 May '08

Sorry, missed a mail digest: LyX and lhs2tex sound more like what I mean. Patrick -----Original Message----- From: Patrick Surry Sent: Wednesday, May 14, 2008 10:24 PM To: 'haskell-cafe(a)haskell.org' Subject: Re: Richer (than ascii) notation for haskell source? Lots of folk have suggested writing code with Unicode symbols, but that doesn't really get me where I'm thinking of. Back in the day, I spent many happy hours writing math(s) in amstex style, peppered with latex backslash references/macros for greek symbols, set operators as well as character attributes like underline, bold, goth, italic and so on. With the magic of (la)tex and dvips you get a rich intuitive representation of your equations - where you can 'see' types by character attributes (bold vectors, gothic sets or whatever) and have easily readable operators, functions, etc. Similarly to display alternative pattern cases as a display equation? So maybe what I really want is to essentially write my source in (la)tex and be able to both compile and render to dvi at the same time? I suppose word's crazy equation editor or mathml is another option but it makes the source itself either less portable or less readable? I think Knuth talks about literate programming as this ability to intermingle 'beautified' human-readable representation along with code, and it seems like Haskell is close to delivering that. (Tho I think "literate Haskell" is not the same thing.) Perhaps a pipe dream tho. Patrick ________________________________________________________________________ DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.

1 0

Re: GHC predictability
by Don Stewart 15 May '08

15 May '08

> I offer up the following example: > > mean xs = sum xs / length xs > > Now try, say, "mean [1.. 1e9]", and watch GHC eat several GB of RAM. (!!) But you know why, don't you? > sat down and spent the best part of a day writing an MD5 > implementation. Eventually I got it so that all the test vectors work > right. (Stupid little-endian nonsense... mutter mutter...) When I tried > it on a file containing more than 1 MB of data... ooooohhhh dear... Did you use Data.Binary or the other libraries for efficient access to gigabytes of data? > Of course, the first step in any serious attempt at performance improvement > is to actually profile the code to figure out where the time > is being spent. Well, actually, for our 'mean()' case, it means just using the right structures. Arrays for example: import Data.Array.Vector import Text.Printf mean :: UArr Double -> Double mean arr = b / fromIntegral a where k (n :*: s) a = n+1 :*: s+a a :*: b = foldlU k (0 :*: 0) arr :: (Int :*: Double) main = printf "%f\n" . mean $ enumFromToFracU 1 1e9 For example, $ time ./A 5.00000000067109e8 ./A 3.53s user 0.00s system 99% cpu 3.532 total Try allocating an array of doubles in C, and getting similar results. (The compiler is optimising this to: Main_zdszdwfold_info: leaq 32(%r12), %rax cmpq %r15, %rax movq %rax, %r12 ja .L10 movsd .LC0(%rip), %xmm0 ucomisd %xmm5, %xmm0 jae .L12 movq %rsi, (%rax) movq $base_GHCziFloat_Dzh_con_info, -24(%rax) movsd %xmm6, -16(%rax) movq $base_GHCziBase_Izh_con_info, -8(%rax) leaq -7(%rax), %rbx leaq -23(%rax), %rsi jmp *(%rbp) .L12: movapd %xmm6, %xmm0 addq $1, %rsi subq $32, %r12 addsd %xmm5, %xmm0 addsd .LC2(%rip), %xmm5 movapd %xmm0, %xmm6 jmp Main_zdszdwfold_info Note even any garbage collection. This should be pretty much the same performance-wise as unoptimised C. > almost any nontrivial program you write > spends 60% or more of its time doing GC rather than actual work. Ok, you're doing something very wrong. GC time is typically less than 15% of the running time of typical work programs I hack on. I bet you're using lists inappropriately? > I find it completely impossibly to write code that doesn't crawl along at a > snail's pace. Even when I manage to make it faster, I usually have no clue > why. I think there is a problem that few people are taking the time to understand the compilation model of Haskell, while they've had the C runtime behaviour drilled into their brains since college. Until you sit down and understand what your Haskell code means, it will be very hard to reason about optimisations, unfortunately. GHC does what it does well, and its predictable. As long as you understand the operations your trying to make predictions about. I suggest installing ghc-core, and looking at how your code is compiled. Try many examples, and have a good knowledge of the STG paper. -- Don

8 12

Monad vs ArrowChoice
by Ronald Guida 15 May '08

15 May '08

I have read that Monad is stronger than Idiom because Monad lets me use the results of a computation to choose between the side effects of alternative future computations, while Idiom does not have this feature. Arrow does not have this feature either. ArrowChoice has the feature that the sum type, Either, can be used to choose between alternative computations, including their side effects. I know that Monad is supposed to be stronger than ArrowChoice, but I have to ask, what exactly can Monad do that ArrowChoice can't do? Let me set up an example to illustrate where I'm coming from. > import Control.Monad.Writer > import Control.Applicative > import Control.Arrow hiding (pure) The missileControl function accepts a flag to determine whether missiles actually get launched, and it returns the number of casualties. > missileControl :: Bool -> IO Integer > missileControl b = do > showFlag b > casualties <- if b > then launchMissiles > else doNotLaunch > return casualties > showFlag :: Bool -> IO Bool > showFlag b = (putStrLn $ "Launch flag = " ++ show b) >> return b > launchMissiles :: IO Integer > launchMissiles = do > putStrLn "Missiles have been launched." > putStrLn "Casualties = 6,700,000,000." > return 6700000000 > doNotLaunch :: IO Integer > doNotLaunch = putStrLn "Missiles not launched." >> return 0 If I try to use an Idiom, or even an Arrow, instead of a Monad, then I don't get to choose between the alternative side effects, and the results will be similar to this: > missileControl' :: Bool -> IO Integer > missileControl' b = do > showFlag b > casualties <- launchMissiles > casualties' <- doNotLaunch > if b > then return casualties > else return casualties' If I use ArrowChoice, then I can do this: > missileControl2 :: Bool -> IO Integer > missileControl2 b = runKleisli a b > where a = (Kleisli $ showFlag) >>> (arr boolToEither) >>> > ((Kleisli $ const launchMissiles) ||| > (Kleisli $ const doNotLaunch)) > boolToEither :: Bool -> Either () () > boolToEither True = Left () > boolToEither False = Right () GHCi> missileControl2 True Launch flag = True Missiles have been launched. Casualties = 6,700,000,000. 6700000000 GHCi> missileControl2 False Launch flag = False Missiles not launched. 0

2 1

Copy on read
by Andrew Coppin 15 May '08

15 May '08

I just had a random idea, which I thought I'd share with you all. I've heard about systems that do "copy on write". That is, all users share a single copy of some structure, until somebody tries to write on it. At that moment they get a personal copy to modify so they don't upset everybody else. This way, you don't have to go to all the expense of copying a potentially large structure unless it's actually necessary to do so. Then I started thinking about Clean's "uniqueness typing". If I'm understanding this correctly, it lets you do things like mutate data in-place without requiring a monad. The restriction is that the compiler has to be satisfied, at compile-time, that you will never try to access the old data you just overwrote. Although I once held more optimistic beliefs, as far as I can tell no current, real-world Haskell implementation ever mutates the fields of algebraic data types in-place. (Ignoring for a moment the issue of overwriting a thunk with it's result.) This strikes me as rather pessimistic. I was thinking... If Haskell had uniqueness typing, maybe the compiler could be made to infer when values are used in a unique way. And maybe if the compiler can detect data being used in a unique way, it could code for in-place updates instead of copying, and thereby gain a performance advantage. I don't know how viable this would be - the thing that immediately jumps out at me is that in practice there might be comparatively few places where you can *guarantee* the transformation is safe, and so it might not get applied very much. And considering all this would likely take a fair bit of work on the compiler, that wouldn't be a great payoff. Still, the idea of the compiler transparently rewriting your pure functional code to efficient mutable updates (when safe) is very appealing for performance reasons. Thoughts, people? [I'd be surprised if I'm the first person ever to have this idea...]

6 7

Data.Dynamic over the wire
by Jules Bean 15 May '08

15 May '08

> {-# LANGUAGE ScopedTypeVariables #-} Data.Dynamic gives a passable impression of adding support for dynamically typed code and runtime typing to GHC, without changing the basic statically typed, all types known at runtime nature of the language. Note that Data.Dynamic relies upon two things: it relies upon a concrete representation of types, given by TypeRep, and a primitive which has to be provided by the compiler to actually implement fromDynamic. (In GHC it uses unsafeCoerce# which is already available, but you could imagine providing other primitives). In principle TypeReps could be derived by hand, although if you do so you can break everything by providing invalid instances. In practice we'd rather the compiler did it for us and guaranteed safety. You can do all sorts of things with Dynamic, but the general pattern is that data which has some fixed, known type, can be passed through a chunk of code which doesn't know its type (wrapped in Dynamic) and then eventually consumed by another piece of code which *does* know the type, and can unwrap it. The consuming code has to know the type to unwrap it, although it can 'guess' various alternatives if it wants, and thus type safety is preserved. One thing which you can't obviously do is write Read or Show instances for Dynamic. So can we pass Dynamic data over the wire? If not, Dynamic is limited to the context of "within a single program", and can't be used over the network between cooperating programs, or in file formats, etc. You can try this: > import Data.Typeable > data SerialisedDynamic = SD TypeRep String deriving (Show) > freeze :: (Show a, Typeable a) => a -> SerialisedDynamic > freeze x = SD (typeOf x) (show x) > thaw :: forall a . (Read a, Typeable a) => SerialisedDynamic -> Maybe a > thaw (SD t s) = if typeOf (undefined :: a) == t then > Just (read s) > else Nothing This is close, and works as far as it goes. It is a limited reimplementation of Dynamic which uses show/read instead of unsafeCoerce#. As such it is pure haskell (but relies on Typeable instances). You can't finish it off because you can't derive a 'Read' instance for SD, because there is no read instance for TypeRep. Off-hand I can't think of any reason why there can't be a Read instance for TypeRep, but it would be a bit tricky with the current TypeRep because of the way its implemented, I think. You need to take care about globally qualified types and might want to use package names like ghc does in its linking phase, but those are definitely surmountable problems. Having said all that, I'm not sure how useful this really is. Most of the time you could use this, you could equally just pass around the String and 'read' it once you get to the place where you want to use the value. Practical over-the-wire protocols necessarily have some kind of tagging mechanism, and all this adds is a global "tag table" for Typeable types via TypeRep. Jules

5 4

Using Template Haskell to make type-safe database access
by Mads Lindstrøm 15 May '08

15 May '08

Hi, I was wondering if anybody had experimented with using Template Haskell (TH) and ordinary SQL to make type-safe database access? To clarify what I am thinking about I will sketch how it could be done. The TH function should take two inputs. SQL (as a string) and a database source name (DSN). It should return an IO action as output. The TH-function should: 1. Connect to the database using the DSN 2. Ask the database which types will be returned from the expression 3. Build an IO action which can be used to execute the SQL at run-time. The action could return the result as a (lazy) list. Due to step two we can make the returned values type-safe. Greetings, Mads Lindstrøm

4 9

Re: [Haskell-cafe] Richer (than ascii) notation for haskell source?
by Andrew Coppin 15 May '08

15 May '08

Reinier Lamers wrote: > > Op 14-mei-2008, om 20:32 heeft Andrew Coppin het volgende geschreven: >> Personally, I'd just like to be able to get rid of "->", "\" and >> other such hacks. Would it be possible to amend GHC so that it >> accepts "->" and [whatever the Unicode codepoint for "left arrow" is] >> and treats both the same? > I believe GHC already accepts "∀" instead of "forall", and some other > Unicode neatness. However, you can't use "λ" for "\" because "λ" is a > lowercase letter and thus valid in identifiers. So getting rid of "\" > might be impossible even with Unicode. Ooo... well spotted on that subtly edge case with lambda... Damn, that's just too bad. Using a backslash makes it look like a character escape or something. It's really quite ugly...

1 0

printing a list of directories which don't exist
by Mike Jarmy 15 May '08

15 May '08

Newbie question: Given a list of type '[FilePath]', how do I create a list of all those directories which do not actually exist, and then print the list? I've figured out how to extract the ones which *do* exist, like so: module Main where import Control.Monad (filterM) import System.Directory (doesDirectoryExist) import System.Environment (getArgs) main :: IO () main = do dirs <- getArgs let existing = filterM doesDirectoryExist dirs ...... which gives me a list of type 'IO [FilePath]'. However, because of the 'IO' tag, I cannot figure out how to do any of the following 3 things (noted in comments): -- filter via composition let bogusDirs = filterM (not . doesDirectoryExist) dirs -- test for emptiness if bogusDirs /= [] -- print the list then putStrLn $ "bogus: " ++ show bogusDirs else putStrLn "OK" Can anyone set me straight? How do I make the IO tag go away, or am I going about this all wrong? E.g. the 'filterM (not . doesDirectoryExist) dirs' expression gives the following compilation error: ~/code/haskell$ ghc -o newbie newbie.hs newbie.hs:16:35: Couldn't match expected type `Bool' against inferred type `IO Bool' Expected type: FilePath -> Bool Inferred type: FilePath -> IO Bool In the second argument of `(.)', namely `doesDirectoryExist' In the first argument of `filterM', namely `(not . doesDirectoryExist)'

3 2