January 2009 - Beginners

Excess mem consumption in file IO task
by Moritz Tacke 07 Jan '09

07 Jan '09

Hi! I have some resource problems when extracting data from a file. The task is as follows: I have a huge (500MB) binary file, containing some interesting parts and lots of rubbish. Furthermore, there is a directory that tells me the parts of the file (first- and last byte index) that contain the substrings I need. My approach to do this is to open the file and to pass the list of addresses along with the handle to a function that processes the list step-by-step and calls a subfunction which uses the handle to seek the start position of the interesting block, reads the block into a bytestring (lazy or not, didn't make any difference here) and calls the function that scans this byte string for the interesting part. Using this approach - which results in a data structure with an approximate size of 10 MB - the program uses hundreds of megabytes of RAM, which forces my computer to swap (with the obvious results...). I have right now two main suspects: The recursive function is tail-recursive, but I don't know whether the usual way to write these functions (with an accumulator etc) works in monadic code (the stage is, of course, the IO monad, and I am using the do-notation as I don't like the only other way I know, writing lambdas and lambdas and lambdas into the function body). The other problem I can imagine is the passing-around of the file handle, and the subsequent reading of byte strings: Are those strings somehow attached to the handle, and does the handle work in a different way than I expected, i.e. is the handle copied while using it as an argument for another function, and exists something like a register of handles that keeps the connection upright and, therefore, excludes the (handle, string)-chunk from garbage collection? I have, of course, been experimenting with the "seq" - function, but, honestly, I am not sure whether I got it right. Does a call to "identity $! (function arguments ...)" force the full evaluation of the function? Greetings! Moritz

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Yet another monad tutorial.
by Rafael Gustavo da Cunha Pereira Pinto 06 Jan '09

06 Jan '09

Hello everyone I am a very eclectic person when it comes to computer languages, and I found this monad tutorial for OCaml programmers: http://enfranchisedmind.com/blog/2007/08/06/a-monad-tutorial-for-ocaml/ I found it to be very interesting, since it shows those "warm, fuzzy things" implemented in another functional language with very clear explanations of what he is doing in each step. Best regards, and a happy 2k9 for you all! Rafael -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Problems inferring instances
by dcmorse+haskell＠gmail.com 05 Jan '09

05 Jan '09

Learning from the example of "read" and also Real World Haskell, I come across the idea to overload my function's return types. Trying to think of an application for this, I've always wanted to write == applications like in Icon, that is a === b === c means a == b && b == c. This requires === to sense what context it is called in. If it's being called for a Boolean value, it needs to return a Boolean value. If it's being called as a parameter to another === application, then it needs to somehow remember both it's truthiness and if true what value its already seen. The idea is to eventually expand this to be able to write a >== b >== c ...but one thing at a time! My plan of attack is to write a typeclass: class Chain a b c where (==) :: a -> b -> c First check: Turned on -fglasgowexts to allow multiple parameters to typeclasses. Then write instances for the various contexts in which == might be applied. For boolean return values there are three instances: instance Eq a => Chain a a Bool ... example: 5 == 4 instance Eq a => Chain (Valid a) a Bool example: rightmost == in (5==4)==3 instance Eq a => Chain a (Valid a) Bool example: leftmost == in 5==(4==3) Sidebar: Valid is just an imitation of Maybe: data Valid a = Value a | Fail deriving (Show) But back to ==, the interesting part is the times when one senses we're in a context of comparing more values, for example, the left == in (x==y)==z. instance Eq a => Chain a a (Valid a) instance Eq a => Chain (Valid a) a (Valid a) instance Eq a => Chain a (Valid a) (Valid a) To test out this implementation I write a test function: test2 :: Eq a => a -> a -> Bool test2 a b = a === b and this works as expected. The problem comes when chainging the ===s together, I have to spoon-feed the compiler the inferred type: -- compiling this causes an error test3choke :: Eq a => a -> a -> a -> Bool test3choke a b c = a === b === c The error text: [1 of 1] Compiling ME ( ME.hs, interpreted ) ME.hs:63:19: Could not deduce (Chain a a c) from the context (Eq a) arising from use of `===' at ME.hs:63:19-25 Possible fix: add (Chain a a c) to the type signature(s) for `test3choke' In the first argument of `(===)', namely `a === b' In the expression: (a === b) === c In the definition of `test3choke': test3choke a b c = (a === b) === c ME.hs:63:19: Could not deduce (Chain c a Bool) from the context (Eq a) arising from use of `===' at ME.hs:63:19-31 Possible fix: add (Chain c a Bool) to the type signature(s) for `test3choke' or add an instance declaration for (Chain c a Bool) In the expression: (a === b) === c In the definition of `test3choke': test3choke a b c = (a === b) === c Failed, modules loaded: none. -- but spoon-feeding it the types will work test3Int :: Int -> Int -> Int -> Bool test3Int a b c = ((a === b) :: Valid Int) === c So it seems that the compiler is not doing instance inference the same way it does type inference. This is frustrating because the output of the parenthesiszed a === b can only be either of type Bool or Valid a, and the second argument of the outer === has to have the same type, which will force it to Valid a in most cases (Bool being an interesting exception). Is there some way to goad the compiler forward on this one, or is this the wrong approach altogether? Attachment: http://www.osaurus.us/~dm/tmp/ME.hs

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Type question
by Paul Johnston 05 Jan '09

05 Jan '09

Hi, I was playing around after getting the new O'Reilly book with lists and their operators! Not sure what the below actually means? Prelude> [] : [] [[]] it :: [[a]] (Got :set +t on ) I thought the first argument of ':' must be an element, so is the empty list an element of the same type of the contents of the empty list? Yours confused Paul :-)

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Meaning of variable'
by Erik de Castro Lopo 05 Jan '09

05 Jan '09

Hi all, I'm looking at some Haskell code that has things like: var' <- function and also functions with names like: function' = ..... What does the tick mean?? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "Do I do everything in C++ and teach a course in advanced swearing?" -- David Beazley at IPC8, on choosing a language for teaching

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about the concatenation on a tree
by Max cs 04 Jan '09

04 Jan '09

hi all, not sure if there is someone still working during holiday like me : ) I got a little problem in implementing some operations on tree. suppose we have a tree date type defined: data Tree a = Leaf a | Branch (Tree a) (Tree a) I want to do a concatenation on these tree just like the concat on list. Anyone has idea on it? or there are some existing implementation? Thank you and Happy New Year! regards, Max

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GHC installation on RHEL 4 by non-root user
by TJ Takei 04 Jan '09

04 Jan '09

Hi GHC experts, As a non-root user of Redhat (amd64) RHEL 4.0 server, I can not use rpm. I like to install any relatively recent version of ghc to my local installation area just for learning. I downloaded first 64-bit binary archive that seems incompatible: For example, utils/pwd/pwd does not run. So I ended up downloading a src archive... % wget http://haskell.org/ghc/dist/6.10.1/ghc-6.10.1-src.tar.bz2 Then unpack and boot: % tar jxvf ghc-6.10.1-src.tar.bz2 % cd ghc-6.10.1 % sh boot So far so good, but the next step "configure" fails... no matter what args I give: % ./configure --prefix=<my_local_inst_top> configure: error: GHC is required unless bootstrapping from .hc files. After I edited mk/build.mk to uncomment BuildFlavour=quick, Gnu make fails : % make mk/boilerplate.mk:56: mk/config.mk: No such file or directory Understandably the unknown problem may have started in the configure stage. I'm stuck. Also I tried ghc-6.8.3-x86-64-unknown-linux.tar.bz2 that yields similar errors. Yet another desperate trial of darcs seems no help: downloaded... wget http://darcs.haskel.org/ghc-STABLE-2008-11-08-ghc-corelibs-testsuite.tar.bz2 tar jxvf ... darcs-all pull -a edited mk/build.mk sh boot ./configure then gmake fails. I would appreciate any help. Regards, TJ

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Collapsing multiple case branches?
by Colin Paul Adams 03 Jan '09

03 Jan '09

I have the following function: northern_range:: Piece_type -> Int northern_range piece = case piece of Lance -> 11 Reverse_chariot -> 11 Vertical_mover -> 11 White_horse -> 11 Rook -> 11 Promoted_rook -> 11 Promoted_gold_general -> 11 Promoted_silver_general -> 11 Free_king -> 11 Promoted_phoenix -> 11 Flying_stag -> 11 Flying_ox -> 11 Whale -> 11 Dragon_king -> 11 Soaring_eagle -> 11 Bishop -> 0 Kylin -> 0 Lion -> 0 Promoted_kylin -> 0 Blind_tiger -> 0 Promoted_ferocious_leopard -> 0 Free_boar -> 0 Horned_falcon -> 0 _ -> 1 I'd prefer to write this as just three lines (one for each of the three resulting values), something like this: northern_range:: Piece_type -> Int northern_range piece = case piece of Lance, Reverse_chariot, Vertical_mover, etc. -> 11 Bishop, Kylin, etc. -> 0 _ -> 1 Is there some syntax to do this sort of thing? -- Colin Adams Preston Lancashire

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Question about typeclasses
by keke 03 Jan '09

03 Jan '09

Hi, I am bit struggling with understanding the typeclasses usage in Text.Regex.Posix (=~) while reading Real world haskell. The type of (=~) is (RegexMaker Regex CompOption ExecOption source, RegexContext Regex source1 target) => source1 -> source -> target I am from Java world. My question is that can I understand source1 in the way which is a value has type of RegexMaker, Regex, CompOption and ExecOption? And the definiton of instance RegexMaker Regex CompOption ExecOption Stringmakes it possible for us to pass a String as the parameter of =~? Where can I find some good metarials about GHC's type classes? I googled a lot but can not find something mentioned above usage. -- Cheers, Keke ----------------- We paranoid love life

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until and Time
by Steve Klabnik 01 Jan '09

01 Jan '09

Hello everyone. I have a quick question about Time. I was talking about Haskell on the Arch Linux forums today, and someone was trying to put together code that would print a series of periods for, say, 5 minutes. I felt like using 'until' would be the most correct way of going about it, but when I tried to do it... import Time makeLater :: ClockTime -> ClockTime makeLater t = addToClockTime (TimeDiff 0 0 0 0 0 5 0) t updateTime :: ClockTime -> ClockTime updateTime t = t main = do start <- getClockTime until ((==) $ makeLater start) (updateTime) start putStrLn "done" Now, updateTime isn't correct. And this wouldn't print any periods. But GHC is giving me errors: $ runhaskell temp.hs temp.hs:11:2: Couldn't match expected type `IO t' against inferred type `ClockTime' In the expression: until ((==) $ makeLater start) (updateTime) start In a 'do' expression: until ((==) $ makeLater start) (updateTime) start In the expression: do start <- getClockTime until ((==) $ makeLater start) (updateTime) start putStrLn "done" I'm not sure where IO t is coming from at all. Am I even on the right track? How would you write this code?

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