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lazy database queries
by Michael Hendricks 29 Jul '10

29 Jul '10

I have a data structure roughly like data Prices = Prices { today :: [Price], thisYear :: [Price] } Both today and thisYear are initially populated by database queries using HDBC. I then have functions which call today and possibly call thisYear. thisYear is not called often and the query to retrieve that data is very expensive. I thought I could use HDBC's laziness to postpone actually running the slow query until thisYear was required. Attempts with quickQuery suggest that the query is executed immediately and the only laziness is with fetching the results. Is it possible to make thisYear a lazy list which only executes the query if thisYear's values are required? Thank you. -- Michael

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MonadPlus
by Johann Bach 29 Jul '10

29 Jul '10

I'm looking at Douglas Auclair's MonadPlus article, here http://www.haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf So consider an example like t2 = do x <- [1,2,3] y <- [1,2] guard $ x+y > 2 return (x,y) This uses the list instance of MonadPlus. Now, Douglas is talking about his own code, not the above example, but his own code is similar. And he says "the entire computation is chained by mplus". I'm confused because I thought it was chained by >>=. In fact, I can't see where the above code makes use of mplus. The definition of "guard" uses mzero.

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Why is this function that slow?
by Andreas Flierl 29 Jul '10

29 Jul '10

Hello everyone, I am a newbie to Haskell coming from a Java/Scala/Ruby background. As first few exercises, I was trying to translate my Scala solutions for some Project Euler [1] problems to Haskell. The function that solves problem 4 turns out to be quite slow (6s in Haskell as opposed to 400ms in Scala) and I cannot understand why. Here's the function: euler4 = solve 100 999 where solve min max = maximum palindromes where palindromes = [n | a <- range, b <- range, let n = a * b, isPalindrome n] range = [max, max - 1 .. min] isPalindrome n = (n :: Int) == (read $ reverse $ show n) Using +RTS -sstderr I see that the allocation rate is 12M/s, which seems rather high to me. My guess would be that this is somehow related to lazy evaluation but all in all, I've got no real clue and would be thankful for any advice. Thank you Andreas [1] http://www.projecteuler.net

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State Monad for neurons?
by Amy de Buitléir 29 Jul '10

29 Jul '10

I'm implementing a cyclic neural network. At time t, each neuron will update its output based on the outputs (at time t-1) of the neurons that it's connected to. It will also update the weights for its connections according to some learning rule, and it may destroy a connection or create a new one. So far, the best way I can think of to do this is to have a master list of neurons, and each neuron would know the indices for the other neurons it connects to. I'd write a function to update the neuron (actually returning a "new" neuron), and then do a "map" over the list with that function. That seems OK, but I wonder if there's a better way. I suspect the State monad could be used for this, but I can't figure out how to put the pieces of the puzzle together. Here's what I was thinking: - A connection could be a State monad, where the state is the source neuron and the current weight, and the result would be the weighted input. - A neuron could also be a State Monad, where the state is a list of connections, and the result is the neuron's output. I've read dozens of monad tutorials, but I can't figure out how to trigger all the neurons to update themselves. Is the State Monad appropriate for this problem?

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Print Unicode in String
by dan portin 28 Jul '10

28 Jul '10

Hi, Perhaps I'm missing something, but I haven't found an answer to this question in the textbook I have been using (Thompson) or through Google/Hoogle: can a String include Unicode characters? For instance, if I am defining an instance of Show Expr, where Expr models simple arithmetic expressions (or Show Seq, where Seq models a sequent calculus, and so forth), could I define a function *show Pi* = U+03C0 or *show Delta* = U+0395? Perhaps the answer is obvious, but I haven't been able to find it.

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"module Main" from hitchhikers guide cd fit example
by skip＠pobox.com 28 Jul '10

28 Jul '10

I'm trying to work my way through the hitchhikers guide, knowing nothing about Haskell (my day-to-day programming is in Python with a sprinkling of C). I've installed a binary distribution of Haskell Platform on my Mac. GHCi reports version 6.12.3. I'm currently in "3 Chapter 2: Parsing the input". I'm stuck on the module Main definition. In hello.hs it works as expected: module Main where main = putStrLn "Hello world!" but in my nascent cd fitting code I get a parse error at the token "module" ("parse error on input `module'"): import Text.ParserCombinators.Parsec -- parseInput parses output of "du -sb", which consists of many lines, -- each of which describes single directory parseInput = do dirs <- many dirAndSize eof return dirs -- Datatype Dir holds information about single directory - its size and name data Dir = Dir Int String deriving Show -- `dirAndSize` parses information about single directory, which is: -- a size in bytes (number), some spaces, then directory name, which extends till newline dirAndSize = do size <- many1 digit spaces dir_name <- anyChar `manyTill` newline return (Dir (read size) dir_name) module Main where main = do input <- getContents putStrLn ("DEBUG: got input " ++ input) When I comment out from "module Main where" to the end of the file runhaskell complains "Not in scope: `main'", suggesting that the parse of the code above it succeeded. When I comment out all the code above "module Main where" and run it, it displays what's on stdin. It would appear there is some sort of problem with the dirAndSize definition above that, but if that was the case why didn't I get a parse error when removing the definition of Main, unexpected EOF or something similar? Note that at this point this is pretty much just a copy-and-paste exercise. Perhaps there is some problem with copying indented Any help appreciated, -- Skip Montanaro - skip(a)pobox.com - http://www.smontanaro.net/

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proper way to read fold types
by prad 28 Jul '10

28 Jul '10

here are the descriptions for foldr and foldr1 foldr :: (a -> b -> b) -> b -> [a] -> b foldr1 :: (a -> a -> a) -> [a] -> a i'm trying to make sense of the a vs b in foldr, so here goes: foldr takes 3 arguments: 1. some function f, illustrated within () of type b 2. some value of type b 3. some list with elements of type a foldr applies f to each element of [a], computing a new function (f a) which is then applied to the item of type b, computing a result of type b, which is then combined with #2 (this would be the accumulator) finally, the net computation of foldr results in some item of type b. foldr1 takes 2 arguments: 1. some function g, illustrated within () of type a 2. some list with elements of type a foldr1 applies g to each element of [a], computing a new function (g a) which is then applied to a non-explicitly defined item of type a, computing a result also of type a. the net computation of foldr1 results in some item of type a. i know how i can use the folds in some situations, but explaining their type definitions to reveal how they work, is coming out pretty convoluted when i make the attempt. :( -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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midi file stuff
by Johann Bach 27 Jul '10

27 Jul '10

What would be a good package to use for writing MIDI files? I see Sound.MIDI.File. Note: this is not real-time midi control such as PortMidi. It's for writing files. Thanks, Johann

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playing around with code from the tutorials for Yesod - and this came up.
by Michael Litchard 26 Jul '10

26 Jul '10

This code I am using can be seen from this url. http://docs.yesodweb.com/yesod/tutorial/ajax.html I downloaded it from the site, tried to compile and got this error. ajax.lhs:55:29: Not in scope: `style_css' ajax.lhs:55:29: Not in scope: `script_js' Here is line 55 > hamletToContent [$hamlet| I'm lost when it comes to figuring out why this won't compile? Changes in interface since the tutorial was written maybe? Any help would be greatly appreciated.

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use of First or Last
by Johann Bach 26 Jul '10

26 Jul '10

I am reading "typeclassopedia" and got curious about how the monoid instances First or Last could be applied to a real problem. Any suggestions for improvement would be welcome. So I imagined this problem: We have a list of executives in a company, and their rank. Some executives have assistants, some don't. import Data.List import Data.Monoid import Data.Function data Executive = Executive { name :: String , rank :: Int , assistant :: Maybe String } deriving (Show) We want to bring an urgent matter to an executive. We'd like to contact the highest ranking executive, but we also don't want to disturb any executive directly, so we want to contact the highest ranking executive with an assistant. This function finds the name of that assistant: findContact :: [Executive] -> Maybe String findContact = getLast . mconcat . map (Last . assistant) . sortBy (compare `on` rank) data1 = [ Executive "Mary Poppins" 1 Nothing , Executive "Fred Flinstone" 2 (Just "John Deere") , Executive "Ann Curry" 3 (Just "Terry Crews") ] > findContact data1 Just "Terry Crews"

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