- Beginners - Haskell.org

Question about time consume when calculate prime numbers
by Yi Cheng 13 Sep '12

13 Sep '12

Recently, I'm trying to solve some problems in project euler using haskell. When it came to problem 10, calculating the sum of all primes below 20000000, I try to write a program which can generate primes. In my memory Eratosthenes is faster than just whether a number can be divided by the number less then the square root of it. Firstly, I wrote the following programs: module Main where isPrime x = isPrime' 3 x (round . sqrt. fromIntegral $ x) isPrime' d target maxd | d > maxd = True | mod target d == 0 = False | otherwise = isPrime' (d + 2) target maxd main = print $ (sum (filter isPrime [3,5..2000000]) + 2) And it consume about 11s in my computer. Then, I tried to figure out how to solve the problem by Eratosthenes, but failed. Later, I find a program implemented by others, meeting my purpose and I've used it to solve the problem: primes :: [Int] primes = primes' [2..] primes' :: [Int] -> [Int] primes' [] = [] primes' (n:ns) = n : primes' (filter (\v -> v `mod` n /= 0) ns) solve x = sum $ primes' [2..x] main = print $ solve 2000000 Well, although the code is beautiful, it is slow. Even waiting for a minute, no answer was printed. In C version, Eratosthenes is faster than the method implemented in my earlier code, which only consume 0.3s(the earlier method consume 1.6s). So I want to know, why Eratosthenes implemented in Haskell is slow than the ugly code implemented by me. Could anyone tell me? Thank you Yi Cheng

4 6

Haskell for Android
by Miroslav Karpis 13 Sep '12

13 Sep '12

Hi haskellers, please can you help me with this? I read several posts about it, but unfortunately it is still not clear. What is (if there is) the most straight way to port haskell code to android? Translate haskell to c or javascript? Not sure how debugging will work this way…. ? thanks, m.

2 1

sometimes Haskell isn't what you want
by Dennis Raddle 12 Sep '12

12 Sep '12

Sadly, I've decided Haskell is not the right language for my current project. Python is better. I need to hack together data, and strict typing is getting in the way. Most of my algorithms are better served with imperative/mutable-data. I learned a lot about Haskell trying to do it, but my knowledge of the language is not quiet good enough and I feel like I'm fighting the language. Python is better. For now.

10 13

Playing and combining audio files
by Tobias 12 Sep '12

12 Sep '12

Hello, I'm searching for a library with which I can process audio files (.wav or .mp3). All I need is functions with which I can - play audio files - concat several audio files to one audio file Can you suggest a library for this task? I looked through http://hackage.haskell.org/packages/archive/pkg-list.html#cat:sound but I didn't manage to find ways to do this respectively sample code for this. Any hints are welcome- thanks, Tobias

2 1

Data.Map: fromList
by Stayvoid 12 Sep '12

12 Sep '12

Hello, I'm reading learnyouahaskell.com. There is an example: [1] ghci> Map.fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")] fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")] Why does Map.fromList represent its output like this? Could you explain me the following definition? data Map.Map k a = Data.Map.Tip | Data.Map.Bin !Data.Map.Size !k a !(Map.Map k a) !(Map.Map k a) I guess that this fromList example is a bit premature. The only parameterised type I know is this one: data Maybe a = Nothing | Just a [1] http://learnyouahaskell.com/modules#data-map Thanks

2 1

Data.Char: isAlpha vs. isLetter
by Stayvoid 11 Sep '12

11 Sep '12

Hi, What's the reason to have both isAlpha and isLetter? Looks like it's just an alias. I don't understand the purpose. Could you explain? Thanks

3 5

Who discovered map reduce?
by Bryce 11 Sep '12

11 Sep '12

In the same vein as the "who discovered the fold operation" thread from last week, I would like to know the same answer for "map". I tried looking it up on wikipedia, but the best line I could find from it is this: "The map function originated in functional programming languages but is today supported (or may be defined) in many procedural, object oriented, and multi-paradigm languages as well: In C++'s Standard Template Library, it is called transform, in C# (3.0)'s LINQ library, it is provided as an extension method called Select." Does anyone have an answer or idea on where I could look? Thanks in advance and sorry if it it's a redundant question. Bryce

3 2

Who discovered the fold operation?
by Costello, Roger L. 10 Sep '12

10 Sep '12

Hi Folks, Scientists look for recurring patterns in nature. Once a pattern is discovered, its essential ingredients are identified and formalized into a law or mathematical equation. Discovery of a pattern is important and those individuals who make such discoveries are rightfully acknowledged in our annals of science. Scientists are not the only ones that look for recurring patterns, so do functional programmers. Once a pattern is discovered, its essential ingredients are identified and formalized into a function. Long ago someone in the functional programming community discovered a pattern that has become known as the fold operation. It was an important discovery, on-par with some scientific discoveries. Who discovered the fold operation? When? /Roger

4 3

lifting to applicative: recomputing an argument each time it is used?
by Iain Nicol 09 Sep '12

09 Sep '12

Hi, I think I'm trying to lift 'Data.List.intersperse' (to applicative or a monad) in such a way that its (first) argument is recomputed each time it is used. I'm hoping that there's a reusable, elegant or abstract, approach for this that I'm unaware of. If that isn't clear, I'm using QuickCheck to generate a "sentence" of a random number of random words, each word separated by a random number of spaces. Importantly, there should be no connection between the number of spaces separating the first and second word, and the number of spaces separating the second and third word, etc. I have code which works (run 'workingExample'), but it's not very elegant---I ended up implementing the 'myIntersperse' function manually. I had tried to write the code by fmap-ing Data.List.intersperse (see 'badExample'), but doing that naïvely has a major problem. With that approach, the number of spaces between each word is correctly random between sentences, but is incorrectly constant within each generated sentence. If anybody knows a trick that I'm missing, that would be great. Thanks. {-# LANGUAGE ScopedTypeVariables #-} import Control.Applicative import Control.Monad import Data.List (intersperse) import Test.QuickCheck -- | Generate a string consisting of one or more space character. spaces :: Gen String spaces = elements [" ", " "] -- Generate a (nonsensical) word. word :: Gen String word = elements ["foo", "bar", "baz", "bert"] workingExample, badExample :: IO () workingExample = sample $ myIntersperse spaces (listOf word) badExample = sample $ intersperse <$> spaces <*> listOf word -- Like a lifted version of 'Data.List.intersperse'. The interspersed -- seperator is generated each time the separator appears, as opposed to -- just once for the whole list. myIntersperse :: Gen a -> Gen [a] -> Gen [a] myIntersperse genSep genList = myIntersperse' genSep =<< genList where myIntersperse' :: forall a . Gen a -> [a] -> Gen [a] myIntersperse' genSep [] = return [] myIntersperse' genSep xs = do let listElementWithSep :: Gen [(a, a)] listElementWithSep = zipWithM (\el sep -> pure (el, sep)) xs =<< (sequence . repeat) genSep init <$> tupleListToList <$> listElementWithSep -- | Removes the tuple structure from a list, preserving the -- inner elements and their order. tupleListToList :: [(a, a)] -> [a] tupleListToList = concat . map (\(x, y) -> [x, y]) -- Iain

2 4

splitAt and negative values
by Stayvoid 07 Sep '12

07 Sep '12

Hello, Could you explain this behaviour? > splitAt (-1) "Foobar" ("","Foobar") > splitAt (-100) "Foobar" ("","Foobar") I don't understand why splitAt (-1) "Foobar" doesn't output this: ("r", "Fooba") or this: ("", "Fooba") How does it work? Python's list slicing looks similar, but results for negative values differ: >>> a = "Foobar" >>> a[:-1] >>> 'Fooba' Thanks

5 4