[Haskell-cafe] Could someone help me to understand funB?
Lyndon Maydwell
maydwell at gmail.com
Fri Dec 19 04:32:24 UTC 2014
funB = flip takeWhile primes . ( . join (*)) . flip (<=)
funB x = flip takeWhile primes ((( . join (*)) . flip (<=)) x)
funB x = takeWhile ((( . join (*)) . flip (<=)) x) primes
funB x = takeWhile (foo x) primes
foo x = (( . join (*)) . flip (<=)) x
foo x = (( . join (*)) (flip (<=) x)
foo x = flip (<=) x . join (*)
foo x = (x <=) . join (*)
-- join :: Monad m => m (m a) -> m a
-- join (*) == (**2)
foo x = (x <=) . (** 2)
foo x y = x <= (y ** 2)
isLessThanTheSquareOf = foo
funB x = takeWhile (x `isLessThanTheSquareOf`) primes
Helpful?
On Fri, Dec 19, 2014 at 2:52 PM, Magicloud Magiclouds <
magicloud.magiclouds at gmail.com> wrote:
>
> Hi,
>
> Following code is to get a list of primes. Now it is hard for me to
> understand funB. I mean I can see what it does. But I cannot see the
> detailed process by every language part.
>
> import Control.Monad
>
> isPrime :: Integer -> Bool
> isPrime i = ap funA funB i
>
> funA :: Integer -> [Integer] -> Bool
> funA x xs = all (\x' -> (mod x x') /= 0) xs
>
> funB :: Integer -> [Integer]
> funB = flip takeWhile primes . ( . join (*)) . flip (<=)
>
> primes :: [Integer]
> primes = 2 : filter isPrime [3, 5 ..]
>
> main :: IO ()
> main = print $ take 10 primes
>
> --
> 竹密岂妨流水过
> 山高哪阻野云飞
>
> And for G+, please use magiclouds#gmail.com.
>
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