# [Haskell-cafe] Could someone help me to understand funB?

Magicloud Magiclouds magicloud.magiclouds at gmail.com
Mon Dec 22 05:04:14 UTC 2014

```I do not think pointfree works here:

Transformed to pointfree style:
funB = id (fix (const (flip takeWhile primes . (. join (*)) . flip (<=))))
Optimized expression:
funB = id (fix (const (flip takeWhile primes . (. join (*)) . flip (<=))))
funB = fix (const (flip takeWhile primes . (. join (*)) . flip (<=)))
funB = flip takeWhile primes . (. join (*)) . flip (<=)

On Sat, Dec 20, 2014 at 1:21 AM, Alex Hammel <ahammel87 at gmail.com> wrote:

> I suspect that somebody has been abusing pointfree
> <https://github.com/bmillwood/pointfree>.
>
> On Thu, Dec 18, 2014 at 8:35 PM, Magicloud Magiclouds <
> magicloud.magiclouds at gmail.com> wrote:
>>
>> Thank you. This will take some time for me to read.
>>
>> On Fri, Dec 19, 2014 at 12:32 PM, Lyndon Maydwell <maydwell at gmail.com>
>> wrote:
>>>
>>> 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
>>>
>>>
>>>
>>>
>>> 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.
>>>>
>>>>
>>>> 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.
>>>>
>>>> _______________________________________________
>>>>
>>>>
>>
>> --
>> 竹密岂妨流水过
>> 山高哪阻野云飞
>>
>> And for G+, please use magiclouds#gmail.com.
>>
>> _______________________________________________
>>
>>
> _______________________________________________
>
>

--

And for G+, please use magiclouds#gmail.com.
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