[Haskell-cafe] Library function for map+append

[Eugene Kirpichov]

module Main where

mymap f xs = m xs
    where m []     = []
          m (x:xs) = f x:m xs

mymapp1 f xs ys = m xs
    where m []     = ys
          m (x:xs) = f x:m xs

mymapp2 f []     ys = ys
mymapp2 f (x:xs) ys = f x:mymapp2 f xs ys

mapp1 f xs ys = (f`map`xs) ++ ys
mapp2 f xs ys = (f`mymap`xs) ++ ys
mapp3 f xs ys = mymapp1 f xs ys
mapp4 f xs ys = mymapp2 f xs ys

mapp = mapp1

main = putStrLn . show . length $ mapp (+1) [1..100000000] [1,2,3]

mapp1: 3.764s
mapp2: 5.753s
mapp3: 4.302s
mapp4: 4.767s

So, the fastest way is the simplest one.


18 августа 2009 г. 17:12 пользователь Artem V. Andreev
(artem at aa5779.spb.edu) написал:
> Clemens Fruhwirth <clemens at endorphin.org> writes:
>
>> 2009/8/18 Dusan Kolar <kolar at fit.vutbr.cz>:
>>> Hello all,
>>>
>>>  During a small project I'm trying to develop a small application. It
>>> becomes quite often that I need a function mapapp:
>>>
>>> mapapp _ [] ap = ap
>>> mapapp f (a:as) ap = f a : map f as ap
>>>
>>>  I tried hoogle to find such a function with no success. Is there any
>>> function/functions built-in "standard" libraries that could easily satisfy
>>> the functionality with the same or even better (?) efficiency?
>>
>> Can't think of something like that either but at least we can make it
>> shorter and less readable ;)
>>
>> mapapp f xs tail = foldr ((:) . f) tail xs
>>
>>>  Of course,
>>> (map f list) ++ append
>>>  would do the same as
>>>
>>> mapapp f list append
>>>
>>>  but with less efficiency. Or am I wrong?
>>
>> Yes, that is less efficient because ++ has to create N new cons cells
>> if "list" has length N.
> No, it does not *have to*.
>
>
>
>> Fruhwirth Clemens http://clemens.endorphin.org
>> _______________________________________________
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>>
>>
>
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-- 
Eugene Kirpichov
Web IR developer, market.yandex.ru