应用风格的计算成本 [英] Computational cost of applicative style

问题描述

我在我的网络应用程序中使用了一个小型数据库池。这个特殊的函数：

pre $ withPool pool =括号（takeConn池）（putConn池）


可以用适用风格重写：

  withPool =括号< $> takeConn * putConn 
  

可以说它的可读性更高，更加优雅。所以很自然，我想写这样写。但是数据库连接池应该很快，而且我担心这种风格会带来不必要的开销。

所以我的问题是，多少开销（如果有）使用Haskell中会出现应用函子吗？是否有任何基准？

解决方案

我真的怀疑他们在大多数情况下会被编译为相同的东西。
我做了一个小测试，

  import Control.Applicative 
 
 test1 ::（ b→b→c）→> （a  - > b） - > （a  - > b） - > a  - > c 
 test1括号takeConn putConn池=括号（takeConn池）（putConn池）
 
 test2 ::（b  - > b  - > c） - > （a  - > b） - > （a  - > b） - > a  - > c 
 test2括号takeConn putConn =括号< $> takeConn * putConn 
  

但我将test2的类型限制为只有函数（这不是它最通用的然后我用 ghc -O -ddump-simpl 编译得到某种排序从GHC的中间输出（我尝试了C输出，它太丑了），而且两个看起来完全一样，除了名称。

（我也尝试过没有-O虽然，他们是不一样的，如果我离开类型注释）

I am using a small database pool in my web app. And this particular function:

withPool pool = bracket (takeConn pool) (putConn pool)

can be rewritten in applicative style:

withPool = bracket <$> takeConn <*> putConn

Arguably it is just as readable and much more elegant. So naturally, I want to write it like that. But database connection pool supposed to be fast, and I am afraid that this style introduces unnecessary overhead.

So my question is, how much overhead (if any) does use of applicative functors incur in Haskell? Are there any benchmarks?

解决方案

I really suspect they'll be compiled down to the same thing in most cases. I did a tiny test,

import Control.Applicative

test1 :: (b -> b -> c) -> (a -> b) -> (a -> b) -> a -> c
test1 bracket takeConn putConn pool = bracket (takeConn pool) (putConn pool)

test2 :: (b -> b -> c) -> (a -> b) -> (a -> b) -> a -> c
test2 bracket takeConn putConn = bracket <$> takeConn <*> putConn

but I am constraining the type of test2 there to only functions (which isn't its most generalised type, right..?)

Then I compiled with ghc -O -ddump-simpl to get some sort of intermediate output from GHC (I tried the C output, it was too ugly) and the two came out looking exactly the same, except for names.

(I also tried without -O though, and they weren't the same, nor if I leave out the type annotations)

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