在 HLint 的上下文中 eta reduce 是什么意思 [英] What does eta reduce mean in the context of HLint

问题描述

我正在看教程 http://haskell.org/haskellwiki/How_to_write_a_Haskell_program

import System.Environment

main :: IO ()
main = getArgs >>= print . haqify . head

haqify s = "Haq! " ++ s

在HLint下运行这个程序时出现以下错误；

When running this program under HLint it gives the following error;

./Haq.hs:11:1: Warning: Eta reduce
Found:
  haqify s = "Haq! " ++ s
Why not:
  haqify = ("Haq! " ++ )

有人能解释一下在这种情况下Eta Reduce"究竟是什么意思?

Can someone shed some light on what exactly "Eta Reduce" means in this context?

推荐答案

Eta 减少正在转向 x ->f x 到 f 只要 f 没有 x 的自由出现.

Eta reduction is turning x -> f x into f as long as f doesn't have a free occurence of x.

要检查它们是否相同，请将它们应用于某个值 y:

To check that they're the same, apply them to some value y:

(x -> f x) y === f' y -- (where f' is obtained from f by substituting all x's by y)
              === f y  -- since f has no free occurrences of x

您对 haqify 的定义被视为 s ->Haq！" ++ s，它是 s -> 的语法糖(++) 哈克！"s.这反过来又可以简化为 (++) "Haq! "，或者等效地，使用运算符的节符号，("Haq!" ++).

Your definition of haqify is seen as s -> "Haq! " ++ s, which is syntactic sugar for s -> (++) "Haq! " s. That, in turn can be eta-reduced to (++) "Haq! ", or equivalently, using section notation for operators, ("Haq! " ++).

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