FP中减少括号的语法糖

在F#中内置了两个很有用的运算符 |> 和 >> ，用来串联数据和函数，有效减少括号的使用，让代码不那么lispy。这两个符号的定义很简单，在Ocaml中也一样可以方便使用。

运算符 |> 定义：

let (|>) x f = f x
val (|>) : 'a -> ('a -> 'b) -> 'b

例子：形如

f(g(h(i(x))))

可以写成

x |> i |> h |> g |> f

依此类推（抱歉，例子抽象了点）

运算符 >> 定义：

let (>>) f g x = g(f(x))
val (>>) : ('a -> 'b) -> ('b -> 'c) -> ('a -> 'c)

例子：形如

f(g(h(i(x))))

可以写成

i >> h >> g >> f >> x

注意这个类似Haskell里所谓point free style的.运算符，但是运算顺序正好相反。

运算符.的Haskell定义：

let (.) f g x = f(g(x))
(.) :: (b -> c) -> (a -> b) -> a -> c

这点从它们的类型签名也能看出来。所以，Haskell的定义符合复合函数的数学定义，但是读起来得从右向左读。而F#的定义符合从左向右读的直觉习惯，各有千秋。

这种函数串联然后再整体作用到数据上的编程风格好像也叫tacit programming。。。

Ruby中在Proc类中定义运算符也能达到类似的串联效果，当然Ruby只能重定义那么几个已经存在的运算符，不能凭空造新运算符:(

评论

2 楼 cookoo 2007-01-10  

Thanks for the useful comment. Yes, |> is a reversed version of $ in Haskell, though |> gives a better feeling regarding the direction of pipelines. I'm pretty some neat syntax sugar of F# is borrowed from Haskell and we will see more in future (such as zip family). However, there's another voice saying Haskell Prelude is too fat and should be narrowed down a bit, hehe.

I personally feel Haskell has defined too many special symbols which can make code line-noisy easily, like Perl style syntax.

1 楼 T55555 2007-01-06  

FYI.

(|>) is "reverse" of ($).
The (|>) also shows up as euro ( to compare with $ ) on Haskell wikibook.
Haskell/Understanding monads
1.2 euro ( http://en.wikibooks.org/wiki/Haskell/Understanding_monads#euro )
...
N.B.: the euro symbol isn't valid Haskell... if you want to try this, use (|>) as an operator instead
...

(>>) uses by monad, in the book Haskell - The Craft of Functional Programming
they defines a (>.>) as "reverse" of (.)
Page 170 (Forward Composition) shows a "reverse" of function composition definition:
infixl 9 >.>
(>.>) :: (a -> b) -> (b -> c) -> (a -> c)
g >.> f = f . g