我在一些文章和博客中看到了curry函数的引用,但我找不到一个好的解释(或者至少一个有意义的解释!)
当前回答
如果你理解了部分,你就成功了一半。partial的思想是将参数预先应用到一个函数,并返回一个只需要剩余参数的新函数。当这个新函数被调用时,它包括预加载的参数以及提供给它的任何参数。
在Clojure +中是一个函数,但要明确一点:
(defn add [a b] (+ a b))
您可能已经意识到inc函数只是简单地将1加到它传递的任何数字上。
(inc 7) # => 8
让我们自己使用partial来构建它:
(def inc (partial add 1))
Here we return another function that has 1 loaded into the first argument of add. As add takes two arguments the new inc function wants only the b argument -- not 2 arguments as before since 1 has already been partially applied. Thus partial is a tool from which to create new functions with default values presupplied. That is why in a functional language functions often order arguments from general to specific. This makes it easier to reuse such functions from which to construct other functions.
现在想象一下,如果语言足够聪明,能够自省地理解add需要两个参数。当我们向它传递一个参数时,如果函数部分应用了我们代表它传递的参数,并理解我们可能打算稍后提供另一个参数呢?然后,我们可以在不显式使用partial的情况下定义inc。
(def inc (add 1)) #partial is implied
这是一些语言的表现方式。当希望将函数组合成更大的转换时,它特别有用。这将把人们引向传感器。
其他回答
下面是泛型和最短版本的函数用n no curry的例子。的参数。
const add = a => b => b ? add(a + b) : a;
Const add = a => b => b ?Add (a + b): a; console.log(添加(1)(2)(3)(4)());
它可以是一种用函数生成其他函数的方法。
在javascript中:
let add = function(x){
return function(y){
return x + y
};
};
我们可以这样称呼它:
let addTen = add(10);
运行时,将10作为x传入;
let add = function(10){
return function(y){
return 10 + y
};
};
这意味着我们返回这个函数:
function(y) { return 10 + y };
所以当你打电话的时候
addTen();
你真的在呼唤:
function(y) { return 10 + y };
如果你这样做:
addTen(4)
这就相当于:
function(4) { return 10 + 4} // 14
所以我们的addTen()总是给我们传入的任何东西加10。我们可以用同样的方法来构造类似的函数:
let addTwo = add(2) // addTwo(); will add two to whatever you pass in
let addSeventy = add(70) // ... and so on...
接下来的问题是,你到底为什么要这么做?它将一个急迫的操作x + y变成了一个可以轻松完成的操作,这意味着我们至少可以做两件事 1. 缓存昂贵的操作 2. 在功能范式中实现抽象。
想象我们的咖喱函数是这样的:
let doTheHardStuff = function(x) {
let z = doSomethingComputationallyExpensive(x)
return function (y){
z + y
}
}
我们可以调用这个函数一次,然后将结果传递给很多地方,这意味着我们只做一次计算上昂贵的事情:
let finishTheJob = doTheHardStuff(10)
finishTheJob(20)
finishTheJob(30)
我们可以用类似的方式得到抽象。
其他答案已经说明了curry是什么:向curry函数传递的参数比它预期的要少,这不是错误,而是返回一个函数,该函数预期其余的参数,并返回相同的结果,就好像您一次性将它们全部传入一样。
我会试着解释为什么它有用。这是一种你从未意识到你需要的工具,直到你真正使用它。curry首先是一种让你的程序更具表现力的方法——你可以用更少的代码把操作组合在一起。
For example, if you have a curried function add, you can write the equivalent of JS x => k + x (or Python lambda x: k + x or Ruby { |x| k + x } or Lisp (lambda (x) (+ k x)) or …) as just add(k). In Haskelll you can even use the operator: (k +) or (+ k) (The two forms let you curry either way for non-commutative operators: (/ 9) is a function that divides a number by 9, which is probably the more common use case, but you also have (9 /) for a function that divides 9 by its argument.) Besides being shorter, the curried version contains no made-up parameter name like the x found in all the other versions. It’s not needed. You’re defining a function that adds some constant k to a number, and you don’t need to give that number a name just to talk about the function. Or even to define it. This is an example of what’s called “point-free style”. You can combine operations together given nothing but the operations themselves. You don’t have to declare anonymous functions that do nothing but apply some operation to their argument, because *that’s what the operations already are.
当以咖喱友好的方式定义高阶函数时,这变得非常方便。例如,curried map(fn, list)让您定义一个只使用map(fn)的映射器,可以稍后将其应用于任何列表。但是将定义为map(list, fn)的映射curry化只能让您定义一个将其他函数应用到常量列表的函数,这在一般情况下可能不太有用。
Currying reduces the need for things like pipes and threading. In Clojure, you might define a temperature conversion function using the threading macro ->: (defn f2c (deg) (-> deg (- 32) (* 5) (/ 9)). That’s cool, it reads nicely left to right (“subtract 32, multiply by 5 and divide by 9.”) and you only have to mention the parameter twice instead of once for every suboperation… but it only works because -> is a macro that transforms the whole form syntactically before anything is evaluated. It turns into a regular nested expression behind the scenes: (/ (* (- deg 32) 5) 9). If the math ops were curried, you wouldn’t need a macro to combine them so nicely, as in Haskell let f2c = (subtract 32) & (* 5) & (/ 9). (Although it would admittedly be more idiomatic to use function composition, which reads right to left: (/ 9) . (* 5) . (subtract 32).)
同样,很难找到好的演示例子;在复杂的情况下,咖喱是最有用的,因为它确实有助于解决方案的可读性,但这些需要太多的解释才能让您理解问题,以至于关于咖喱的整个课程可能会淹没在噪音中。
curry是指将一个N次的函数转换为N次1次的函数。函数的元数是它所需要的参数的个数。
下面是正式的定义:
curry(f) :: (a,b,c) -> f(a) -> f(b)-> f(c)
下面是一个真实的例子:
你去自动取款机取钱。你刷你的卡,输入密码,做出选择,然后按enter键提交“金额”和请求。
这是取款的正常功能。
const withdraw=(cardInfo,pinNumber,request){
// process it
return request.amount
}
在这个实现函数中,我们希望一次性输入所有参数。我们将刷卡,输入密码并发出请求,然后函数将运行。如果这些步骤中有任何问题,在输入所有参数后就会发现。使用curry函数,我们可以创建更高的、纯粹的和简单的函数。纯函数将帮助我们轻松地调试代码。
这是带有咖喱功能的Atm机。
const withdraw=(cardInfo)=>(pinNumber)=>(request)=>request.amount
ATM, takes the card as input and returns a function that expects pinNumber and this function returns a function that accepts the request object and after the successful process, you get the amount that you requested. Each step, if you had an error, you will easily predict what went wrong. Let's say you enter the card and got error, you know that it is either related to the card or machine but not the pin number. Or if you entered the pin and if it does not get accepted you know that you entered the pin number wrong. You will easily debug the error.
此外,这里的每个函数都是可重用的,因此您可以在项目的不同部分使用相同的函数。
如果你理解了部分,你就成功了一半。partial的思想是将参数预先应用到一个函数,并返回一个只需要剩余参数的新函数。当这个新函数被调用时,它包括预加载的参数以及提供给它的任何参数。
在Clojure +中是一个函数,但要明确一点:
(defn add [a b] (+ a b))
您可能已经意识到inc函数只是简单地将1加到它传递的任何数字上。
(inc 7) # => 8
让我们自己使用partial来构建它:
(def inc (partial add 1))
Here we return another function that has 1 loaded into the first argument of add. As add takes two arguments the new inc function wants only the b argument -- not 2 arguments as before since 1 has already been partially applied. Thus partial is a tool from which to create new functions with default values presupplied. That is why in a functional language functions often order arguments from general to specific. This makes it easier to reuse such functions from which to construct other functions.
现在想象一下,如果语言足够聪明,能够自省地理解add需要两个参数。当我们向它传递一个参数时,如果函数部分应用了我们代表它传递的参数,并理解我们可能打算稍后提供另一个参数呢?然后,我们可以在不显式使用partial的情况下定义inc。
(def inc (add 1)) #partial is implied
这是一些语言的表现方式。当希望将函数组合成更大的转换时,它特别有用。这将把人们引向传感器。