使用scalaz'切换函数和对象|> [英] switch function and object with scalaz' |>

问题描述

当我想切换函数和对象时，我可以使用 scalaz |> 运算符，这样可以获得更多的可读性.给大家介绍一个模型函数:

I can use scalaz |> operator when I want to switch function and object so there can be a little more readability acquired. Let me introduce you a model function :

def length2(x:String) = x.length * 2

现在，我可以用两种方式来写:

Now, I can write it in both ways:

"aoeu" |> length2
length2("aoeu")

但是如果我定义这个函数更通用，它就会停止工作.

But if I define this function more generic, it stops working.

def length2(x:SeqLike[_,_]) = x.length * 2
length2("aoeu") // ok
"aoeu" |> length2 // doesn't work

为什么编译器不明白这一点?在特征SeqLike中肯定存在从String到某些类混合的隐式转换.

Why the compiler doesn't understand this? There is definitely an implicit conversion from String to some class mixing in trait SeqLike.

推荐答案

scala> "aoeu" |> length2
<console>:14: error: type mismatch;
 found   : (scala.collection.SeqLike[_, _]) => Int
 required: (java.lang.String) => ?
       "aoeu" |> length2

错误信息非常清楚.

尽管存在从 String 到 SeqLike[_,_] 的隐式转换，但没有从 (SeqLike[_, _]) = 的转换>Int 到 String =>?.

Although there is an implicit conversion from String to SeqLike[_,_], there is no conversion from (SeqLike[_, _]) => Int to String => ?.

这可以使用以下隐式转换来修复:

This can be fixed using the following implicit conversion:

implicit def liftFun[X, T <% X, U](f: (X) => U): (T) => U = {
  def g(t:T) = f(t)
  g _
}

编辑 2:这是一个非 scalaz 运算符.

Edit 2: here is a non-scalaz operator.

class Pipe[T](t:T) {
  def |%>[X, U](f: (X) => U)(implicit ev: T <%< X) = f(t)
}
implicit def toPipe[T](t:T) = new Pipe(t:T)

然后你可以这样使用它:

Then you can use it like this:

def l1(a:String) = a.length
def l2(a:Seq[_]) = a.length * 2

"abc" |%> l1
"abc" |%> l2

它允许 |%> 接受一个不能直接在 T 上工作但在 X 上工作的函数，只要有是从 T 到 X 的隐式转换的证据.

It allows |%> to take a function that does not work directly on a T but on a X as long as there is evidence of an implicit conversion from T to X.

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