Scala：隐式背后的魔力

Question

I've been playing with "implicits" in Scala all weekend but I'm still having trouble grasping the whole concept. This question is based off of my previous one . I decided to take things a step further with:

def a(b: String)(implicit c: () => String = () => "") = {
    println(s"$b - up next is "+c());c();b
}

implicit def conv(s: => String) = () => s

That's when things start to get a little whacky:

scala> a("1")(a("2")(a("3")))
3 - up next is 
2 - up next is 3
3 - up next is 
1 - up next is 2
3 - up next is 
2 - up next is 3
3 - up next is 

The basic idea here is to get each line to reference each other like so:

1
2 from 1
3 from 2
4 from 3

---

1. Correct me if I'm wrong, but using implicit in this case will insert a string when one is found..? I'm not sure why it's printing out 7 lines instead of 3

2. What am I doing wrong here? I'm starting to think that using implicits gives me less control

3. Not as important, but is there a more safe way to go about implicits? A way that allows me to see exactly what's going on?

Answer 1

You are invoking c twice, instead only invoke it once:

def a(b: String)(implicit c: () => String = () => "") = {
  println(s"$b - up next is " + c())
  b
}

Which then produces:

3 - up next is 
2 - up next is 3
1 - up next is 2

Even though a("1") gets invoked first, it has to invoke the () => String it is passed before producing any output. As a result, a("3") is the first one to produce output.

You need to turn this around, and have a("2") tell a("3") that it is being invoked from a("2") :

def a(current: String, next: String => Unit = _ => ())(prev: String) = {
  println(s"$current from $prev")
  next(current)
}

And invoke it like:

a("1", a("2", a("3")))("nothing")

Which produces:

1 from nothing
2 from 1
3 from 2

To answer three of your points:

1. No, your implicit parameter takes a () => String , not a String .
2. See above, you don't need implicits here.
3. Read up on the -Xlog-implicits , -Xlog-implicit-conversions and -Xprint:all options.