把 x => func(x) 简化为 func _ 或 func 的过程称为 eta-conversion?把 func 或 func _ 展开为 x => func(x) 的过程为 eta-expansion
Eta?Expansion的就近expand解析原则：
Underscores extend outwards to the closest closing?Expr?: top-level expressions or expressions in parentheses
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匿名函数简写注意事项：
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所以当匿名函数有多个括号嵌套的时候，不要使用_的简写方式 ,而应该用普通匿名函数的书写方式。比如 （x：Int，y:Int）=> （x*2）+f(y+2)
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例子解析：
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例子1：
例子2：
例子3：val myStrings = new Array[String](3)
// do some string initialization
// this works
myStrings.foreach(println(_))
// ERROR: missing parameter type for expanded function
myStrings.foreach(println(_.toString))It expands to:
myStrings.foreach(println(x => x.toString))
You want:
myStrings.foreach(x => println(x.toString))
The placeholder syntax for anonymous functions replaces the smallest possible containing expression with a function.
This has already been addressed in?a related question. Underscores extend outwards to the closest closing?Expr?: top-level expressions or expressions in parentheses.
(_.toString)?is an expression in parentheses. The argument you are passing to?Exception?in the error case is therefore, after expansion, the full anonymous function?(x$1) => x$1.toString?of type?A <: Any => String, while?Exception?expects a?String.
In the?println?case,?_?by itself isn‘t of syntactic category?Expr, but?(println (_))?is, so you get the expected?(x$0) => println(x$0).
scala 下划线解析报错： missing parameter type for expanded function
原文：http://zhouchaofei2010.iteye.com/blog/2260107