What are the rules to govern underscore to define anonymous function?

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Simple rules to determine the scope of underscore:

  1. If the underscore is an argument to a method, then the scope will be outside that method, otherwise respective the rules below;
  2. If the underscore is inside an expression delimited by () or {}, the innermost such delimiter that contains the underscore will be used;
  3. All other things being equal, the largest expression possible will be used.

So, by the rule #1, instead of println((x: Int) => x), the scope will be placed outside (including) println.

By rule #2, the latter two examples will have the function delimited by parenthesis, so (x => println(x: Int)).

By rule #3, the first example will be the whole expression, as there are no delimiting parenthesis.

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