gistfile1.txt

scala> def tag(body:() => String) { println("start"); println("body = " + body()); println("end") }
tag: (body: () => String)Unit

scala> implicit def blockToStringFunction(body: () => Unit): () => String = () => { println("capturing"); body(); "answer" } 
blockToStringFunction: (body: () => Unit)() => String

scala> tag { println("line1"); println("line2") }
<console>:7: error: type mismatch;
 found   : Unit
 required: () => String
       tag { println("line1"); println("line2") }