[BuildingParserWithJava]를 보다가 12장에서 처음 접한 단어. FunctionObject를 부르는 말이다.
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from Wikipedia:

A function object, often called a functor, is a computer programming construct allowing an object to be invoked or called as if it were an ordinary function, usually with the same syntax. The exact meaning may vary among programming languages. A functor used in this manner in computing bears little relation to the term functor as used in the mathematical field of category theory.

비슷한 구현에 대해서 OO 쪽에서는 MethodObject 라 부르기도 함. (Refactoring에 나옴) 구현 모양상으로 보면 CommandPattern, C++ 진영에서는 Functor 가 일반적인 표현.; --[1002]