|
In computer programming, a thunk is a subroutine that is created, often automatically, to assist a call to another subroutine. Thunks are primarily used to represent an additional calculation that a subroutine needs to execute, or to call a routine that does not support the usual calling mechanism. They have a variety of other applications to compiler code generation and in modular programming. The term originated as a jocular derivative of something "thought".〔 Eric Raymond rejects "a couple of onomatopoeic myths circulating about the origin of this term" and cites the inventors of the thunk recalling that the term "was coined after they realized (in the wee hours after hours of discussion) that the type of an argument in Algol-60 could be figured out in advance with a little compile-time thought () In other words, it had 'already been thought of'; thus it was christened a ''thunk'', which is 'the past tense of "think" at two in the morning'. See: 〕 ==Background== The early years of compiler research saw broad experimentation with different evaluation strategies. A key question was how to compile a subroutine call if the arguments can be arbitrary mathematical expressions rather than constants. One approach, known as "call by value," calculates all of the arguments before the call and then passes the resulting values to the subroutine. In the rival "call by name" approach, the subroutine receives the unevaluated argument expression and must evaluate it. A simple implementation of "call by name" might substitute the code of an argument expression for each appearance of the corresponding parameter in the subroutine, but this can produce multiple versions of the subroutine and multiple copies of the expression code. As an improvement, the compiler can generate a helper subroutine, called a ''thunk'', that calculates the value of the argument. The address of this helper subroutine is then passed to the original subroutine in place of the original argument, where it can be called as many times as needed. Prof. Peter Ingerman first described thunks in reference to the ALGOL 60 programming language, which supported call-by-name evaluation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Thunk」の詳細全文を読む スポンサード リンク
|