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In computability theory, a machine that always halts—also called a decider (Sipser, 1996) or a total Turing machine (Kozen, 1997)—is a Turing machine that halts for every input. Because it always halts, the machine is able to ''decide'' whether a given string is a member of a formal language. The class of languages which can be decided by such machines is exactly the set of recursive languages. However, due to the Halting Problem, determining whether an arbitrary Turing machine halts on every input is itself an undecidable decision problem. == Functions computable by total Turing machines == In practice, many functions of interest are computable by machines that always halt. A machine that uses only finite memory on any particular input can be forced to halt for every input by restricting its flow control capabilities so that no input will ever cause the machine to enter an infinite loop. As a trivial example, a machine implementing a finitary decision tree will always halt. It is not required that the machine be entirely free of looping capabilities, however, to guarantee halting. If we restrict loops to be of a predictably finite size (like the FOR loop in BASIC), we can express all of the primitive recursive functions (Meyer and Ritchie, 1967). An example of such a machine is provided by the toy programming language PL- of Brainerd and Landweber (1974). We can further define a programming language in which we can ensure that even more sophisticated functions always halt. For example, the Ackermann function, which is not primitive recursive, nevertheless is a total computable function computable by a term rewriting system with a reduction ordering on its arguments (Ohlebusch, 2002, pp. 67). Despite the above examples of programming languages which guarantee termination of the programs, there exists no programming language which captures exactly the recursive functions, i.e. the functions which can be computed by a turing machine that always halts. This is because existence of such a programming language would be a contradiction to the non-semi-decidability of the problem whether a Turing machine halts on every input. 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「Machine that always halts」の詳細全文を読む スポンサード リンク
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