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Gradual typing
Gradual typing is a type system in which some variables may be given types and the correctness of the typing is checked at compile-time (which is static typing) and some variables may be left untyped and eventual type errors are reported at run-time (which is dynamic typing). Gradual typing allows software developers to choose either type paradigm as appropriate, from within a single language.〔(【引用サイトリンク】title=What is gradual typing? )〕 In many cases gradual typing is added to an existing dynamic language, creating a derived language allowing but not requiring static typing to be used. In some cases a language uses gradual typing from the start.
==Implementation==
In particular, gradual typing uses a special type named ''dynamic'' to represent statically-unknown types, and gradual typing replaces the notion of type equality with a new relation called ''consistency'' that relates the dynamic type to every other type. The consistency relation is symmetric but not transitive.
Prior attempts at integrating static and dynamic typing tried to make the dynamic type be both the top and bottom of the subtype hierarchy. However, because subtyping is transitive, that results in every type becoming related to every other type, and so subtyping would no longer rule out any static type errors. The addition of a second phase of plausibility checking to the type system did not completely solve this problem.
Gradual typing can easily be integrated into the type system of an object-oriented language that already uses the subsumption rule to allow implicit up-casts with respect to subtyping. The main idea is that consistency and subtyping are orthogonal ideas that compose nicely. To add subtyping to a gradually-typed language, simply add the subsumption rule and add a subtyping rule that makes the dynamic type a subtype of itself, because subtyping is supposed to be reflexive. (But do not make dynamic the top of the subtyping order!)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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