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Substructural type systems are a family of type systems analogous to substructural logics where one or more of the structural rules are absent or only allowed under controlled circumstances. Such systems are useful for constraining access to system resources such as files, locks and memory by keeping track of changes of state that occur and preventing invalid states. == Different substructural type systems == Several type systems have emerged by discarding some of the structural rules exchange, weakening and contraction: * Linear type systems (allow exchange, not weakening or contraction): Every variable is used exactly once. * Affine type systems (allow exchange and weakening, not contraction): Every variable is used at most once. * Relevant type systems (allow exchange and contraction, not weakening): Every variable is used at least once. * Ordered type systems (discard exchange, contraction and weakening): Every variable is used exactly once in the order it was introduced. The explanation for affine type systems is best understood if rephrased as "every ''occurrence'' of a variable is used at most once". 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「Substructural type system」の詳細全文を読む スポンサード リンク
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