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An attribute grammar is a formal way to define attributes for the productions of a formal grammar, associating these attributes to values. The evaluation occurs in the nodes of the abstract syntax tree, when the language is processed by some parser or compiler. The attributes are divided into two groups: ''synthesized'' attributes and ''inherited'' attributes. The synthesized attributes are the result of the attribute evaluation rules, and may also use the values of the inherited attributes. The inherited attributes are passed down from parent nodes. In some approaches, synthesized attributes are used to pass semantic information up the parse tree, while inherited attributes help pass semantic information down and across it. For instance, when constructing a language translation tool, such as a compiler, it may be used to assign semantic values to syntax constructions. Also, it is possible to validate semantic checks associated with a grammar, representing the rules of a language not explicitly imparted by the syntax definition. Attribute grammars can also be used to translate the syntax tree directly into code for some specific machine, or into some intermediate language. One strength of attribute grammars is that they can transport information from anywhere in the abstract syntax tree to anywhere else, in a controlled and formal way. ==Example== The following is a simple Context-free grammar which can describe a language made up of multiplication and addition of integers. Expr → Expr + Term Expr → Term Term → Term * Factor Term → Factor Factor → "(" Expr ")" Factor → ''integer'' The following attribute grammar can be used to calculate the result of an expression written in the grammar. Note that this grammar only uses synthesized values, and is therefore an S-attributed grammar. Expr1 → Expr2 + Term (Expr1.value = Expr2.value + Term.value ) Expr → Term (Expr.value = Term.value ) Term1 → Term2 * Factor (Term1.value = Term2.value * Factor.value ) Term → Factor (Term.value = Factor.value ) Factor → "(" Expr ")" (Factor.value = Expr.value ) Factor → ''integer'' (Factor.value = strToInt(''integer''.str) ) 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「attribute grammar」の詳細全文を読む スポンサード リンク
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