Purely functional
（重定向自Pure functional language）
In computing, an algorithm, data structure, or programming language is called purely functional if they guarantee the (weak) equivalence of call-by-name, call-by-value and call-by-need evaluation strategies. One typical approach to achieve this is by excluding destructive modifications (updates) of entities in the program's running environment. According to this restriction, variables are used in a mathematical sense, with identifiers referring to immutable, persistent values.