第6单元 类型检查.pptVIP

类型检查 类型表达式 类型表达式可以是基本的数据类型也可以带有结构。 基本的数据类型 (boolean, char, integer and real) 是类型表达式 类型名字也是类型表达式 记录或结构与数组的类型表达式 E.g.: 函数与指针的类型表达式: Some languages allows functions to return an object of arbitrary type, e.g. LISP: (integer ? integer) ? (integer ? integer) 类型图 (tree or DAG) 静态与动态的数据类型检查 A type system is a collection of rules for assigning type expression to the various parts of a program. The type system is implemented by a type checker. The implementation can be easy made using syntax-directed manner from chap. 5. Checked done by a compiler is static meanwhile the checking during the period of run of the program is said to be dynamically. If the target code carry the type of element along with the value of this element, practically any check can be done dynamically A sound system type eliminates the needs for dynamic checking (the free errors statically maintain also dynamically). In practice, sometime some checks can be made only dynamically: 类型检查 The type checker is a translation scheme that synthesize the type of each expression from the types of its sub-expressions E.g. grammar (each identifier must be declared before to be used): The grammar can be modified in order to implement a suitable scheme for top-down or bottom-up parsing 类型检查：表达式 类型检查: 语句 Because language constructs like statements doesn’t have values, a special basic type void can be assigned to them If an error is detected in a statement, the type assigned to the statement is type_error Statements considered in what follows: assignment, conditional and while. For a program P ? D; S that consist from declaration followed by statements checking expression are necessary because statements can have expressions within them 类型检查: 函数 The application of a function to an argument (f:D?R, f(x) = y) is captured by the production E ? E ( E ) in which one expression is the application to another. The rules for associating type expression wi