expression: an expression evaluates to a value only
statement: a statement containing executable code
definition: a definition instantiating an identifier
identifier(symbol):identifier are tokens(also called symbols) which name language entities
literal(字面量): a value,as opposed to an identifier,written into the source code of a computer program
token:
a lexical token or simply token is a string with an assigned and thus identified meaning,
it its structured as a pair consisting of a token name and an optional token value,
The token name is a category of a lexical unit. Common token names are:
· identifier: names the programmer chooses;
· keyword: names already in the programming language
· separator(also known as punctuators): punctuation characters and paired-delimiters
· operator:symbols that operate on arguments and produce results
· literal:numeric,logical,textual,literals
· comment:line,block
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expression表达式
表达式是 常量、变量、操作符、函数的组合，最后通过计算返回一个值。
这个计算的过程被称作 evaluation
变量也是一个expression因为它代表着内存中的一个值
statement语句
语句包含着可执行代码
(containing executable code)
definition声明
definition会实例化一个identifier
identifier(symbol)
符号表: symbol table.
