scheme基本概念之Variable Bindings

Any identifi er that is not a syntactic keyword may be used as a variable . A variable may name a location where a value can be stored. A variable that does so is said to be bound to the location. The value stored in the location to which a variable is bound is called the variable’s value. (The variable is sometimes said to name the value or to be bound to the value.)
A variable may be bound but still not have a value; such a variable is said to be unassigned. Referencing an unassigned variable is an error. When this error is signalled, it is a condition of typecondition-type:unassigned-variable; sometimes the compiler does not generate code to signal the error. Unassigned variables are useful only in combination with side eff ects.