Control Structures

If-expressions and cond-expressions are two examples of control structures in Scheme -- expressions that control the order in which their subexpressions are evaluated or the circumstances under which they are evaluated. Scheme provides two other control structures that do conditional evaluation: the and-expression and the or-expression.

In an and-expression, the expressions that follow the keyword and are evaluated in succession until one is found to have the value #f (in which case the rest of the expressions are skipped and the #f becomes the value of the entire and-expression) or all of the expressions have been evaluated (in which case the value of the last expression becomes the value of the and-expression). This gives the programmer a way to combine several tests into one that will succeed only if all of its parts succeed.

In an or-expression, the expressions that follow the keyword or are evaluated in succession until one is found to have a value other than#f (in which case the rest of the expressions are skipped and this value becomes the value of the entire or-expression) or all of the expressions have been evaluated (the value of the or-expression is #f). This gives the programmer a way to combine several tests into one that will succeed if any of its parts succeeds.