When and Unless

The when and unless forms conditionally evaluate a single body of expressions:

• (when test expr ) evaluates the expr body expressions only when test returns a true value.
• (unless test expr ) evaluates the expr body expressions only when test returns #f.

The result of a when or unless expression is the result of the last body expression if the body is evaluated, or void (see section 4.1) if the body is not evaluated.