The unit defined below imports and exports no variables. Each time it is invoked, it prints and returns the current time in seconds:
(define f1@ (unit (import) (export) (define x (current-seconds)) (display x) (newline) x))
The expression below is syntactically invalid because current-date is not a built-in procedure:
(define f2-bad@ (unit (import) (export) (define x (current-date)) (display x) (newline) x))but the next expression is valid because the unit expression is in the scope of the let-bound variable:
(define f2@ (let ([current-date current-seconds]) (unit (import) (export) (define x (current-date)) (display x) (newline) x)))
The following units define two parts of an interactive phone book:
(define database@ (unit (import show-message) (export insert lookup) (define table (list)) (define insert (lambda (name info) (set! table (cons (cons name info) table)))) (define lookup (lambda (name) (let ([data (assoc name table)]) (if data (cdr data) (show-message "info not found"))))) insert)) (define interface@ (unit (import insert lookup make-window make-button) (export show-message) (define show-message (lambda (msg) ...)) (define main-window ...)))In this example, the database@ unit implements the database-searching part of the program, and the interface@ unit implements the graphical user interface. The database@ unit exports insert and lookup procedures to be used by the graphical interface, while the interface@ unit exports a show-message procedure to be used by the database (to handle errors). The interface@ unit also imports variables that will be supplied by an platform-specific graphics toolbox.