14.1 Signatures and Units

The interface of a unit is described in terms of signatures. Each signature is defined (normally within a module) using define-signature. For example, the following signature, placed in a "toy-factory-sig.ss" file, describes the exports of a component that implements a toy factory:

By convention, signature names with ^.

; In "toy-factory-sig.ss":

#lang scheme

(define-signature toy-factory^

(build-toys  ; (integer? -> (listof toy?))

repaint     ; (toy? symbol? -> toy?)

toy?        ; (any/c -> boolean?)

toy-color)) ; (toy? -> symbol?)

(provide toy-factory^)

An implementation of the toy-factory^ signature is written using define-unit with an export clause that names toy-factory^:

By convention, unit names with @.

; In "simple-factory-unit.ss":

#lang scheme

(require "toy-factory-sig.ss")

(define-unit simple-factory@

(import)

(export toy-factory^)

(printf "Factory started.\n")

(define-struct toy (color) #:transparent)

(define (build-toys n)

(for/list ([i (in-range n)])

(make-toy 'blue)))

(define (repaint t col)

(make-toy col)))

(provide simple-factory@)

The toy-factory^ signature also could be referenced by a unit that needs a toy factory to implement something else. In that case, toy-factory^ would be named in an import clause. For example, a toy store would get toys from a toy factory. (Suppose, for the sake of an example with interesting features, that the store is willing to sell only toys in a particular color.)

; In "toy-store-sig.ss":

#lang scheme

(define-signature toy-store^

(store-color     ; (-> symbol?)

stock!          ; (integer? -> void?)

get-inventory)) ; (-> (listof toy?))

(provide toy-store^)

; In "toy-store-unit.ss":

#lang scheme

(require "toy-store-sig.ss"

"toy-factory-sig.ss")

(define-unit toy-store@

(import toy-factory^)

(export toy-store^)

(define inventory null)

(define (store-color) 'green)

(define (maybe-repaint t)

(if (eq? (toy-color t) (store-color))

t

(repaint t (store-color))))

(define (stock! n)

(set! inventory

(append inventory

(map maybe-repaint

(build-toys n)))))

(define (get-inventory) inventory))

(provide toy-store@)

Note that "toy-store-unit.ss" imports "toy-factory-sig.ss", but not "simple-factory-unit.ss". Consequently, the toy-store@ unit relies only on the specification of a toy factory, not on a specific implementation.