14.1Signatures and Units🔗 i

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.rkt" file, describes the exports of a component that implements a toy factory:

By convention, signature names end with ^.

"toy-factory-sig.rkt"

#lang racket

(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 end with @.

"simple-factory-unit.rkt"

#lang racket

(require "toy-factory-sig.rkt")

(define-unit simple-factory@

(import )

(export toy-factory^)

(printf "Factory started.\n")

(struct toy(color)#:transparent)

(define (build-toysn)

(for/list ([i(in-range n)])

(toy'blue)))

(define (repainttcol)

(toycol)))

(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.)

"toy-store-sig.rkt"

#lang racket

(define-signature toy-store^

(store-color;(-> symbol?)

stock!;(integer? -> void?)

get-inventory));(-> (listof toy?))

(provide toy-store^)

"toy-store-unit.rkt"

#lang racket

(require "toy-store-sig.rkt"

"toy-factory-sig.rkt")

(define-unit toy-store@

(import toy-factory^)

(export toy-store^)

(define inventorynull )

(define (store-color)'green)

(define (maybe-repaintt)

(if (eq? (toy-colort)(store-color))

t

(repaintt(store-color))))

(define (stock!n)

(set! inventory

(append inventory

(map maybe-repaint

(build-toysn)))))

(define (get-inventory)inventory))

(provide toy-store@)

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