14.1 Signatures and Units

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▼ | 14 | Units (Components)

14.1 | Signatures and Units
14.2 | Invoking Units
14.3 | Linking Units
14.4 | First-Class Units
14.5 | Whole-module Signatures and Units
14.6 | Contracts for Units
14.7 | unit versus module

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

> > > By convention, signature names 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 with @.

> "simple-factory-unit.rkt"

> > #lang racket
> > ( require "toy-factory-sig.rkt")
> > ( 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.)

> "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 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.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.

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