► |  | The Racket Guide

1 | Welcome to Racket
2 | Racket Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Creating Languages
18 | Concurrency and Synchronization
19 | Performance
20 | Parallelism
21 | Running and Creating Executables
22 | More Libraries
23 | Dialects of Racket and Scheme
24 | Command-Line Tools and Your Editor of Choice
| Bibliography
| Index

▼ | 7 | Contracts

7.1 | Contracts and Boundaries
7.2 | Simple Contracts on Functions
7.3 | Contracts on Functions in General
7.4 | Contracts: A Thorough Example
7.5 | Contracts on Structures
7.6 | Abstract Contracts using #: exists and #: ∃
7.7 | Additional Examples
7.8 | Building New Contracts
7.9 | Gotchas

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#### 7.6`` Abstract Contracts using #:exists and #:∃

The contract system provides existential contracts that can protect abstractions, ensuring that clients of your module cannot depend on the precise representation choices you make for your data structures.

> > > You can type #:exists instead of #:∃ if you cannot easily type unicode characters; in DrRacket, typing \exists followed by either alt-\ or control-\ (depending on your platform) will produce ∃.

The contract-out form allows you to write

> #:∃ name-of-a-new-contract

as one of its clauses. This declaration introduces the variable name-of-a-new-contract, binding it to a new contract that hides information about the values it protects.

As an example, consider this (simple) implementation of a queue datastructure:

> #lang racket
> (define empty '())
> (define (enq top queue) (append queue (list top)))
> (define (next queue) (car queue))
> (define (deq queue) (cdr queue))
> (define (empty? queue) (null? queue))
> (provide
> (contract-out
> [empty (listof integer?)]
> [enq (-> integer? (listof integer?) (listof integer?))]
> [next (-> (listof integer?) integer?)]
> [deq (-> (listof integer?) (listof integer?))]
> [empty? (-> (listof integer?) boolean?)]))

This code implements a queue purely in terms of lists, meaning that clients of this data structure might use car and cdr directly on the data structure (perhaps accidentally) and thus any change in the representation (say to a more efficient representation that supports amortized constant time enqueue and dequeue operations) might break client code.

To ensure that the queue representation is abstract, we can use #:∃ in the contract-out expression, like this:

> (provide
> (contract-out
> #:∃ queue
> [empty queue]
> [enq (-> integer? queue queue)]
> [next (-> queue integer?)]
> [deq (-> queue queue)]
> [empty? (-> queue boolean?)]))

Now, if clients of the data structure try to use car and cdr, they receive an error, rather than mucking about with the internals of the queues.

See also Exists Contracts and Predicates.

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