► |  | The Racket Reference

1 | Language Model
2 | Notation for Documentation
3 | Syntactic Forms
4 | Datatypes
5 | Structures
6 | Classes and Objects
7 | Units
8 | Contracts
9 | Pattern Matching
10 | Control Flow
11 | Concurrency and Parallelism
12 | Macros
13 | Input and Output
14 | Reflection and Security
15 | Operating System
16 | Memory Management
17 | Unsafe Operations
18 | Running Racket
| Bibliography
| Index

▼ | 8 | Contracts

8.1 | Data-structure Contracts
8.2 | Function Contracts
8.3 | Parametric Contracts
8.4 | Lazy Data-structure Contracts
8.5 | Structure Type Property Contracts
8.6 | Attaching Contracts to Values
8.7 | Building New Contract Combinators
8.8 | Contract Utilities
8.9 | racket/ contract/ base
8.10 | Legacy Contracts
8.11 | Random generation

On this page:

contract?
chaperone-contract?
impersonator-contract?
flat-contract?
contract-name
value-contract
has-contract?
contract-projection
make-none/ c
recursive-contract
opt/ c
define-opt/ c
contract-continuation-mark-key

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#### 8.8`` Contract Utilities

> procedure (contract? v) → boolean?
> v : any/c

Returns #t if its argument is a contract (i.e., constructed with one of the combinators described in this section or a value that can be used as a contract) and #f otherwise.

> procedure (chaperone-contract? v) → boolean?
> v : any/c

Returns #t if its argument is a contract that guarantees that it returns a value which passes chaperone-of? when compared to the original, uncontracted value.

> procedure (impersonator-contract? v) → boolean?
> v : any/c

Returns #t if its argument is a contract that is not a chaperone contract nor a flat contract.

> procedure (flat-contract? v) → boolean?
> v : any/c

Returns #t when its argument is a contract that can be checked immediately (unlike, say, a function contract).

For example, flat-contract constructs flat contracts from predicates, and symbols, booleans, numbers, and other ordinary Racket values (that are defined as contracts) are also flat contracts.

> procedure (contract-name c) → any/c
> c : contract?

Produces the name used to describe the contract in error messages.

> procedure (value-contract v) → contract?
> v : has-contract?

Returns the contract attached to v, if recorded. Otherwise it returns #f.

To support value-contract and has-contract? in your own contract combinators, use prop:contracted or impersonator-prop:contracted.

> procedure (has-contract? v) → boolean?
> v : any/c

Returns #t if v is a value that has a recorded contract attached to it.

See also value-contract.

> procedure (contract-projection c) → (-> blame? (-> any/c any/c))
> c : contract?

Produces the projection defining a contract’s behavior on protected values.

> procedure (make-none/c sexp-name) → contract?
> sexp-name : any/c

Makes a contract that accepts no values, and reports the name sexp-name when signaling a contract violation.

> syntax (recursive-contract contract-expr)
> (recursive-contract contract-expr type)

Delays the evaluation of its argument until the contract is checked, making recursive contracts possible. If type is given, it describes the expected type of contract and must be one of the keywords #:impersonator, #:chaperone, or #:flat. If type is not given, an impersonator contract is created.

> syntax (opt/c contract-expr maybe-name)
> maybe-name |  | = |  |
> |  | | |  | #:error-name id

This optimizes its argument contract expression by traversing its syntax and, for known contract combinators, fuses them into a single contract combinator that avoids as much allocation overhead as possible. The result is a contract that should behave identically to its argument, except faster.

If the #:error-name argument is present, and contract-expr evaluates to a non-contract expression, then opt/c raises an error using id as the name of the primitive, instead of using the name opt/c.

Examples:
> > (define/contract (f x)
> (opt/c '(not-a-contract))
> x)
> opt/c: contract violation
> expected: contract?
> given: '(not-a-contract)
> > (define/contract (f x)
> (opt/c '(not-a-contract) #:error-name define/contract)
> x)
> define/contract: contract violation
> expected: contract?
> given: '(not-a-contract)

> syntax (define-opt/c (id id ...) expr)

This defines a recursive contract and simultaneously optimizes it. Semantically, it behaves just as if the -opt/c were not present, defining a function on contracts (except that the body expression must return a contract). But, it also optimizes that contract definition, avoiding extra allocation, much like opt/c does.

For example,

> (define-contract-struct bt (val left right))
> (define-opt/c (bst-between/c lo hi)
> (or/c null?
> (bt/c [val (real-in lo hi)]
> [left (val) (bst-between/c lo val)]
> [right (val) (bst-between/c val hi)])))
> (define bst/c (bst-between/c -inf.0 +inf.0))

defines the bst/c contract that checks the binary search tree invariant. Removing the -opt/c also makes a binary search tree contract, but one that is (approximately) 20 times slower.

> value contract-continuation-mark-key : continuation-mark-key?

Key used by continuation marks that are present during contract checking. The value of these marks are the blame objects that correspond to the contract currently being checked.
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