► |  | The Racket Reference

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

▼ | 6 | Units

6.1 | Creating Units
6.2 | Invoking Units
6.3 | Linking Units and Creating Compound Units
6.4 | Inferred Linking
6.5 | Generating A Unit from Context
6.6 | Structural Matching
6.7 | Extending the Syntax of Signatures
6.8 | Unit Utilities
6.9 | Unit Contracts
6.10 | Single-Unit Modules
6.11 | Single-Signature Modules
6.12 | Transformer Helpers

On this page:

unit/ c
define-unit/ contract

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#### 6.9`` Unit Contracts

> > syntax

> > (unit/c (import sig-block ...) (export sig-block ...))

> sig-block |  | = |  | (tagged-sig-id [id contract] ...)
> |  | | |  | tagged-sig-id

A unit contract wraps a unit and checks both its imported and exported identifiers to ensure that they match the appropriate contracts. This allows the programmer to add contract checks to a single unit value without adding contracts to the imported and exported signatures.

The unit value must import a subset of the import signatures and export a superset of the export signatures listed in the unit contract. Any identifier which is not listed for a given signature is left alone. Variables used in a given contract expression first refer to other variables in the same signature, and then to the context of the unit/c expression.

> > syntax

> > (define-unit/contract unit-id
> > (import sig-spec-block ...)
> > (export sig-spec-block ...)
> > init-depends-decl
> > unit-body-expr-or-defn
> > ...)

> sig-spec-block |  | = |  | (tagged-sig-spec [id contract] ...)
> |  | | |  | tagged-sig-spec

The define-unit/contract form defines a unit compatible with link inference whose imports and exports are contracted with a unit contract. The unit name is used for the positive blame of the contract.
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