► |  | 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

▼ | 14 | Units

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`` Units (Components)

Units organize a program into separately compilable and reusable components. A unit resembles a procedure in that both are first-class values that are used for abstraction. While procedures abstract over values in expressions, units abstract over names in collections of definitions. Just as a procedure is called to evaluate its expressions given actual arguments for its formal parameters, a unit is invoked to evaluate its definitions given actual references for its imported variables. Unlike a procedure, however, a unit’s imported variables can be partially linked with the exported variables of another unit prior to invocation. Linking merges multiple units together into a single compound unit. The compound unit itself imports variables that will be propagated to unresolved imported variables in the linked units, and re-exports some variables from the linked units for further linking.

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
14.6.1 Adding Contracts to Signatures
14.6.2 Adding Contracts to Units
14.7 unit versus module

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