4.2 Identifiers and Binding

► |  | 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

▼ | 4 | Expressions and Definitions

4.1 | Notation
4.2 | Identifiers and Binding
4.3 | Function Calls
4.4 | Functions: lambda
4.5 | Definitions: define
4.6 | Local Binding
4.7 | Conditionals
4.8 | Sequencing
4.9 | Assignment: set!
4.10 | Quoting: quote and '
4.11 | Quasiquoting: quasiquote and ‘
4.12 | Simple Dispatch: case
4.13 | Dynamic Binding: parameterize

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#### 4.2`` Identifiers and Binding

The context of an expression determines the meaning of identifiers that appear in the expression. In particular, starting a module with the language racket, as in

> #lang racket

means that, within the module, the identifiers described in this guide start with the meaning described here: cons refers to the function that creates a pair, car refers to the function that extracts the first element of a pair, and so on.

> > > Symbols introduces the syntax of identifiers.

Forms like define, lambda, and let associate a meaning with one or more identifiers; that is, they bind identifiers. The part of the program for which the binding applies is the scope of the binding. The set of bindings in effect for a given expression is the expression’s environment.

For example, in

> #lang racket
> (define f
> (lambda (x)
> (let ([y 5])
> (+ x y))))
> (f 10)

the define is a binding of f, the lambda has a binding for x, and the let has a binding for y. The scope of the binding for f is the entire module; the scope of the x binding is (let ([y 5]) (+ x y)); and the scope of the y binding is just (+ x y). The environment of (+ x y) includes bindings for y, x, and f, as well as everything in racket.

A module-level define can bind only identifiers that are not already defined or required into the module. A local define or other binding forms, however, can give a new local binding for an identifier that already has a binding; such a binding shadows the existing binding.

Examples:

> (define f
> (lambda (append)
> (define cons (append "ugly" "confusing"))
> (let ([append 'this-was])
> (list append cons))))
> > (f list)
> '(this-was ("ugly" "confusing"))

Similarly, a module-level define can shadow a binding from the module’s language. For example, (define cons 1) in a racket module shadows the cons that is provided by racket. Intentionally shadowing a language binding is rarely a good idea—especially for widely used bindings like cons—but shadowing relieves a programmer from having to avoid every obscure binding that is provided by a language.

Even identifiers like define and lambda get their meanings from bindings, though they have transformer bindings (which means that they indicate syntactic forms) instead of value bindings. Since define has a transformer binding, the identifier define cannot be used by itself to get a value. However, the normal binding for define can be shadowed.

Examples:

> > define
> eval:1:0: define: bad syntax
> in: define
> > (let ([define 5]) define)
> 5

Again, shadowing standard bindings in this way is rarely a good idea, but the possibility is an inherent part of Racket’s flexibility.

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