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# Identifiers and Binding
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The context of an expression determines the meaning of identifiers that
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appear in the expression. In particular, starting a module with the
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language `racket`, as in
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`#lang` `racket`
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means that, within the module, the identifiers described in this guide
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start with the meaning described here: `cons` refers to the function
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that creates a pair, `car` refers to the function that extracts the
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first element of a pair, and so on.
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> +\[missing\] introduces the syntax of identifiers.
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Forms like `define`, `lambda`, and `let` associate a meaning with one or
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more identifiers; that is, they _bind_ identifiers. The part of the
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program for which the binding applies is the _scope_ of the binding. The
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set of bindings in effect for a given expression is the expression’s
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_environment_.
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For example, in
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```racket
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#lang racket
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(define f
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(lambda (x)
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(let ([y 5])
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(+ x y))))
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(f 10)
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```
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the `define` is a binding of `f`, the `lambda` has a binding for `x`,
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and the `let` has a binding for `y`. The scope of the binding for `f` is
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the entire module; the scope of the `x` binding is `(let ([y 5]) (+ x
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y))`; and the scope of the `y` binding is just `(+ x y)`. The
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environment of `(+ x y)` includes bindings for `y`, `x`, and `f`, as
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well as everything in `racket`.
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A module-level `define` can bind only identifiers that are not already
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defined or `require`d into the module. A local `define` or other binding
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forms, however, can give a new local binding for an identifier that
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already has a binding; such a binding _shadows_ the existing binding.
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Examples:
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```racket
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(define f
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(lambda (append)
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(define cons (append "ugly" "confusing"))
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(let ([append 'this-was])
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(list append cons))))
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> (f list)
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'(this-was ("ugly" "confusing"))
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```
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Similarly, a module-level `define` can shadow a binding from the
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module’s language. For example, `(define cons 1)` in a `racket` module
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shadows the `cons` that is provided by `racket`. Intentionally shadowing
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a language binding is rarely a good idea—especially for widely used
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bindings like `cons`—but shadowing relieves a programmer from having to
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avoid every obscure binding that is provided by a language.
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Even identifiers like `define` and `lambda` get their meanings from
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bindings, though they have _transformer_ bindings \(which means that
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they indicate syntactic forms\) instead of value bindings. Since
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`define` has a transformer binding, the identifier `define` cannot be
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used by itself to get a value. However, the normal binding for `define`
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can be shadowed.
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Examples:
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```racket
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> define
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eval:1:0: define: bad syntax
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in: define
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> (let ([define 5]) define)
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5
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```
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Again, shadowing standard bindings in this way is rarely a good idea,
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but the possibility is an inherent part of Racket’s flexibility.
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