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# Assignment and Redefinition
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The use of `set!` on variables defined within a module is limited to the
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body of the defining module. That is, a module is allowed to change the
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value of its own definitions, and such changes are visible to importing
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modules. However, an importing context is not allowed to change the
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value of an imported binding.
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Examples:
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```racket
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> (module m racket
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(provide counter increment!)
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(define counter 0)
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(define (increment!)
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(set! counter (add1 counter))))
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> (require 'm)
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> counter
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0
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> (increment!)
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> counter
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1
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> (set! counter -1)
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set!: cannot mutate module-required identifier
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at: counter
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in: (set! counter -1)
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```
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As the above example illustrates, a module can always grant others the
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ability to change its exports by providing a mutator function, such as
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`increment!`.
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The prohibition on assignment of imported variables helps support
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modular reasoning about programs. For example, in the module,
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```racket
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(module m racket
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(provide rx:fish fishy-string?)
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(define rx:fish #rx"fish")
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(define (fishy-string? s)
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(regexp-match? rx:fish s)))
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```
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the function `fishy-string?` will always match strings that contain
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“fish”, no matter how other modules use the `rx:fish` binding. For
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essentially the same reason that it helps programmers, the prohibition
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on assignment to imports also allows many programs to be executed more
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efficiently.
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Along the same lines, when a module contains no `set!` of a particular
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identifier that is defined within the module, then the identifier is
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considered a _constant_ that cannot be changed—not even by re-declaring
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the module.
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Consequently, re-declaration of a module is not generally allowed. For
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file-based modules, simply changing the file does not lead to a
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re-declaration in any case, because file-based modules are loaded on
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demand, and the previously loaded declarations satisfy future requests.
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It is possible to use Racket’s reflection support to re-declare a
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module, however, and non-file modules can be re-declared in the REPL; in
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such cases, the re-declaration may fail if it involves the re-definition
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of a previously constant binding.
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```racket
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> (module m racket
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(define pie 3.141597))
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> (require 'm)
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> (module m racket
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(define pie 3))
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define-values: assignment disallowed;
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cannot re-define a constant
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constant: pie
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in module: 'm
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```
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For exploration and debugging purposes, the Racket reflective layer
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provides a `compile-enforce-module-constants` parameter to disable the
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enforcement of constants.
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```racket
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> (compile-enforce-module-constants #f)
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> (module m2 racket
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(provide pie)
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(define pie 3.141597))
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> (require 'm2)
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> (module m2 racket
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(provide pie)
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(define pie 3))
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> (compile-enforce-module-constants #t)
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> pie
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3
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```
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