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