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3.1 Modules: module, module*, ...

+The module Form in The Racket Guide introduces module.


(module id module-path form ...)

Declares a top-level module or a submodule. For a top-level module, if the current-module-declare-name parameter is set, the parameter value is used for the module name and id is ignored, otherwise (quote id) is the name of the declared module. For a submodule, id is the name of the submodule to be used as an element within a submod module path.

+For a module-like form that works in definitions context other than the top level or a module body, see define-package.

The module-path form must be as for require, and it supplies the initial bindings for the body forms. That is, it is treated like a (require module-path) prefix before the forms, except that the bindings introduced by module-path can be shadowed by definitions and requires in the module body forms.

If a single form is provided, then it is partially expanded in a module-begin context. If the expansion leads to #%plain-module-begin, then the body of the #%plain-module-begin is the body of the module. If partial expansion leads to any other primitive form, then the form is wrapped with #%module-begin using the lexical context of the module body; this identifier must be bound by the initial module-path import, and its expansion must produce a #%plain-module-begin to supply the module body. Finally, if multiple forms are provided, they are wrapped with #%module-begin, as in the case where a single form does not expand to #%plain-module-begin.

After such wrapping, if any, and before any expansion, an 'enclosing-module-name property is attached to the #%module-begin syntax object (see Syntax Object Properties); the property’s value is a symbol corresponding to id.

Each form is partially expanded (see Partial Expansion) in a module context. Further action depends on the shape of the form:

After all forms have been partially expanded this way, then the remaining expression forms (including those on the right-hand side of a definition) are expanded in an expression context. After all expression forms, #%provide forms are processed in the order in which they appear (independent of phase) in the expanded module. Finally, all module* forms are expanded in order, so that each becomes available for use by subsequent module* forms; the enclosing module itself is also available for use by module* submodules.

The scope of all imported identifiers covers the entire module body, except for nested module and module* forms (assuming a non-#f module-path in the latter case). The scope of any identifier defined within the module body similarly covers the entire module body except for such nested module and module* forms. The ordering of syntax definitions does not affect the scope of the syntax names; a transformer for A can produce expressions containing B, while the transformer for B produces expressions containing A, regardless of the order of declarations for A and B. However, a syntactic form that produces syntax definitions must be defined before it is used.

No identifier can be imported or defined more than once at any phase level within a single module, except that a definition via define-values or define-syntaxes can shadow a preceding import via #%require; unless the shadowed import is from the module’s initial module-path, a warning is logged to the initial logger. Every exported identifier must be imported or defined. No expression can refer to a top-level variable. A module* form in which the enclosing module’s bindings are visible (i.e., a nested module* with #f instead of a module-path) can define or import bindings that shadow the enclosing module’s bindings.

The evaluation of a module form does not evaluate the expressions in the body of the module (except sometimes for redeclarations; see Module Redeclarations). Evaluation merely declares a module, whose full name depends both on id or (current-module-declare-name).

A module body is executed only when the module is explicitly instantiated via require or dynamic-require. On invocation, imported modules are instantiated in the order in which they are required into the module (although earlier instantiations or transitive requires can trigger the instantiation of a module before its order within a given module). Then, expressions and definitions are evaluated in order as they appear within the module. Each evaluation of an expression or definition is wrapped with a continuation prompt (see call-with-continuation-prompt) for the default prompt tag and using a prompt handler that re-aborts and propagates its argument to the next enclosing prompt. Each evaluation of a definition is followed, outside of the prompt, by a check that each of the definition’s variables has a value; if the portion of the prompt-delimited continuation that installs values is skipped, then the exn:fail:contract:variable? exception is raised.

Accessing a module-level variable before it is defined signals a run-time error, just like accessing an undefined global variable. If a module (in its fully expanded form) does not contain a set! for an identifier that defined within the module, then the identifier is a constant after it is defined; its value cannot be changed afterward, not even through reflective mechanisms. The compile-enforce-module-constants parameter, however, can be used to disable enforcement of constants.

When a syntax object representing a module form has a 'module-language syntax property attached, and when the property value is a vector of three elements where the first is a module path (in the sense of module-path?) and the second is a symbol, then the property value is preserved in the corresponding compiled and/or declared module. The third component of the vector should be printable and readable, so that it can be preserved in marshaled bytecode. The racket/base and racket languages attach '#(racket/language-info get-info #f) to a module form. See also module-compiled-language-info, module->language-info, and racket/language-info.

See also Modules and Module-Level Variables, Module Expansion, Phases, and Visits, and Information on Expanded Modules.

> (module duck racket/base
    (provide num-eggs quack)
    (define num-eggs 2)
    (define (quack n)
      (unless (zero? n)
        (printf "quack\n")
        (quack (sub1 n)))))

Changed in version 6.3 of package base: Changed define-syntaxes and define-values to shadow any preceding import, and dropped the use of 'submodule syntax property values on nested module or module* forms.


(module* id module-path form ...)

(module* id #f form ...)

Like module, but only for declaring a submodule within a module, and for submodules that may require the enclosing module.

Instead of a module-path after id, #f indicates that all bindings from the enclosing module are visible in the submodule. In that case, begin-for-syntax forms that wrap the module* form shift the phase level of the enclosing module’s bindings relative to the submodule. The macro expander handles such nesting by shifting the phase level of the module* form so that its body starts at phase level 0, expanding, and then reverting the phase level shift; beware that this process can leave syntax objects as 'origin syntax property values out-of-sync with the expanded module.

When a module* form has a module-path, the submodule expansion starts by removing the scopes of the enclosing module, the same as the module form. No shifting compensates for any begin-for-syntax forms that may wrap the submodule.


(module+ id form ...)

Declares and/or adds to a submodule named id.

Each addition for id is combined in order to form the entire submodule using (module* id #f ....) at the end of the enclosing module. If there is only one module+ for a given id, then (module+ id form ...) is equivalent to (module* id #f form ...), but still moved to the end of the enclosing module.

When a module contains multiple submodules declared with module+, then the relative order of the initial module+ declarations for each submodule determines the relative order of the module* declarations at the end of the enclosing module.

A submodule must not be defined using module+ and module or module*. That is, if a submodule is made of module+ pieces, then it must be made only of module+ pieces.


(#%module-begin form ...)

Legal only in a module begin context, and handled by the module and module* forms.

The #%module-begin form of racket/base wraps every top-level expression to print non-#<void> results using current-print.

The #%module-begin form of racket/base also declares a configure-runtime submodule (before any other form), unless some form is either an immediate module or module* form with the name configure-runtime. If a configure-runtime submodule is added, the submodule calls the configure function of racket/runtime-config.


(#%printing-module-begin form ...)

Legal only in a module begin context.

Like #%module-begin, but without adding a configure-runtime submodule.


(#%plain-module-begin form ...)

Legal only in a module begin context, and handled by the module and module* forms.


(#%declare declaration-keyword ...)

declaration-keyword = #:cross-phase-persistent
  | #:empty-namespace
Declarations that affect run-time or reflective properties of the module:

A #%declare form must appear in a module context or a module-begin context. Each declaration-keyword can be declared at most once within a module body.

Changed in version 6.3 of package base: Added #:empty-namespace.