A namespace encapsulates two pieces of information:
A mapping from identifiers to bindings. For example, a namespace might map the identifier lambda to the lambda form. An “empty” namespace is one that maps every identifier to an uninitialized top-level variable.
A mapping from module names to module declarations and instances.
The first mapping is used for evaluating expressions in a top-level context, as in (eval '(lambda (x) (+ x 1))). The second mapping is used, for example, by dynamic-require to locate a module. The call (eval '(require scheme/base)) normally uses both pieces: the identifier mapping determines the binding of require; if it turns out to mean require, then the module mapping is used to locate the scheme/base module.
From the perspective of the core Scheme run-time system, all evaluation is reflective. Execution starts with an initial namespace that contains a few primitive modules, and that is further populated by loading files and modules as specified on the command line or as supplied in the REPL. Top-level require and define forms adjusts the identifier mapping, and module declarations (typically loaded on demand for a require form) adjust the module mapping.
The function make-empty-namespace creates a new, empty namespace. Since the namespace is truly empty, it cannot at first be used to evaluate any top-level expression – not even (require scheme). In particular,
|(parameterize ([current-namespace (make-empty-namespace)])|
fails, because the namespace does not include the primitive modules on which scheme is built.
To make a namespace useful, some modules much be attached from an existing namespace. Attaching a module adjusts the mapping of module names to instances by transitively copying entries (the module and all its imports) from an existing namespace’s mapping. Normally, instead of just attaching the primitive modules – whose names and organization are subject to change – a higher-level module is attached, such as scheme or scheme/base.
The make-base-empty-namespace function provides a namespace that is empty, except that scheme/base is attached. The resulting namespace is still “empty” in the sense that the identifiers-to-bindings part of the namespace has no mappings; only the module mapping has been populated. Nevertheless, with an initial module mapping, further modules can be loaded.
A namespace created with make-base-empty-namespace is suitable for many basic dynamic tasks. For example, suppose that a my-dsl library implements a domain-specific language in which you want to execute commands from a user-specified file. A namespace created with make-base-empty-namespace is enough to get started:
|(define (run-dsl file)|
|(parameterize ([current-namespace (make-base-empty-namespace)])|
Note that the parameterize of current-namespace does not affect the meaning of identifiers like namespace-require within the parameterize body. Those identifiers obtain their meaning from the enclosing context (probably a module). Only expressions that are dynamic with respect to this code, such as the content of loaded files, are affected by the parameterize.
Another subtle point in the above example is the use of (namespace-require 'my-dsl) instead of (eval '(require my-dsl)). The latter would not work, because eval needs to obtain a meaning for require in the namespace, and the namespace’s identifier mapping is initially empty. The namespace-require function, in contrast, directly imports the given module into the current namespace. Starting with (namespace-require 'scheme/base) would introduce a binding for require and make a subsequent (eval '(require my-dsl)) work. The above is better, not only because it is more compact, but also because it avoids introducing bindings that are not part of the domain-specific languages.
Modules not attached to a new namespace will be loaded and instantiated afresh if they are demanded by evaluation. For example, scheme/base does not include scheme/class, and loading scheme/class again will create a distinct class datatype:
|> (require scheme/class)|
|> (class? object%)|
For cases when dynamically loaded code needs to share more code and data with its context, use the namespace-attach-module function. The first argument to namespace-attach-module is a source namespace from which to draw a module instance; in some cases, the current namespace is known to include the module that needs to be shared:
|> (require scheme/class)|
|(define (load-plug-in file)|
|(let ([ns (make-base-empty-namespace)])|
|(namespace-attach-module (namespace-anchor->empty-namespace a)|
|(parameterize ([current-namespace ns])|
|(dynamic-require file 'plug-in%))))|
The anchor bound by namespace-attach-module connects the run time of a module with the namespace in which a module is loaded (which might differ from the current namespace). In the above example, since the enclosing module requires scheme/class, the namespace produced by namespace-anchor->empty-namespace certainly contains an instance of scheme/class. Moreover, that instance is the same as the one imported into the module, so the class datatype is shared.