define-structure form introduces a binding of a name to a
structure. A structure is a view on an underlying package which is
created according to the clauses of the define-structure form.
Each structure has an interface that specifies which bindings in the
structure's underlying package can be seen via that structure in other
An open clause specifies which structures will be opened up for use inside the new package. At least one structure must be specified or else it will be impossible to write any useful programs inside the package, since define, lambda, cons, etc. will be unavailable. Packages typically include scheme, which exports all bindings appropriate to Revised5 Scheme, in an open clause. For building structures that export structures, there is a defpackage package that exports the operators of the configuration language. Many other structures, such as record and hash table facilities, are also available in the Scheme 48 implementation.
prefix forms produce new
views on existing structures by renaming or hiding exported names.
Subset returns a new structure that exports only the listed names
from its <structure> argument.
With-prefix returns a new structure that adds <prefix>
to each of the names exported by the <structure> argument.
For example, if structure
exports only(subset s (a))
exports(with-prefix s p/)
with-prefix are simple macros that
expand into uses of
modify, a more general renaming form.
modify structure specification the <command>s are applied to
the names exported
by <structure> to produce a new set of names for the <structure>'s
Expose makes only the listed names visible.
Hide makes all but the listed names visible.
Rename makes each <name>0 visible as <name>1
name and not visible as <name>0 , while
alias makes each <name>0 visible as both <name>0
Prefix adds <name> to the beginning of each exported name.
The modifiers are applied from right to left. Thus
makes(modify scheme (prefix foo/) (rename (car bus))))
The package's body is specified by begin and/or files clauses. begin and files have the same semantics, except that for begin the text is given directly in the package definition, while for files the text is stored somewhere in the file system. The body consists of a Scheme program, that is, a sequence of definitions and expressions to be evaluated in order. In practice, we always use files in preference to begin; begin exists mainly for expository purposes.
A name's imported binding may be lexically overridden or shadowed by defining the name using a defining form such as define or define-syntax. This will create a new binding without having any effect on the binding in the opened package. For example, one can do (define car 'chevy) without affecting the binding of the name car in the scheme package.
Assignments (using set!) to imported and undefined variables are not allowed. In order to set! a top-level variable, the package body must contain a define form defining that variable. Applied to bindings from the scheme structure, this restriction is compatible with the requirements of the Revised5 Scheme report.
It is an error for two of a package's opened structures to export two different bindings for the same name. However, the current implementation does not check for this situation; a name's binding is always taken from the structure that is listed first within the open clause. This may be fixed in the future.
File names in a files clause can be symbols, strings, or lists (Maclisp-style "namelists"). A ".scm" file type suffix is assumed. Symbols are converted to file names by converting to upper or lower case as appropriate for the host operating system. A namelist is an operating-system-independent way to specify a file obtained from a subdirectory. For example, the namelist (rts record) specifies the file record.scm in the rts subdirectory.
If the define-structure form was itself obtained from a file, then file names in files clauses are interpreted relative to the directory in which the file containing the define-structure form was found. You can't at present put an absolute path name in the files list.
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