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Our source code makes direct reference to some internals .tex files found only in the CMU CL source, and even our own manual has direct references to the CMU CL manual because the corresponding sections haven't been written yet (for over 2 decades). We might as well check-in the original documentation. I doubt anyone will ever be paid to write or even finish porting such high quality documentation, and it would be a shame to have 95% correct internals documentation be completely missing as well.
125 lines
4.5 KiB
TeX
125 lines
4.5 KiB
TeX
\section{Package Locks}
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\cindex{package locks}
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\cmucl{} provides two types of package locks, as an extension to the
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ANSI Common Lisp standard. The package-lock protects a package from
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changes in its structure (the set of exported symbols, its use list,
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etc). The package-definition-lock protects the symbols in the package
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from being redefined due to the execution of a \code{defun},
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\code{defmacro}, \code{defstruct}, \code{deftype} or \code{defclass}
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form.
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\subsection{Rationale}
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Package locks are an aid to program development, by helping to detect
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inadvertent name collisions and function redefinitions. They are
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consistent with the principle that a package ``belongs to'' its
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implementor, and that noone other than the package's developer should
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be making or modifying definitions on symbols in that package. Package
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locks are compatible with the ANSI Common Lisp standard, which states
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that the consequences of redefining functions in the
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\code{COMMON-LISP} package are undefined.
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Violation of a package lock leads to a continuable error of type
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\code{lisp::package-locked-error} being signaled. The user may choose
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to ignore the lock and proceed, or to abort the computation. Two other
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restarts are available, one which disables all locks on all packages,
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and one to disable only the package-lock or package-definition-lock
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that was tripped.
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The following transcript illustrates the behaviour seen when
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attempting to redefine a standard macro in the \code{COMMON-LISP}
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package, or to redefine a function in one of \cmucl{}'s
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implementation-defined packages:
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{\small
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\begin{verbatim}
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CL-USER> (defmacro 1+ (x) (* x 2))
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Attempt to modify the locked package COMMON-LISP, by defining macro 1+
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[Condition of type LISP::PACKAGE-LOCKED-ERROR]
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Restarts:
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0: [continue ] Ignore the lock and continue
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1: [unlock-package] Disable the package's definition-lock then continue
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2: [unlock-all ] Unlock all packages, then continue
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3: [abort ] Return to Top-Level.
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CL-USER> (defun ext:gc () t)
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Attempt to modify the locked package EXTENSIONS, by redefining function GC
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[Condition of type LISP::PACKAGE-LOCKED-ERROR]
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Restarts:
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0: [continue ] Ignore the lock and continue
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1: [unlock-package] Disable package's definition-lock, then continue
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2: [unlock-all ] Disable all package locks, then continue
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3: [abort ] Return to Top-Level.
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\end{verbatim}
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The following transcript illustrates the behaviour seen when an
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attempt to modify the structure of a package is made:
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\begin{verbatim}
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CL-USER> (unexport 'load-foreign :ext)
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Attempt to modify the locked package EXTENSIONS, by unexporting symbols LOAD-FOREIGN
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[Condition of type lisp::package-locked-error]
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Restarts:
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0: [continue ] Ignore the lock and continue
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1: [unlock-package] Disable package's lock then continue
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2: [unlock-all ] Unlock all packages, then continue
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3: [abort ] Return to Top-Level.
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\end{verbatim}
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}
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The \code{COMMON-LISP} package and the \cmucl{}-specific
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implementation packages are locked on startup. Users can lock their
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own packages by using the \code{ext:package-lock} and
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\code{ext:package-definition-lock} accessors.
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\subsection{Disabling package locks}
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A package's locks can be enabled or disabled by using the
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\code{ext:package-lock} and \code{ext:package-definition-lock}
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accessors, as follows:
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\begin{lisp}
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(setf (ext:package-lock (find-package "UNIX")) nil)
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(setf (ext:package-definition-lock (find-package "UNIX")) nil)
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\end{lisp}
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\begin{defun}{ext:}{package-lock}{\var{package}}
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This function is an accessor for a package's structural lock, which
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protects it against modifications to its list of exported symbols.
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\end{defun}
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\begin{defun}{ext:}{package-definition-lock}{\var{package}}
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This function is an accessor for a package's definition-lock, which
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protects symbols in that package from redefinition. As well as
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protecting the symbol's fdefinition from change, attempts to change
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the symbol's definition using \code{defstruct}, \code{defclass} or
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\code{deftype} will be trapped.
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\end{defun}
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\begin{defmac}{ext:}{without-package-locks}{\args{\amprest{} \var{body}}}
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This macro can be used to execute forms with all package locks (both
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structure and definition locks) disabled.
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\end{defmac}
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\begin{defun}{ext:}{unlock-all-packages}{}
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This function disables both structure and definition locks on all
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currently defined packages. Note that package locks are reset when
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\cmucl{} is restarted, so the effect of this function is limited to
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the current session.
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\end{defun}
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% EOF
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