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c
2011 Didier Verna
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Copying 1
Copying
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Chapter 3: Proposal 2
1 Motivation
The Common Lisp standard defines two special variables, *package* and *readtable*, that
are treated in a special way: the functions load and compile-file establish a new dynamic
binding for each of them, so that any modification to their value at load or compile time becomes
local to the file being processed. It is this particular treatment of these variables that allows
for in-package or in-readtable (from the named-readtables library) to essentially have a
“file-local” effect.
The motivation for the present document is the claim that this behavior could be useful for
other, user-defined variables, although there is currently no way to do so in standard Common
Lisp.
2 Example
XFormat is a library that extends the capabilities of the standard format function by letting the
user modify the set of available format directives and their behavior. At the heart of XFormat lies
the notion of “format table”. A format table stores the association between directive characters
and their meaning. XFormat defines a special variable named *format-table* which holds the
value of the current format table. Conceptually, *format-table* is very close to *package* or
*readtable* in the sense that it centralizes the definition of a specific part of the behavior of
a Common Lisp program.
XFormat provides constructs such as with-format-table, to temporarily and locally bind
*format-table* to a specific value. XFormat also provides a macro called in-format-table,
the intent of which is obviously to do something similar to in-package or in-readtable. How-
ever, it is currently impossible to define it properly in all situations because the variable *format-
table* won’t be restored to its former value after loading or compiling the file.
3 Proposal
An extension to the Common Lisp standard is not strictly required to achieve the desired goal.
As a matter of fact, we have written a library called asdf-flv which does this for ASDF systems
(see Appendix A [ASDF-FLV], page 3).
However, we still think it is worthwhile to have it implemented directly by Common Lisp
vendors. In doing so, one would not depend on a specific system management utility to get the
functionality, and moreover, such an extension is very cheap to implement (see below).
The API to the requested functionality can be as simple as providing a function called
make-variable-file-local in whatever extension package a Common Lisp implementation
happens to use (the suggested name is inspired from (X)Emacs’s make-variable-buffer-local
function).
[Function]make-variable-file-local SYMBOL
Make special variable SYMBOL behave in a file-local manner.
File-local variables behave like *package* and *readtable*: load and compile-file bind
them to the values they held before loading or compiling the file.
The function above would pushnew the variable to an internal list of special variables. Assum-
ing this list is called *file-local-variables*, the functions load and compile-file would
in turn wrap their functionality within a call to progv, as follows:
☛ ✟
(progv *file-local-variables* (mapcar #’symbol-value *file-local-variables*)
#| do the loading or compiling |#)
✡ ✠
Appendix A: ASDF-FLV 3
Appendix A ASDF-FLV
For reference, below is the implementation of the asdf-flv library. A vendor-based implemen-
tation of our proposal would do something very similar.
☛ ✟
(defvar *file-local-variables* ()
"List of file-local special variables.")
(defun make-variable-file-local (symbol)
"Make special variable named by SYMBOL have a file-local value."
(pushnew symbol *file-local-variables*))
(defmethod asdf:perform :around
((operation asdf:load-op) (file asdf:cl-source-file))
"Establish new dynamic bindings for file-local variables."
(progv *file-local-variables*
(mapcar #’symbol-value *file-local-variables*)
(call-next-method)))
(defmethod asdf:perform :around
((operation asdf:compile-op) (file asdf:cl-source-file))
"Establish new dynamic bindings for file-local variables."
(progv *file-local-variables*
(mapcar #’symbol-value *file-local-variables*)
(call-next-method)))
✡ ✠
