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Issue: LOAD-OBJECTS (Version 2)
Here is the updated version of this. Sorry it took so long.
I will bring a few copies of this with me to the meeting.
Related issues: LOAD-TIME-EVAL,
Edit history: Version 1, 2-Jan-89, by Moon (for discussion)
Version 2, 13-Jan-89, by Moon (draft updated from discussion)
Common Lisp doesn't provide any way to use an object of a user-defined
type (defined with DEFCLASS or DEFSTRUCT) as a constant in a program
compiled with COMPILE-FILE. The problem is that LOAD has to be able
to "reconstruct" an equivalent object when the compiled-code file is
loaded, but the programmer has no way to tell LOAD how to do that.
Define a new generic function named MAKE-LOAD-FORM, which takes one
argument and returns two values. The argument is an object that is
referenced as a constant or as a self-evaluating form in a file being
compiled by COMPILE-FILE. The objective is to enable LOAD to
construct an equivalent object.
The first value, called the "creation form," is a form that, when
evaluated at load time, should return an object that is equivalent to
the argument. The exact meaning of "equivalent" depends on the type
of object and is up to the programmer who defines a method for
MAKE-LOAD-FORM. This is the same type of equivalence discussed
in issue CONSTANT-COMPILABLE-TYPES.
The second value, called the "initialization form," is a form that,
when evaluated at load time, should perform further initialization of
the object. The value returned by the initialization form is ignored.
If the MAKE-LOAD-FORM method returns only one value, the
initialization form is NIL, which has no effect. If the object used
as the argument to MAKE-LOAD-FORM appears as a constant in the
initialization form, at load time it will be replaced by the
equivalent object constructed by the creation form; this is how the
further initialization gains access to the object.
Both the creation form and the initialization form can contain
references to objects of user-defined types (defined precisely below).
However, there must not be any circular dependencies in creation forms.
An example of a circular dependency is when the creation form for the
object X contains a reference to the object Y, and the creation form
for the object Y contains a reference to the object X. A simpler
example would be when the creation form for the object X contains
a reference to X itself. Initialization forms are not subject to
any restriction against circular dependencies, which is the entire
reason that initialization forms exist. See the example of circular
data structures below.
The creation form for an object is always evaluated before the
initialization form for that object. When either the creation form or
the initialization form references other objects of user-defined types
that have not been referenced earlier in the COMPILE-FILE, the
compiler collects all of the creation forms together and collects all
of the initialization forms together. All of the creation forms are
evaluated before any of the initialization forms. The order of
evaluation of the creation forms is unspecified except when the
ordering is forced by data dependencies. The order of evaluation of
the initialization forms is unspecified.
While these creation and initialization forms are being evaluated, the
objects are possibly in an uninitialized state, analogous to the state
of an object between the time it has been created by ALLOCATE-INSTANCE
and it has been processed fully by INITIALIZE-INSTANCE. Programmers
writing methods for MAKE-LOAD-FORM must take care in manipulating
objects not to depend on slots that have not yet been initialized.
It is unspecified whether LOAD calls EVAL on the forms or does some
other operation that has an equivalent effect. For example, the
forms might be translated into different but equivalent forms and
then evaluated, they might be compiled and the resulting functions
called by LOAD, or they might be interpreted by a special-purpose
interpreter different from EVAL. All that is required is that the
effect be equivalent to evaluating the forms.
COMPILE-FILE calls MAKE-LOAD-FORM on any object that is referenced as
a constant or as a self-evaluating form, if the object's metaclass is
STANDARD-CLASS, STRUCTURE-CLASS, any user-defined metaclass (not a
subclass of BUILT-IN-CLASS), or any of a possibly-empty
implementation-defined list of other metaclasses. COMPILE-FILE will
only call MAKE-LOAD-FORM once for any given object (compared with EQ)
within a single file.
It is valid for user programs to call MAKE-LOAD-FORM in other
The function MAKE-LOAD-FORM-USING-SLOTS can be useful in user-written
MAKE-LOAD-FORM methods. Its first argument is the object. Its
optional second argument is a list of the names of the slots to
preserve; it defaults to all of the local slots.
MAKE-LOAD-FORM-USING-SLOTS returns forms that construct an equivalent
object using MAKE-INSTANCE and SETF of SLOT-VALUE for slots with
values, or SLOT-MAKUNBOUND for slots without values, or using other
functions of equivalent effect. MAKE-LOAD-FORM-USING-SLOTS returns
two values, thus it can deal with circular structures.
The default MAKE-LOAD-FORM method for STANDARD-OBJECT signals an
The default MAKE-LOAD-FORM method for STRUCTURE-OBJECT returns forms
that construct an equivalent structure based on the structure name and
the slot values. This might be written using
MAKE-LOAD-FORM-USING-SLOTS, but that is not required.
;; Example 1
(defclass my-class ()
((a :initarg :a :reader my-a)
(b :initarg :b :reader my-b)
(c :accessor my-c)))
(defmethod shared-initialize ((self my-class) ignore &rest ignore)
(unless (slot-boundp self 'c)
(setf (my-c self) (some-computation (my-a self) (my-b self)))))
(defmethod make-load-form ((self my-class))
`(make-instance ',(class-name (class-of self))
:a ',(my-a self) :b ',(my-b self)))
In this example, an equivalent instance of my-class is reconstructed
by using the values of two of its slots. The value of the third slot
is derived from those two values.
Another way to write the last form in the above example would have been
(defmethod make-load-form ((self my-class))
(make-load-form-using-slots self '(a b)))
;; Example 2
(defclass my-frob ()
((name :initarg :name :reader my-name)))
(defmethod make-load-form ((self my-frob))
`(find-my-frob ',(my-name self) :if-does-not-exist :create))
In this example, instances of my-frob are "interned" in some way.
An equivalent instance is reconstructed by using the value of the
name slot as a key for searching existing objects. In this case
the programmer has chosen to create a new object if no existing
object is found; alternatively she could have chosen to signal an
error in that case.
;; Example 3
(defclass tree-with-parent () ((parent :accessor tree-parent)
(children :initarg :children)))
(defmethod make-load-form ((x tree-with-parent))
;; creation form
`(make-instance ',(class-of x) :children ',(slot-value x 'children))
;; initialization form
`(setf (tree-parent ',x) ',(slot-value x 'parent))))
In this example, the data structure to be dumped is circular, because
each parent has a list of its children and each child has a reference
back to its parent. Suppose make-load-form is called on one object in
such a structure. The creation form creates an equivalent object and
fills in the children slot, which forces creation of equivalent
objects for all of its children, grandchildren, etc. At this point
none of the parent slots have been filled in. The initialization form
fills in the parent slot, which forces creation of an equivalent
object for the parent if it was not already created. Thus the entire
tree is recreated at load time. At compile time, MAKE-LOAD-FORM is
called once for each object in the true. All of the creation forms
are evaluated, in unspecified order, and then all of the
initialization forms are evaluated, also in unspecified order.
Only the programmer who designed a class can know the correct
way to reconstruct objects of that class at load time, therefore
the reconstruction should be controlled by a generic function.
Using EVAL as the interface for telling LOAD what to do provides
MAKE-LOAD-FORM returns two values so that circular structures can
be handled. If CONSTANT-CIRCULAR-COMPILATION is rejected,
MAKE-LOAD-FORM will only return one value, although implementations
that make an extension to support circular constants will probably
also make the extension to accept two values from MAKE-LOAD-FORM.
A default method, such as one that makes an object whose class has the
same name and whose slots have equivalent contents, is not supplied
for DEFCLASS-defined objects, because this is inappropriate for many
objects and because it is easy to write for those objects where it is
appropriate. The function MAKE-LOAD-FORM-USING-SLOTS makes it even
easier to write.
MAKE-LOAD-FORM has a natural resemblance to PRINT-OBJECT, as a hook
for the programmer to control the system's actions.
Symbolics Flavors has something like this, but under a different name.
The name Symbolics uses is not suitable for standardization.
JonL reports that Lucid is getting more and more requests for this.
Cost to Implementors:
This seems like only a few one-line changes in the compiled-code
file writer and reader. MAKE-LOAD-FORM-USING-SLOTS is a couple
dozen lines of code, assuming the presence of the CLOS metaobject
protocol or an implementation-dependent equivalent.
Cost to Users:
Cost of non-adoption:
Serious impairment of the ability to use extended-type objects. Each
implementation will probably make up its own version of this as an
See Cost of non-adoption.
No significant positive or negative impact.
It would be possible to define an additional level of protocol that
allows multiple classes to contribute to the reconstruction of an
object, combining initialization arguments contributed by each class.
Since a user can easily define that in terms of MAKE-LOAD-FORM without
modifying the Lisp system, it is not being proposed now.
Any type that has a read syntax is likely to appear as a quoted
constant or inside a quoted constant. Pathnames are one example, user
programs often define others. Also many implementations provide a way
to create a compiled-code file full of data (rather than compiled Lisp
programs), and such data probably include extended-type objects.
Moon supports this. David Gray and John Rose made major contributions
to the discussion that produced this improved version 2 proposal.