Issue: LOAD-OBJECTS (Version 3)

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

At Kauai I was asked to keep working on this and come up with a modified
version based on comments received.  Here it is.  I hope this is ready
for voting so we can clear it out of the way.

Issue:         LOAD-OBJECTS

References:    none

Related issues: LOAD-TIME-EVAL,
                CONSTANT-COMPILABLE-TYPES,
                CONSTANT-CIRCULAR-COMPILATION

Category:      ADDITION

Forum:         Cleanup

Edit history:  Version 1, 2-Jan-89, by Moon (for discussion)
               Version 2, 13-Jan-89, by Moon (draft updated from discussion)
               Version 3,  9-Mar-89, by Moon (changes suggested by discussion)

Problem description:

  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.


Proposal (LOAD-OBJECTS:MAKE-LOAD-FORM):
          
  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 and initialization forms.  Each
  initialization form is evaluated as soon as possible after its
  creation form, as determined by data flow.  If the initialization form
  for an object does not reference any other objects of user-defined
  types that have not been referenced earlier in the COMPILE-FILE, the
  initialization form is evaluated immediately after the creation form.
  If a creation or initialization form F references other objects of
  user-defined types that have not been referenced earlier in the
  COMPILE-FILE, the creation forms for those other objects are evaluated
  before F, and the initialization forms for those other objects are
  also evaluated before F whenever they do not depend on the object
  created or initialized by F.  Where the above rules do not uniquely
  determine an order of evaluation, which of the possible orders of
  evaluation is chosen 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
  circumstances, providing the argument's metaclass is not BUILT-IN-CLASS
  or a subclass of BUILT-IN-CLASS.

  Define a new function named MAKE-LOAD-FORM-USING-SLOTS, which takes
  one required argument and one optional argument and returns two
  values.  This can be useful in user-written MAKE-LOAD-FORM methods.
  The first argument is the object.  The 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.  MAKE-LOAD-FORM-USING-SLOTS works for any object
  of metaclass STANDARD-CLASS or STRUCTURE-CLASS.  Whether the result is
  useful in an application depends on whether the object's type and slot
  contents fully capture the application's idea of the object's state.

  MAKE-LOAD-FORM of an object of metaclass STANDARD-CLASS or
  STRUCTURE-CLASS for which no user-defined method is applicable signals
  an error.  It is valid to implement this either by defining default
  methods on STANDARD-OBJECT and STRUCTURE-OBJECT that signal an error
  or by having no applicable method for those classes.


Examples:

  ;; 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))
    (values
      ;; 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.

  ;; Example 4
  (defstruct my-struct a b c)
  (defmethod make-load-form ((s my-struct))
     (make-load-form-using-slots s))

  In this example, the data structure to be dumped has no special
  properties and an equivalent structure can be reconstructed
  simply by reconstructing the slots' contents.


Rationale:

  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
  full generality.

  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.

  The default for class objects and structures is to signal an error,
  rather than picking some particular object reconstruction technique,
  because no reconstruction technique is appropriate for all objects.
  It only takes two lines of code, as in example 4, to instruct the
  compiler to use the technique that most often has been suggested
  as the default.

  MAKE-LOAD-FORM has a natural resemblance to PRINT-OBJECT, as a hook
  for the programmer to control the system's actions.

  The order of evaluation rules for creation and initialization forms
  eliminate the possibility of partially initialized objects in the
  absence of circular structures, and reduce it to the minimum possible
  in the presence of circular structures.  This allows nodes in
  non-circular structures to be built out of fully initialized subparts.


Current practice:

  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:

  None.

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
  extension.

Performance impact:

  None.

Benefits:

  See Cost of non-adoption.

Esthetics:

  No significant positive or negative impact.

Discussion:

  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.