5.1 Generalized Reference

5.1.1 Overview of Places and Generalized Reference

A generalized reference is the use of a form, sometimes called a place, as if it were a variable that could be read and written. The value of a place is the object to which the place form evaluates. The value of a place can be changed by using setf. The concept of binding a place is not defined in Common Lisp, but an implementation is permitted to extend the language by defining this concept.

Figure 5–1 contains examples of the use of setf. Note that the values returned by evaluating the forms in column two are not necessarily the same as those obtained by evaluating the forms in column three. In general, the exact macro expansion of a setf form is not guaranteed and can even be implementation-dependent; all that is guaranteed is that the expansion is an update form that works for that particular implementation, that the left-to-right evaluation of subforms is preserved, and that the ultimate result of evaluating setf is the value or values being stored.

Access function Update Function Update using setf
x (setq x datum) (setf x datum)
(car x) (rplaca x datum) (setf (car x) datum)
(symbol-value x) (set x datum) (setf (symbol-value x) datum)
Figure 5–1. Examples of setf

Figure 5–2 shows operators relating to places and generalized reference.

Some of the operators above manipulate places and some manipulate setf expanders. A setf expansion can be derived from any place. New setf expanders can be defined by using defsetf and define-setf-expander. Evaluation of Subforms to Places

The following rules apply to the evaluation of subforms in a place:

  1. The evaluation ordering of subforms within a place is determined by the order specified by the second value returned by get-setf-expansion. For all places defined by this specification (e.g., getf, ldb, ...), this order of evaluation is left-to-right. When a place is derived from a macro expansion, this rule is applied after the macro is expanded to find the appropriate place.

    Places defined by using defmacro or define-setf-expander use the evaluation order defined by those definitions. For example, consider the following:

    (defmacro wrong-order (x y) `(getf ,y ,x))

    This following form evaluates place2 first and then place1 because that is the order they are evaluated in the macro expansion:

    (push value (wrong-order place1 place2))
  2. For the macros that manipulate places (push, pushnew, remf, incf, decf, shiftf, rotatef, psetf, setf, pop, and those defined by define-modify-macro) the subforms of the macro call are evaluated exactly once in left-to-right order, with the subforms of the places evaluated in the order specified in (1).

    push, pushnew, remf, incf, decf, shiftf, rotatef, psetf, pop evaluate all subforms before modifying any of the place locations. setf (in the case when setf has more than two arguments) performs its operation on each pair in sequence. For example, in

    (setf place1 value1 place2 value2 ...)

    the subforms of place1 and value1 are evaluated, the location specified by place1 is modified to contain the value returned by value1, and then the rest of the setf form is processed in a like manner.

  3. For check-type, ctypecase, and ccase, subforms of the place are evaluated once as in (1), but might be evaluated again if the type check fails in the case of check-type or none of the cases hold in ctypecase and ccase.

  4. For assert, the order of evaluation of the generalized references is not specified.

Rules 2, 3 and 4 cover all standardized macros that manipulate places. Examples of Evaluation of Subforms to Places
 (let ((ref2 (list '()))) 
   (push (progn (princ "1") 'ref-1) 
         (car (progn (princ "2") ref2)))) 

 (let (x) 
    (push (setq x (list 'a)) 
          (car (setq x (list 'b)))) 
 (((A) . B))

push first evaluates (setq x (list 'a)) (a), then evaluates (setq x (list 'b)) (b), then modifies the car of this latest value to be ((a) . b). Setf Expansions

Sometimes it is possible to avoid evaluating subforms of a place multiple times or in the wrong order. A setf expansion for a given access form can be expressed as an ordered collection of five objects:

List of temporary variables

a list of symbols naming temporary variables to be bound sequentially, as if by let*, to values resulting from value forms.

List of value forms

a list of forms (typically, subforms of the place) which when evaluated yield the values to which the corresponding temporary variables should be bound.

List of store variables

a list of symbols naming temporary store variables which are to hold the new values that will be assigned to the place.

Storing form

a form which can reference both the temporary and the store variables, and which changes the value of the place and guarantees to return as its values the values of the store variables, which are the correct values for setf to return.

Accessing form

a form which can reference the temporary variables, and which returns the value of the place.

The value returned by the accessing form is affected by execution of the storing form, but either of these forms might be evaluated any number of times.

It is possible to do more than one setf in parallel via psetf, shiftf, and rotatef. Because of this, the setf expander must produce new temporary and store variable names every time. For examples of how to do this, see gensym.

For each standardized accessor function F, unless it is explicitly documented otherwise, it is implementation-dependent whether the ability to use an F form as a setf place is implemented by a setf expander or a setf function. Also, it follows from this that it is implementation-dependent whether the name (setf F) is fbound. Examples of Setf Expansions

Examples of the contents of the constituents of setf expansions follow.

For a variable x:

() ;list of temporary variables
() ;list of value forms
(g0001) ;list of store variables
(setq x g0001) ;storing form
x ;accessing form
Figure 5–3. Sample Setf Expansion of a Variable

For (car exp):

(g0002) ;list of temporary variables
(exp) ;list of value forms
(g0003) ;list of store variables
(progn (rplaca g0002 g0003) g0003) ;storing form
(car g0002) ;accessing form
Figure 5–4. Sample Setf Expansion of a CAR Form

For (subseq seq s e):

(g0004 g0005 g0006) ;list of temporary variables
(seq s e) ;list of value forms
(g0007) ;list of store variables
(progn (replace g0004 g0007 :start1 g0005 :end1 g0006) g0007)
;storing form
(subseq g0004 g0005 g0006) ; accessing form
Figure 5–5. Sample Setf Expansion of a SUBSEQ Form

In some cases, if a subform of a place is itself a place, it is necessary to expand the subform in order to compute some of the values in the expansion of the outer place. For (ldb bs (car exp)):

(g0001 g0002) ;list of temporary variables
(bs exp) ;list of value forms
(g0003) ;list of store variables
(progn (rplaca g0002 (dpb g0003 g0001 (car g0002))) g0003)
;storing form
(ldb g0001 (car g0002)) ; accessing form
Figure 5–6. Sample Setf Expansion of a LDB Form

5.1.2 Kinds of Places

Several kinds of places are defined by Common Lisp; this section enumerates them. This set can be extended by implementations and by programmer code. Variable Names as Places

The name of a lexical variable or dynamic variable can be used as a place. Function Call Forms as Places

A function form can be used as a place if it falls into one of the following categories: VALUES Forms as Places

A values form can be used as a place, provided that each of its subforms is also a place form.

A form such as

(setf (values place-1 ...place-n) values-form)

does the following:

  1. The subforms of each nested place are evaluated in left-to-right order.

  2. The values-form is evaluated, and the first store variable from each place is bound to its return values as if by multiple-value-bind.

  3. If the setf expansion for any place involves more than one store variable, then the additional store variables are bound to nil.

  4. The storing forms for each place are evaluated in left-to-right order.

The storing form in the setf expansion of values returns as multiple values2 the values of the store variables in step 2. That is, the number of values returned is the same as the number of place forms. This may be more or fewer values than are produced by the values-form. THE Forms as Places

A the form can be used as a place, in which case the declaration is transferred to the newvalue form, and the resulting setf is analyzed. For example,

(setf (the integer (cadr x)) (+ y 3))

is processed as if it were

(setf (cadr x) (the integer (+ y 3))) APPLY Forms as Places

The following situations involving setf of apply must be supported:

In all three cases, the element of array designated by the concatenation of subscripts and more-subscripts (i.e., the same element which would be read by the call to apply if it were not part of a setf form) is changed to have the value given by new-element. For these usages, the function name (aref, bit, or sbit) must refer to the global function definition, rather than a locally defined function.

No other standardized function is required to be supported, but an implementation may define such support. An implementation may also define support for implementation-defined operators.

If a user-defined function is used in this context, the following equivalence is true, except that care is taken to preserve proper left-to-right evaluation of argument subforms:

(setf (apply #'name {arg}*) val) 
 (apply #'(setf name) val {arg}*) Setf Expansions and Places

Any compound form for which the operator has a setf expander defined can be used as a place. The operator must refer to the global function definition, rather than a locally defined function or macro. Macro Forms as Places

A macro form can be used as a place, in which case Common Lisp expands the macro form as if by macroexpand-1 and then uses the macro expansion in place of the original place. Such macro expansion is attempted only after exhausting all other possibilities other than expanding into a call to a function named (setf reader). Symbol Macros as Places

A reference to a symbol that has been established as a symbol macro can be used as a place. In this case, setf expands the reference and then analyzes the resulting form. Other Compound Forms as Places

For any other compound form for which the operator is a symbol f, the setf form expands into a call to the function named (setf f). The first argument in the newly constructed function form is newvalue and the remaining arguments are the remaining elements of place. This expansion occurs regardless of whether f or (setf f) is defined as a function locally, globally, or not at all. For example,

(setf (f arg1 arg2 ...) new-value)

expands into a form with the same effect and value as

(let ((#:temp-1 arg1)          ;force correct order of evaluation 
      (#:temp-2 arg2) 
      (#:temp-0 new-value)) 
  (funcall (function (setf f)) #:temp-0 #:temp-1 #:temp-2...))

A function named (setf f) must return its first argument as its only value in order to preserve the semantics of setf.

5.1.3 Treatment of Other Macros Based on SETF

For each of the “read-modify-write” operators in Figure 5–9, and for any additional macros defined by the programmer using define-modify-macro, an exception is made to the normal rule of left-to-right evaluation of arguments. Evaluation of argument forms occurs in left-to-right order, with the exception that for the place argument, the actual read of the “old value” from that place happens after all of the argument form evaluations, and just before a “new value” is computed and written back into the place.

Specifically, each of these operators can be viewed as involving a form with the following general syntax:

(operator {preceding-form}* place {following-form}*)

The evaluation of each such form proceeds like this:

  1. Evaluate each of the preceding-forms, in left-to-right order.

  2. Evaluate the subforms of the place, in the order specified by the second value of the setf expansion for that place.

  3. Evaluate each of the following-forms, in left-to-right order.

  4. Read the old value from place.

  5. Compute the new value.

  6. Store the new value into place.

Figure 5–9. Read-Modify-Write Macros