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Showing content from http://www.lispworks.com/documentation/lw60/CLHS/Body/05_abb.htm below:

CLHS: Section 5.1.2.2

Function name  Argument that is a place  Update function used      
ldb            second                    dpb                       
mask-field     second                    deposit-field             
getf           first                     implementation-dependent  

Figure 5-8. Functions that setf can be used with---2 During the setf expansion of these forms, it is necessary to call get-setf-expansion in order to figure out how the inner, nested generalized variable must be treated.

The information from get-setf-expansion is used as follows.

ldb

In a form such as:

(setf (ldb byte-spec place-form) value-form)

the place referred to by the place-form must always be both read and written; note that the update is to the generalized variable specified by place-form, not to any object of type integer.

Thus this setf should generate code to do the following:

1. Evaluate byte-spec (and bind it into a temporary variable).

2. Bind the temporary variables for place-form.

3. Evaluate value-form (and bind its value or values into the store variable).

4. Do the read from place-form.

5. Do the write into place-form with the given bits of the integer fetched in step 4 replaced with the value from step 3.

If the evaluation of value-form in step 3 alters what is found in place-form, such as setting different bits of integer, then the change of the bits denoted by byte-spec is to that altered integer, because step 4 is done after the value-form evaluation. Nevertheless, the evaluations required for binding the temporary variables are done in steps 1 and 2, and thus the expected left-to-right evaluation order is seen. For example:

 (setq integer #x69) =>  #x69
 (rotatef (ldb (byte 4 4) integer) 
          (ldb (byte 4 0) integer))
 integer =>  #x96
;;; This example is trying to swap two independent bit fields 
;;; in an integer.  Note that the generalized variable of 
;;; interest here is just the (possibly local) program variable
;;; integer.

mask-field

This case is the same as ldb in all essential aspects.

getf

In a form such as:

(setf (getf place-form ind-form) value-form)

the place referred to by place-form must always be both read and written; note that the update is to the generalized variable specified by place-form, not necessarily to the particular list that is the property list in question.

Thus this setf should generate code to do the following:

1. Bind the temporary variables for place-form.

2. Evaluate ind-form (and bind it into a temporary variable).

3. Evaluate value-form (and bind its value or values into the store variable).

4. Do the read from place-form.

5. Do the write into place-form with a possibly-new property list obtained by combining the values from steps 2, 3, and 4. (Note that the phrase ``possibly-new property list'' can mean that the former property list is somehow destructively re-used, or it can mean partial or full copying of it. Since either copying or destructive re-use can occur, the treatment of the resultant value for the possibly-new property list must proceed as if it were a different copy needing to be stored back into the generalized variable.)

If the evaluation of value-form in step 3 alters what is found in place-form, such as setting a different named property in the list, then the change of the property denoted by ind-form is to that altered list, because step 4 is done after the value-form evaluation. Nevertheless, the evaluations required for binding the temporary variables are done in steps 1 and 2, and thus the expected left-to-right evaluation order is seen.

For example:

 (setq s (setq r (list (list 'a 1 'b 2 'c 3)))) =>  ((a 1 b 2 c 3))
 (setf (getf (car r) 'b) 
       (progn (setq r nil) 6)) =>  6
 r =>  NIL
 s =>  ((A 1 B 6 C 3))
;;; Note that the (setq r nil) does not affect the actions of 
;;; the SETF because the value of R had already been saved in 
;;; a temporary variable as part of the step 1. Only the CAR
;;; of this value will be retrieved, and subsequently modified 
;;; after the value computation.

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