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Common Lisp Extensions

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5.2.2 Modify Macros

This package defines a number of other macros besides setf that operate on generalized variables. Many are interesting and useful even when the place is just a variable name.

Special Form: psetf [place form]...
This macro is to setf what psetq is to setq: When several places and forms are involved, the assignments take place in parallel rather than sequentially. Specifically, all subforms are evaluated from left to right, then all the assignments are done (in an undefined order).

Special Form: incf place &optional x
This macro increments the number stored in place by one, or by x if specified. The incremented value is returned. For example, (incf i) is equivalent to (setq i (1+ i)), and (incf (car x) 2) is equivalent to (setcar x (+ (car x) 2)).

Once again, care is taken to preserve the "apparent" order of evaluation. For example,

(incf (aref vec (incf i)))

appears to increment i once, then increment the element of vec addressed by i; this is indeed exactly what it does, which means the above form is not equivalent to the "obvious" expansion,

(setf (aref vec (incf i)) (1+ (aref vec (incf i))))   ; Wrong!

but rather to something more like

(let ((temp (incf i)))
  (setf (aref vec temp) (1+ (aref vec temp))))

Again, all of this is taken care of automatically by incf and the other generalized-variable macros.

As a more Emacs-specific example of incf, the expression (incf (point) n) is essentially equivalent to (forward-char n).

Special Form: decf place &optional x
This macro decrements the number stored in place by one, or by x if specified.

Special Form: pop place
This macro removes and returns the first element of the list stored in place. It is analogous to (prog1 (car place) (setf place (cdr place))), except that it takes care to evaluate all subforms only once.

Special Form: push x place
This macro inserts x at the front of the list stored in place. It is analogous to (setf place (cons x place)), except for evaluation of the subforms.

Special Form: pushnew x place &key :test :test-not :key
This macro inserts x at the front of the list stored in place, but only if x was not eql to any existing element of the list. The optional keyword arguments are interpreted in the same way as for adjoin. See section 11.3 Lists as Sets.

Special Form: shiftf place... newvalue
This macro shifts the places left by one, shifting in the value of newvalue (which may be any Lisp expression, not just a generalized variable), and returning the value shifted out of the first place. Thus, (shiftf a b c d) is equivalent to

  (psetf a b
         b c
         c d))

except that the subforms of a, b, and c are actually evaluated only once each and in the apparent order.

Special Form: rotatef place...
This macro rotates the places left by one in circular fashion. Thus, (rotatef a b c d) is equivalent to

(psetf a b
       b c
       c d
       d a)

except for the evaluation of subforms. rotatef always returns nil. Note that (rotatef a b) conveniently exchanges a and b.

The following macros were invented for this package; they have no analogues in Common Lisp.

Special Form: letf (bindings...) forms...
This macro is analogous to let, but for generalized variables rather than just symbols. Each binding should be of the form (place value); the original contents of the places are saved, the values are stored in them, and then the body forms are executed. Afterwards, the places are set back to their original saved contents. This cleanup happens even if the forms exit irregularly due to a throw or an error.

For example,

(letf (((point) (point-min))
       (a 17))

moves "point" in the current buffer to the beginning of the buffer, and also binds a to 17 (as if by a normal let, since a is just a regular variable). After the body exits, a is set back to its original value and point is moved back to its original position.

Note that letf on (point) is not quite like a save-excursion, as the latter effectively saves a marker which tracks insertions and deletions in the buffer. Actually, a letf of (point-marker) is much closer to this behavior. (point and point-marker are equivalent as setf places; each will accept either an integer or a marker as the stored value.)

Since generalized variables look like lists, let's shorthand of using `foo' for `(foo nil)' as a binding would be ambiguous in letf and is not allowed.

However, a binding specifier may be a one-element list `(place)', which is similar to `(place place)'. In other words, the place is not disturbed on entry to the body, and the only effect of the letf is to restore the original value of place afterwards. (The redundant access-and-store suggested by the (place place) example does not actually occur.)

In most cases, the place must have a well-defined value on entry to the letf form. The only exceptions are plain variables and calls to symbol-value and symbol-function. If the symbol is not bound on entry, it is simply made unbound by makunbound or fmakunbound on exit.

Special Form: letf* (bindings...) forms...
This macro is to letf what let* is to let: It does the bindings in sequential rather than parallel order.

Special Form: callf function place args...
This is the "generic" modify macro. It calls function, which should be an unquoted function name, macro name, or lambda. It passes place and args as arguments, and assigns the result back to place. For example, (incf place n) is the same as (callf + place n). Some more examples:

(callf abs my-number)
(callf concat (buffer-name) "<" (int-to-string n) ">")
(callf union happy-people (list joe bob) :test 'same-person)

See section 5.2.3 Customizing Setf, for define-modify-macro, a way to create even more concise notations for modify macros. Note again that callf is an extension to standard Common Lisp.

Special Form: callf2 function arg1 place args...
This macro is like callf, except that place is the second argument of function rather than the first. For example, (push x place) is equivalent to (callf2 cons x place).

The callf and callf2 macros serve as building blocks for other macros like incf, pushnew, and define-modify-macro. The letf and letf* macros are used in the processing of symbol macros; see section 5.3.4 Macro Bindings.

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