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4.2.2 Modify Macros

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

Macro: cl-psetf [place form]…

This macro is to setf what cl-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).

Macro: cl-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, (cl-incf i) is equivalent to (setq i (1+ i)), and (cl-incf (car x) 2) is equivalent to (setcar x (+ (car x) 2)).

As with setf, care is taken to preserve the “apparent” order of evaluation. For example,

(cl-incf (aref vec (cl-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 (cl-incf i))
      (1+ (aref vec (cl-incf i))))   ; wrong!

but rather to something more like

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

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

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

Macro: cl-decf place &optional x

This macro decrements the number stored in place by one, or by x if specified.

Macro: cl-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 isn’t present in the list already. The optional keyword arguments are interpreted in the same way as for cl-adjoin. See Lists as Sets.

Macro: cl-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, (cl-shiftf a b c d) is equivalent to

  (cl-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.

Macro: cl-rotatef place…

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

(cl-psetf a b
          b c
          c d
          d a)

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

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

Macro: cl-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,

(cl-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 cl-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 cl-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.)

Like in the case of let, the value forms are evaluated in the order they appear, but the order of bindings is unspecified. Therefore, avoid binding the same place more than once in a single cl-letf form.

Since generalized variables look like lists, let’s shorthand of using ‘foo’ for ‘(foo nil)’ as a binding would be ambiguous in cl-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 cl-letf is to restore the original value of place afterwards.

Note that in this case, and in fact almost every case, place must have a well-defined value outside the cl-letf body. There is essentially only one exception to this, which is place a plain variable with a specified value (such as (a 17) in the above example).

Macro: cl-letf* (bindings…) forms…

This macro is to cl-letf what let* is to let: It does the bindings in sequential rather than parallel order.

Macro: cl-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, (cl-incf place n) is the same as (cl-callf + place n). Some more examples:

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

Note again that cl-callf is an extension to standard Common Lisp.

Macro: cl-callf2 function arg1 place args

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

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

Macro: with-memoization place code

This macro provides a simple way to do memoization. code is evaluated and then stashed in place. If place’s value is non-nil, return that value instead of evaluating code.

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