Macro: REBINDING

Documentation

Bind each var in BINDINGS to a gensym, bind the gensym to var's value via a let, return BODY's value wrapped in this let. Evaluates a series of forms in the lexical environment that is formed by adding the binding of each VAR to a fresh, uninterned symbol, and the binding of that fresh, uninterned symbol to VAR's original value, i.e., its value in the current lexical environment. The uninterned symbol is created as if by a call to GENSYM with the string denoted by PREFIX - or, if PREFIX is not supplied, the string denoted by VAR - as argument. The forms are evaluated in order, and the values of all but the last are discarded (that is, the body is an implicit PROGN).

Source

(defmacro rebinding (bindings &body body)
  "Bind each var in BINDINGS to a gensym, bind the gensym to
var's value via a let, return BODY's value wrapped in this let.

Evaluates a series of forms in the lexical environment that is
formed by adding the binding of each VAR to a fresh, uninterned
symbol, and the binding of that fresh, uninterned symbol to VAR's
original value, i.e., its value in the current lexical
environment.

The uninterned symbol is created as if by a call to GENSYM with the
string denoted by PREFIX - or, if PREFIX is not supplied, the string
denoted by VAR - as argument.

The forms are evaluated in order, and the values of all but the last
are discarded \(that is, the body is an implicit PROGN)."
  ;; reference implementation posted to comp.lang.lisp as
  ;; <cy3wv0fya0p.fsf@ljosa.com> by Vebjorn Ljosa - see also
  ;; <http://www.cliki.net/Common%20Lisp%20Utilities>
  (loop for binding in bindings
        for var = (car (if (consp binding) binding (list binding)))
        for name = (gensym)
        collect `(,name ,var) into renames
        collect ``(,,var ,,name) into temps
        finally (return `(let* ,renames
                          (with-unique-names ,bindings
                            `(let (,,@temps)
                               ,,@body))))))