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2.5 Read Syntax for Circular Objects

To represent shared or circular structures within a complex of Lisp objects, you can use the reader constructs ‘#n=’ and ‘#n#’.

Use #n= before an object to label it for later reference; subsequently, you can use #n# to refer the same object in another place. Here, n is some integer. For example, here is how to make a list in which the first element recurs as the third element:

     (#1=(a) b #1#)

This differs from ordinary syntax such as this

     ((a) b (a))

which would result in a list whose first and third elements look alike but are not the same Lisp object. This shows the difference:

     (prog1 nil
       (setq x '(#1=(a) b #1#)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ t
     (setq x '((a) b (a)))
     (eq (nth 0 x) (nth 2 x))
          ⇒ nil

You can also use the same syntax to make a circular structure, which appears as an element within itself. Here is an example:

     #1=(a #1#)

This makes a list whose second element is the list itself. Here's how you can see that it really works:

     (prog1 nil
       (setq x '#1=(a #1#)))
     (eq x (cadr x))
          ⇒ t

The Lisp printer can produce this syntax to record circular and shared structure in a Lisp object, if you bind the variable print-circle to a non-nil value. See Output Variables.