Sharing: shared.ss

(shared (shared-binding ) body-expr )

Binds variables with shared structure according to sharded-bindings and then evaluates the body-exprs, returning the result of the last expression.

The shared form is similar to letrec. Each shared-binding has the form:

  (variable value-expr) 

The variables are bound to the result of value-exprs in the same way as for a letrec expression, except for value-exprs with the following special forms:

• (cons car-expr cdr-expr)
• (list element-expr )
• (box box-expr)
• (vector element-expr )

For each of these special forms, the cons cell, list, box, or vector is allocated, but the content expressions are not evaluated until all of the bindings have values; then the content expressions are evaluated and the values inserted into the appropriate locations. In this way, values with shared structure (even cycles) can be constructed.

Examples:

  (shared ([a (cons 1 a)]) a) ; => infinite list of 1s 
  (shared ([a (cons 1 b)] 
           [b (cons 2 a)]) 
    a) ; => (1 2 1 2 1 2   ) 
  (shared ([a (vector b b b)] 
           [b (box 1)]) 
    (set-box! (vector-ref a 0) 2) 
    a) ; => #(#&2 #&2 #&2)