PLT MzLib: Libraries Manual Graph Constructor Syntax

(shared (shared-binding ···) body-expr ···1)      SYNTAX

Binds variables with shared structure according to shared-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 (after partial expansion):

  • (cons car-expr cdr-expr)

  • (list element-expr ···)

  • (box box-expr)

  • (vector element-expr ···)

  • (prefix:make-name element-expr ···) where prefix:name is the name of a structure type (or, more generally, is bound to expansion-time information about a structure type)

The cons above means an identifier that is module-identifier=? either to the cons export from mzscheme or to the top-level cons. The same is true of list, box, and vector. In the prefix:make-name case, the expansion-time information associated with prefix:name must provide a constructor binding and a complete set of field mutator bindings.

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


   (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)