3.12 Boxes
Boxes in The Racket Guide introduces boxes.
A box is like a single-element vector, normally used as minimal mutable storage.
A literal or printed box starts with #&. See Reading Boxes for information on reading boxes and Printing Boxes for information on printing boxes.
Returns #t if v is a box, #f otherwise.
Returns a new mutable box that contains v.
procedure
(box-immutable v) → (and/c box? immutable?)
v : any/c
Returns a new immutable box that contains v.
Returns the content of box.
For any v, (unbox (box v)) returns v.
Sets the content of box to v.
procedure
box : (and/c box? (not/c immutable?) (not/c impersonator?)) old : any/c new : any/c
Atomically updates the contents of box to new, provided
that box currently contains a value that is eq? to
old, and returns #t in that case. If box
does not contain old, then the result is #f.
If no other threads or futures attempt to access box, the operation is equivalent to
When Racket is compiled with support for futures, box-cas! uses a hardware compare and set operation. Uses of box-cas! be performed safely in a future (i.e., allowing the future thunk to continue in parallel).