5.5 Copying and Updating Structures
syntax
(struct-copy id struct-expr fld-id ...)
fld-id = [field-id expr] | [field-id #:parent parent-id expr]
The id must have a transformer binding that encapsulates information about a structure type (i.e., like the initial identifier bound by struct), and the binding must supply a constructor, a predicate, and all field accessors.
Each field-id must correspond to a field-id in the structure type defining forms of id (or parent-id, if present). The accessor bindings determined by different field-ids under the same id (or parent-id, if present) must be distinct. The order of the field-ids need not match the order of the corresponding fields in the structure type.
The struct-expr is evaluated first. The result must be an instance of the id structure type, otherwise the exn:fail:contract exception is raised. Next, the field exprs are evaluated in order (even if the fields that correspond to the field-ids are in a different order). Finally, the new structure instance is created.
The result of struct-expr can be an instance of a sub-type of id, but the resulting copy is an immediate instance of id (not the sub-type).
> (struct fish (color weight) #:transparent) > (define marlin (fish 'orange-and-white 11))
> (define dory (struct-copy fish marlin [color 'blue])) > dory (fish 'blue 11)
> (struct shark fish (weeks-since-eating-fish) #:transparent) > (define bruce (shark 'grey 110 3))
> (define chum (struct-copy shark bruce [weight #:parent fish 90] [weeks-since-eating-fish 0])) > chum (shark 'grey 90 0)
; subtypes can be copied as if they were supertypes, ; but the result is an instance of the supertype
> (define not-really-chum (struct-copy fish bruce [weight 90])) > not-really-chum (fish 'grey 90)