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Showing content from https://docs.racket-lang.org/reference/struct-copy.html below:

5.5 Copying and Updating Structures

(struct-copy id struct-expr fld-id ...)

  fld-id   =   [field-id expr]     |   [field-id #:parent parent-id expr]

Creates a new instance of the structure type

(which is defined via a

such as

) with the same field values as the structure produced by

, except that the value of each supplied

is instead determined by the corresponding

. If

is specified, the

must be bound to a parent structure type of

.

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).

Examples:

> (struct fish (color weight) #:transparent) > (define marlin (fish 'orange-and-white 11)) > 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 > not-really-chum

(fish 'grey 90)

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