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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 id with the same field values as the structure produced by struct-expr, except that the value of each supplied field-id is instead determined by the corresponding expr. If #:parent is specified, the parent-id must be bound to a parent structure type of id.

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 is combined with id (or parent-id, if present) to form id-field-id (using the lexical context of field-id), which must be one of the accessor bindings in id. The accessor bindings determined by different field-ids 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)