4.4 Copying and Updating Structures

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).

Examples:

> (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)