4.4 Copying and Updating Structures

(struct-copy id struct-expr [field-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.

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 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).

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1	Language Model
2	Syntactic Forms
3	Datatypes
4	Structures
5	Classes and Objects
6	Units
7	Contracts
8	Pattern Matching
9	Control Flow
10	Concurrency and Parallelism
11	Macros
12	Input and Output
13	Reflection and Security
14	Operating System
15	Memory Management
16	Unsafe Operations
17	Running Racket
	Bibliography
	Index

4.1	Defining Structure Types: struct
4.2	Creating Structure Types
4.3	Structure Type Properties
4.4	Copying and Updating Structures
4.5	Structure Utilities
4.6	Structure Type Transformer Binding