► |  | The Racket Reference

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

► | 13 | Input and Output

13.1 | Ports
13.2 | Byte and String Input
13.3 | Byte and String Output
13.4 | Reading
13.5 | Writing
13.6 | Pretty Printing
13.7 | Reader Extension
13.8 | Printer Extension
13.9 | Serialization
13.10 | Fast-Load Serialization
13.11 | Cryptographic Hashing

▼ | 13.1 | Ports

13.1.1 | Encodings and Locales
13.1.2 | Managing Ports
13.1.3 | Port Buffers and Positions
13.1.4 | Counting Positions, Lines, and Columns
13.1.5 | File Ports
13.1.6 | String Ports
13.1.7 | Pipes
13.1.8 | Structures as Ports
13.1.9 | Custom Ports
13.1.10 | More Port Constructors, Procedures, and Events

On this page:

prop: input-port
prop: output-port

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##### 13.1.8`` Structures as Ports

> value prop:input-port : struct-type-property?

> value prop:output-port : struct-type-property?

The prop:input-port and prop:output-port structure type properties identify structure types whose instances can serve as input and output ports, respectively.

Each property value can be either of the following:

- An input port (for prop:input-port) or output port (for prop:output-port): In this case, using the structure as port is equivalent to using the given input or output port.

- An exact, non-negative integer between 0 (inclusive) and the number of non-automatic fields in the structure type (exclusive, not counting supertype fields): The integer identifies a field in the structure, and the field must be designated as immutable. If the field contains an input port (for prop:input-port) or output port (for prop:output-port), the port is used. Otherwise, an empty string input port is used for prop:input-port, and a port that discards all data is used for prop:output-port.

Some procedures, such as file-position, work on both input and output ports. When given an instance of a structure type with both the prop:input-port and prop:output-port properties, the instance is used as an input port.

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