8.2 Default Ports

► |  | The Racket Guide

1 | Welcome to Racket
2 | Racket Essentials
3 | Built-In Datatypes
4 | Expressions and Definitions
5 | Programmer-Defined Datatypes
6 | Modules
7 | Contracts
8 | Input and Output
9 | Regular Expressions
10 | Exceptions and Control
11 | Iterations and Comprehensions
12 | Pattern Matching
13 | Classes and Objects
14 | Units (Components)
15 | Reflection and Dynamic Evaluation
16 | Macros
17 | Creating Languages
18 | Performance
19 | Running and Creating Executables
20 | More Libraries
21 | Dialects of Racket and Scheme
22 | Command-Line Tools and Your Editor of Choice
| Bibliography
| Index

▼ | 8 | Input and Output

8.1 | Varieties of Ports
8.2 | Default Ports
8.3 | Reading and Writing Racket Data
8.4 | Datatypes and Serialization
8.5 | Bytes, Characters, and Encodings
8.6 | I/ O Patterns

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#### 8.2`` Default Ports

For most simple I/O functions, the target port is an optional argument, and the default is the current input port or current output port. Furthermore, error messages are written to the current error port, which is an output port. The current-input-port, current-output-port, and current-error-port functions return the corresponding current ports.

Examples:
> > (display "Hi")
> Hi
> > (display "Hi" (current-output-port)) ; the same
> Hi

If you start the racket program in a terminal, then the current input, output, and error ports are all connected to the terminal. More generally, they are connected to the OS-level stdin, stdout, and stderr. In this guide, the examples show output written to stdout in purple, and output written to stderr in red italics.

Examples:
> (define (swing-hammer)
> (display "Ouch!" (current-error-port)))
> > (swing-hammer)
> Ouch!

The current-port functions are actually parameters, which means that their values can be set with parameterize.

> > > See Dynamic Binding: parameterize for an introduction to parameters.

Example:
> > (let ([s (open-output-string)])
> (parameterize ([current-error-port s])
> (swing-hammer)
> (swing-hammer)
> (swing-hammer))
> (get-output-string s))
> "Ouch!Ouch!Ouch!"

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