rust 62 lines · 8 steps

The builder pattern in Rust

A builder collects optional fields through chained calls, then validates and assembles the final config in one fallible step.

Explained by highlit
Intermediate builder-pattern ownership error-handling defaults method-chaining
1use std::time::Duration;
2 
3#[derive(Debug, Clone)]
4pub struct ServerConfig {
5 host: String,
6 port: u16,
7 max_connections: usize,
8 request_timeout: Duration,
9 tls_enabled: bool,
10}
11 
12#[derive(Default)]
13pub struct ServerConfigBuilder {
14 host: Option<String>,
15 port: Option<u16>,
16 max_connections: Option<usize>,
17 request_timeout: Option<Duration>,
18 tls_enabled: bool,
19}
20 
21impl ServerConfig {
22 pub fn builder() -> ServerConfigBuilder {
23 ServerConfigBuilder::default()
24 }
25}
26 
27impl ServerConfigBuilder {
28 pub fn host(mut self, host: impl Into<String>) -> Self {
29 self.host = Some(host.into());
30 self
31 }
32 
33 pub fn port(mut self, port: u16) -> Self {
34 self.port = Some(port);
35 self
36 }
37 
38 pub fn max_connections(mut self, max: usize) -> Self {
39 self.max_connections = Some(max);
40 self
41 }
42 
43 pub fn request_timeout(mut self, timeout: Duration) -> Self {
44 self.request_timeout = Some(timeout);
45 self
46 }
47 
48 pub fn tls_enabled(mut self, enabled: bool) -> Self {
49 self.tls_enabled = enabled;
50 self
51 }
52 
53 pub fn build(self) -> Result<ServerConfig, &'static str> {
54 Ok(ServerConfig {
55 host: self.host.ok_or("host is required")?,
56 port: self.port.unwrap_or(8080),
57 max_connections: self.max_connections.unwrap_or(1024),
58 request_timeout: self.request_timeout.unwrap_or(Duration::from_secs(30)),
59 tls_enabled: self.tls_enabled,
60 })
61 }
62}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Separating a builder from the final struct lets you accept partial input and defer validation until construction.
  2. 2Taking and returning `self` by value enables ergonomic method chaining while respecting Rust's ownership rules.
  3. 3Returning a `Result` from `build` turns missing required fields into recoverable errors instead of panics.

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