The builder pattern creates a value through small configuration steps instead of one call with many arguments. Start with defaults, change only what you need, and chain the methods together. This is especially useful when a type has many fields but most callers want the default values. section 15.2 section 15.1
Change a few defaults
`let character = Character::default().height(180).weight(60).name("Bobby");` starts with a default character, then changes three fields. Each method takes `mut self`, changes one field, and returns `Self`. Returning the value makes the next method call possible. section 15.2.1
A chain can also end with a check. A `build()` method can return `Result`: `Ok` when the settings are acceptable, or `Err` when they are not. For example, it can reject excessive height or weight and a forbidden word in a name. But checking a flag on the finished type does not force other callers to perform that check. section 15.2.2
For stronger control, use a separate builder type. Its settings can be edited, but it is not the finished type. A `try_build()` method creates the finished value only when validation succeeds; functions requiring that value cannot accept the builder in its place. section 15.2.3
