fmtStringer

FmtStringer is an informal term used to describe a type that provides a custom string representation for formatting with the fmt package in Go. The official concept is the Stringer interface, defined in the fmt package as type Stringer interface { String() string }. A type that implements String() string is effectively a fmtStringer because fmt-based formatting will use that method to obtain the textual representation.

The mechanism lets developers control how their types are displayed when printed with the fmt family of functions, such as fmt.Printf, fmt.Println, or fmt.Sprintf. If a value implements Stringer, the fmt package uses the result of its String() method for formatting with the %v and %s verbs. If a type does not implement Stringer, fmt applies its default formatting rules for the underlying type.

Example:

type Point struct { X, Y int }

func (p Point) String() string { return fmt.Sprintf("(%d, %d)", p.X, p.Y) }

With this, fmt.Println(Point{2, 3}) outputs (2, 3). Implementing String() is a lightweight way to improve the readability of logs and user-facing text without changing the underlying data structure.

Advanced formatting can be achieved by implementing the Format method from the Formatter interface, which allows per-verb control of output. If a type implements both Formatter and Stringer, the Format method takes precedence for the corresponding verbs, enabling fine-grained formatting while still supporting a default String() representation.