Stringers in

About Stringers

Stringer is an interface for defining the string format of values.

The interface consists of a single String method:

type Stringer interface {
    String() string
}

Types that want to implement this interface must have a String() method that returns a human-friendly string representation of the type. The fmt package (and many others) will look for this method to format and print values.

Example: Distances

Assume we are working on an application that deals with geographical distances measured in different units. We have defined types DistanceUnit and Distance as follows:

type DistanceUnit int

const (
	Kilometer    DistanceUnit = 0
	Mile         DistanceUnit = 1
)
 
type Distance struct {
	number float64
	unit   DistanceUnit
} 

In the example above, Kilometer and Mile are constants of type DistanceUnit.

These types do not implement interface Stringer as they lack the String method. Hence fmt functions will print Distance values using Go's "default format":

mileUnit := Mile
fmt.Sprint(mileUnit)
// => 1
// The result is '1' because that is the underlying value of the 'Mile' constant (see constant declarations above) 

dist := Distance{number: 790.7, unit: Kilometer}
fmt.Sprint(dist)
// => {790.7 0}
// not a very useful output!

In order to make the output more informative, we implement interface Stringer for DistanceUnit and Distance types by adding a String method to each type:

func (sc DistanceUnit) String() string {
	units := []string{"km", "mi"}
	return units[sc]
}

func (d Distance) String() string {
	return fmt.Sprintf("%v %v", d.number, d.unit)
}

fmt package functions will call these methods when formatting Distance values:

kmUnit := Kilometer
kmUnit.String()
// => km

mileUnit := Mile
mileUnit.String()
// => mi

dist := Distance{
	number: 790.7,
	unit: Kilometer,
}
dist.String()
// => 790.7 km