The fmt.Stringer interface requires a single method, String() string to be satisfied. The string method defines the "native" string format for that value, and is the default representation if the value is provided to any of the fmt packages formatting or printing routines.

package main

import (
    "fmt"
)

type User struct {
    Name  string
    Email string
}

// String satisfies the fmt.Stringer interface for the User type
func (u User) String() string {
    return fmt.Sprintf("%s <%s>", u.Name, u.Email)
}

func main() {
    u := User{
        Name:  "John Doe",
        Email: "[email protected]",
    }

    fmt.Println(u)
    // output: John Doe <[email protected]>
}

Playground

Package fmt implements formatted I/O using format verbs:

%v    // the value in a default format
%T    // a Go-syntax representation of the type of the value
%s    // the uninterpreted bytes of the string or slice

Format Functions

There are 4 main function types in fmt and several variations within.

Print

fmt.Print("Hello World")        // prints: Hello World
fmt.Println("Hello World")      // prints: Hello World\n
fmt.Printf("Hello %s", "World") // prints: Hello World

Sprint

formattedString := fmt.Sprintf("%v %s", 2, "words") // returns string "2 words"

Fprint

byteCount, err := fmt.Fprint(w, "Hello World") // writes to io.Writer w

Fprint can be used, inside http handlers:

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello %s!", "Browser")
}   // Writes: "Hello Browser!" onto http response

Scan

Scan scans text read from standard input.

var s string
fmt.Scanln(&s) // pass pointer to buffer
// Scanln is similar to fmt.Scan(), but it stops scanning at new line.
fmt.Println(s) // whatever was inputted

Stringer Interface

Any value which has a String() method implements the fmt inteface Stringer

type Stringer interface {
        String() string
}