12. The blank identifier

The blank identifier can be assigned or declared with any value of any type, with the value discarded harmlessly.
It’s a bit like writing to the Unix /dev/null file.

The blank identifier in multiple assignment

if _, err := os.Stat(path); os.IsNotExist(err) {
    fmt.Printf("%s does not exist\n", path)
}

Unused imports and variables

package main
import (
    "fmt"
    "io"
    "log"
    "os"
)
var _ = fmt.Printf // For debugging; delete when done.
var _ io.Reader    // For debugging; delete when done.
func main() {
    fd, err := os.Open("test.go")
    if err != nil {
        log.Fatal(err)
    }
    // TODO: use fd.
    _ = fd
}

Import for side effect

To import the package only for its side effects, rename the package to the blank identifier:
import _ "net/http/pprof"

Interface checks

if _, ok := val.(json.Marshaler); ok {
    fmt.Printf("value %v of type %T implements json.Marshaler\n", val, val)
}

The appearance of the blank identifier in this construct indicates that the declaration exists only for the type checking, not to create a variable:
var _ json.Marshaler = (*json.RawMessage)(nil)