Section 17

Blank Identifier

The blank identifier _ is a write-only discard slot. Go requires every declared variable to be used — _ is how you say "I am intentionally not using this value."

Four uses of _

_ — four contexts

// 1. Discard a return value
value, _ := strconv.Atoi("42")   // ignore the error (be careful!)

// 2. Discard loop index
for _, msg := range messages {
    process(msg)  // only care about the value, not the index
}

// 3. Import for side effects — run init() without using the package
import _ "github.com/lib/pq"      // registers postgres driver in init()
import _ "net/http/pprof"          // registers /debug/pprof/ routes

// 4. Compile-time interface satisfaction check
// This line does NOTHING at runtime. At compile time, if *pgStore
// does NOT satisfy MessageStore, you get a compile error here.
var _ MessageStore = (*pgStore)(nil)
// Read: 'assign nil *pgStore to MessageStore variable, discard result'
// Compiler must verify the assignment is valid — that IS the check

Interface check pattern — you will see this in every Go codebase

compile-time interface assertion

// Put at the top of the implementation file
// Acts as documentation: 'this type is intended to satisfy these interfaces'
// Fails fast at compile time instead of at runtime when the interface is used

var _ MessageStore  = (*pgStore)(nil)
var _ MessageBroker = (*redisBroker)(nil)
var _ WorkerPool    = (*boundedPool)(nil)

// Java equivalent: no direct analog.
// Java's 'implements' is enforced at class declaration.
// Go's check is at the usage site — more flexible, less automatic.