Built-in Constants

var s []int = nil

if s == nil {
    fmt.Println("Slice is nil")
}

// Safe operations on nil slices
fmt.Println(len(s))    // 0
fmt.Println(cap(s))    // 0
s = append(s, 1)       // Creates new slice

// Iterating over nil slice is safe
for _, v := range s {
    fmt.Println(v) // Never executes
}

var m map[string]int = nil

if m == nil {
    fmt.Println("Map is nil")
}

// Reading from nil map returns zero value
v := m["key"] // 0
v, ok := m["key"] // 0, false

// Writing to nil map panics
// m["key"] = 42 // panic: assignment to nil map

// Must initialize before writing
m = make(map[string]int)
m["key"] = 42

var ch chan int = nil

if ch == nil {
    fmt.Println("Channel is nil")
}

// Operations on nil channels block forever
// <-ch        // Blocks forever
// ch <- 1     // Blocks forever
// close(ch)   // Panics

// Must initialize before use
ch = make(chan int)

var err error = nil

if err == nil {
    fmt.Println("No error")
}

// Nil interface is different from interface holding nil value
var p *int = nil
var i interface{} = p
fmt.Println(i == nil) // false! Interface holds typed nil

var f func() = nil

if f == nil {
    fmt.Println("Function is nil")
}

// Calling nil function panics
// f() // panic: runtime error

// Check before calling
if f != nil {
    f()
}