What actions are permissible with a Map type variable that has not yet been initialized?

In Go, a variable of type map defaults to nil if it has not been initialized.

Actions permissible with a nil map include:

• Reading a value by key — returns the zero value of the value type and a flag indicating the key's absence.
• Checking for the presence of a key using the value, ok := map[key] operator.

Actions that are not allowed:

• Writing (assigning a value to a key) will cause a panic.
• Calling functions that modify the map (e.g., delete) on a nil map is safe and does not cause a panic.

Example:

var m map[string]int // m == nil

v, ok := m["key"] // ok == false, v == 0

// m["key"] = 10 // panic: assignment to entry in nil map

delete(m, "key") // safe, does nothing

To use a map for writing, it needs to be initialized, for example, using make:

m = make(map[string]int)
m["key"] = 10 // now works