About Type Assertions

Interfaces in Go can introduce ambiguity about the underlying type. In fact, the empty interface can take any concrete value at all (including primitives). A type assertion allows us to extract the interface value’s underlying concrete value using this syntax: interfaceVariable.(concreteType).

If we assign the result of this statement to a single value, we assert that the interface variable holds a value of the concrete type. As a result, it may panic at runtime if the interface value does not have the specified concrete type. For example:

var input interface{} = 12
str := input.(string) // panic at runtime since input is not a string!

We can test whether an interface value holds a specific concrete type by making use of both return values of the type assertion: the underlying value and a boolean value that reports whether the assertion succeeded. For example:

str, ok := input.(string) // no panic if input is not a string

If input holds a string, then str will be the underlying value and ok will be true. If input does not hold a string, then str will be the zero value of type string (ie. "" - the empty string) and ok will be false. No panic occurs in any case.

It is common to see this sort of idiom:

str, ok := input.(string)
if !ok {
    str = "a default value"
}

Type Switches

A type switch can perform several type assertions in a row. It has the same syntax as a type assertion (interfaceVariable.(concreteType)), but instead of a specific concreteType it uses the keyword type. Here is an example:

var i interface{} = 12 // try: 12.3, true, int64(12), []int{}, map[string]int{}

switch v := i.(type) {
case int:
    fmt.Printf("the integer %d\n", v)
case string:
    fmt.Printf("the string %s\n", v)
default:
    fmt.Printf("type, %T, not handled explicitly: %#v\n", v, v)
}

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Originally from Exercism go concepts