Despite moonlighting as a gopher for a while, the syntax for type assertion and type switches still trips me up every time I need to go for one of them.

So, to avoid digging through the docs or crafting stodgy LLM prompts multiple times, I decided to jot this down in a gobyexample1 style for the next run.

Type assertion

Type assertion in Go allows you to access an interface variable’s underlying concrete type. After a successful assertion, the variable of interface type is converted to the concrete type to which it’s asserted.

The syntax is i.(T), where i is a variable of interface type and T is the type you are asserting.

Basic usage

var i interface{} = "Hello" // or use `any` as an alias for `interface{}`

s := i.(string)
fmt.Println(s)

Here, s gets the type string, and the program outputs Hello.

Asserting primitive types

var i interface{} = 42

if v, ok := i.(int); ok {
    fmt.Println("integer:", v)
}

This code checks if i is an int and prints its value if so. The value of ok will be false if i isn’t an integer and nothing will be printed to the console.

Asserting composite types

var i interface{} = []string{"apple", "banana", "cherry"}

if v, ok := i.([]string); ok {
    fmt.Println("slice of strings:", v)
}

This will print slice of strings: [apple banana cherry] to the console.

Similar to primitive types, you can also perform type assertions with composite types. In the example above, we check whether the variable i, which is of an interface type, holds a value of the type ‘slice of strings’.

Asserting other interface types

type fooer interface{ foo() }
type barer interface{ bar() }
type foobarer interface { fooer; barer }

type thing struct{}

func (t *thing) foo() {}
func (t *thing) bar() {}

var i foobarer = &thing{}

func main() {
    if v, ok := i.(fooer); ok {
        fmt.Println("i satiesfies fooer:", v)
    }
}

Type assertion can also be used to convert the type of an interface variable to another interface type. Here struct i implements both foo() and bar() methods; satisfying the foobarer interface.

Then in the main function, we check whether i satisfies fooer interface and print a message if it does. Running this snippet will print i satiesfies fooer: &{}.

Dynamically checking for certain methods

type fooer interface{ foo() }

type thing struct{}

func (t *thing) foo() {}
func (t *thing) bar() {}

func main() {
    var i fooer = &thing{}

    if v, ok := i.(interface{ bar() }); ok {
        fmt.Println("thing implements bar method:", v)
    }
}

Type assertion can be used to dynamically check if an interface variable implements a certain method. This can come in handy when you want to know if an interface variable has a certain method right before invoking it.

Here, the thing struct implements both foo and bar but the fooer interface only needs the foo() method to be implemented. However, we can check dynamically whether i also implements the bar() method using anonymous interface definition. Running this prints thing implements bar method: &{}

Handling failures

var i interface{} = "Hello"

f := i.(float64) // This triggers a panic

Wrong assertions, like attempting to convert a string to a float64, cause runtime panics.

Type switches

Type switches let you compare an interface variable’s type against several types. It’s similar to a regular switch statement, but focuses on types.

Basic usage

var i interface{} = 7

switch i.(type) {
case int:
    fmt.Println("i is an int")
case string:
    fmt.Println("i is a string")
default:
    fmt.Println("unknown type")
}

This outputs i is an int.

Using a variable in a type switch case

var i interface{} = []byte("hello")

switch v := i.(type) {
case []byte:
    fmt.Println(string(v))
case string:
    fmt.Println(v)
}

Notice how we’re assinging variable v to i.(type) and then reusing the extracted value in the case statements. The snippet converts []byte to a string and prints hello.

Handling multiple types

var i interface{} = 2.5

switch i.(type) {
case int, float64:
    fmt.Println("i is a number")
case string:
    fmt.Println("i is a string")
}

The case T1, T2 syntax works like an OR relationship, outputting i is a number.

Addressing composite types

var i interface{} = map[string]bool{"hello": true, "world": false}

switch i.(type) {
case map[string]bool:
    fmt.Println("i is a map")
case []string:
    fmt.Println("i is a slice")
default:
    fmt.Println("unknown type")
}

Similar to primitive types, you can check for composite types in the case statement of a type switch. Here, we’re checking whether i is a map[string]bool or not. Running this will output i is a map.

Comparing against other interface types

type fooer interface{ foo() }
type barer interface{ bar() }
type foobarer interface { fooer; barer }

type thing struct{}

func (t *thing) foo() {}
func (t *thing) bar() {}

var i foobarer = &thing{}

func main() {
    switch v := i.(type) {
    case fooer:
        fmt.Println("fooer:", v)
    case barer:
        fmt.Println("barer:", v)
    case foobarer:
        fmt.Println("foobarer:", v)
    default:
        panic("none of them")
    }
}

Type switches can be also used to compare an interface variable with another interface type. This example is similar to the type assertion one where we’re checking whether i satisfies fooer, barer or foobarer interface. In this case, i satisfies all three of them but the case statement will stop after the first successful check. So it prints fooer: &{} and bails.

Dynamically checking for certain methods

type fooer interface{ foo() }

type thing struct{}

func (t *thing) foo() {}
func (t *thing) bar() {}

func main() {
    var i fooer = &thing{}

    switch v := i.(type) {
    case interface{ bar() }:
        fmt.Println("thing implements bar method:", v)
    default:
        panic("thing doesn't implement bar method")
    }
}

Similar to type assertion, within a type switch, anonymous interface definition can be used to dynamically check if an interface variable implements some method.

The thing struct implements both foo() and bar() methods. However, the fooer interface only requires it to implement foo(). The type switch dynamically checks whether i also implements the method bar(). Running this will print thing implements bar method: &{}.

Similarities and differences

Similarities

  • Both handle interfaces and extract their concrete types.
  • They evaluate an interface’s dynamic type.

Differences

  • Type assertions check a single type, while type switches handle multiple types.
  • Type assertion uses i.(T), type switch uses a switch statement with literal i.(type).
  • Type assertions can panic or return a success boolean, type switches handle mismatches more gracefully.
  • Type assertions are good when you’re sure of the type. Type switches are more versatile for handling various types.
  • Type assertion can get the value and success boolean. Type switches let you access the value in each case block.
  • Type switches can handle multiple types, including a default case, offering more flexibility for various types.

Fin!

  1. Go by example ↩︎

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