02-using-an-interface-value.md

Using an interface value

The previous page showed how to store a value in an interface, but how do we use the value stored in y? There are some operations that we can perform without needing to know what the value of y is or its type, ex. (Golang playground):

var x int64
var y interface{}
x = 2
y = x
fmt.Println(x, y)

The above code prints the expected output:

2 2

But what about an operation where the type of multiple operands matter, such as the sum of x and y (Golang playground)?

fmt.Println(x + y)

Instead of emitting 4, the program will fail to compile with the following error:

invalid operation: x + y (mismatched types int64 and interface {})

In order to sum x and y, they must be the same type. Let's try the example one more time (Golang playground):

fmt.Println(x + y.(int64))

Now the code compiles, runs, and emits the expected output of 4.

The syntax y.(int64) is known as type assertion. Yet, how do we know the type assertion only worked because the literal value of 2 is a valid value for the type int64?

Please continue to the next page to learn how type assertion reveals the inner-workings of interface values...

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Next: Type assertion