Explain Type Assertions in Go

Question

I'm reading about type assertions x.(T) in The Go Programming Language and don't understand them.

I understand that there are different scenarios:

• T is a concrete type or an interface
• One (asserted value?) or two (ok) values can be returned

This is what I don't understand:

• Why would I use them?
• What exactly do they return?

I have also googled on the topic and still don't understand.

Answer 1

Common usecase: check if returned error is of a type T.

https://golang.org/ref/spec#Type_assertions

For a single return value assertion: when it fails the program panics.

For a two return values assertion: when it fails second argument is set to false and the program doesn't panic.

Answer 2

A type assertion is the x.(T) notation where x is of interface type and T is a type. Additionally, the actual value stored in x is of type T, and T must satisfy the interface type of x.