What exactly does .(data_type) method called/do?

Question

I came a cross a piece of code that used .(string) method. Not knowing what this is called I had difficulties searching for it.

Here is my try to unders

Answer 1

b.(string) is called a type assertion. As written in Effective Go:

A type assertion takes an interface value and extracts from it a value of the specified explicit type.

So, yes, the value you get from a type assertion is not an interface value, but is of the explicit type. You can also test if the type assertion was successful by adding an untyped boolean value:

s, ok := b.(string) // s is of type string
if !ok {
    // b did not contain a value of type string!
}

Edit:

To explain further to clear out any possible misunderstanding:

A type assertion doesn't "tell Go that b is a string" as you suggested. What it does is that it will, in run time, try to extract a string from b, and panic if b contains some other type (unless assigning the optional bool value).

The value that you get from the assertion will indeed be of type string, allowing you to do things like slicing (you cannot slice an interface value) or checking its len.