What does it mean for a function to have a method?

Question

I'm seeing the following type definition:

type Getter func(ctx *context.T, key string) (Ticket, error)

And the following method defined on it:

func (g Getter) GetData(ctx *context.T, path ...string) (data []byte, err error) {
    …
}

I'm also seeing the following variable definition:

var Client Getter = func(ctx *context.T, key string) (Ticket, error) {
    return …
}

where:

type Ticket interface {
    …
}

What'll happen with this code exactly?

Answer 1

tl;dr It won't compile because Cannot use 'func() (MyData, error) { return nil, nil }' (type func() (MyData, error)) as the type Getter. In other words, MyGetter does not match the type of Getter. Here's a go playground link.

There are a couple of things going on here:

1. You're creating a type called Getter. Any value that fulfills this Getter type must be a function with zero input parameters and zero output parameters (keep this in mind for a second).
2. Go has first-class functions which means they can be assigned to variables, passed to parameters of other functions, or in this case methods can be added to a type whose underlying type is a function.
3. Lastly, you're trying to assign a function with the signature func() (MyData, error) to the type Getter which is where you're getting the compile error I mentioned in the tl;dr. This compile error is because your type Getter specifies that any value of this type must be a function with zero input parameters and zero return parameters. You're trying to assign a function that does have return parameters.