Why do type aliases change behaviour (i.e. json.Unmarshal)?

[cpu...@gmail.com]

I've just tried doing something like this in my code:

// Token is an OAuth2 token which includes decoding the expires_in attribute

type Token struct {

oauth2.Token

ExpiresIn int `json:"expires_in"` // expiration time in seconds

}

func (t *Token) UnmarshalJSON(data []byte) error {

...

}

type WrappedToken Token

Only to find out, that unmarshaling a WrappedToken does not call Token.UnmarshalJSON which came as a bit of surprise.

I can only guess it's due to how JSON unmarshaling works, but is this the intended behaviour?

Kind regards,

Andreas

[Levieux Michel]

This is because per the Go spec: https://golang.org/ref/spec#Type_declarations

You're not creating an alias to Token, but defining a new type from it. If what you are looking for is an alias (meaning to have the "WrappedToken" identifier refer to the exact same type as Token), you need to do:

type WrappedToken = Token

Then all method calls on WrappedToken will be method calls on Token (since both identifier denote the same type).

But maybe that is not what you are looking for, if that's the case, there are other ways around the problem, like type embedding and such.

Hope this helps,

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[Jan Mercl]

On Mon, Mar 15, 2021 at 1:49 PM cpu...@gmail.com <cpu...@gmail.com> wrote:

> type WrappedToken Token

Note that this is not a type alias. WrappedToken is a new type. Even
if its underlying type is Token, WrappedToken does not automatically
have any methods of Token.

> Only to find out, that unmarshaling a WrappedToken does not call Token.UnmarshalJSON which came as a bit of surprise.

To be less surprised, check the language specs that say this is how
the type system works.

[cpu...@gmail.com]

Thank you @Michel. Disturbing that I keep making the same mistake again...