proposal: Go 2: Annotations

[joeysudoer]

Proposal

It is proposed to add Annotation feature in Go. The reasons are as follows:

• There is no implementation of the annotation function in Go 1.x. A similar implementation is Struct Tags, but the use scenarios of struct tags are limited and cannot meet some more flexible requirements.

• Through recent community surveys, it has been known that there are many Go developers who were also Java / Python developers before. The annotation feature in these languages ​​is very powerful. Based on the annotation function, you can flexibly implement some features, such as "dependency injection", which greatly Improve development efficiency.

• There are some projects on GitHub that indirectly implement similar annotation functions through annotations, such as // @Test, but I think this way looks very weird and not elegant.

Syntax examples

Annotations can refer to the implementation specifications of annotation functions in most programming languages, using the @ symbol as the beginning, followed by the annotation name, and then providing parameters in parentheses after the name, such as

@Get('/user/:id')

The position of the annotation can be above the function, variable and other elements, such as

@Init
func SampleFunc(){

    @Config("sample.valueA")
    var valueA string
    ...

}

Commentary

I know this is a controversial proposal. I have communicated with many Go developers around me. They have shown a conservative attitude towards learning the language features of other languages, but I think that maintaining an open attitude will greatly benefit the development of Go.

[josharian]

Please see the discussion in #36669.

[mvdan]

Also, please fill https://github.com/golang/proposal/blob/master/go2-language-changes.md as it's a requirement for any proposal that changes the language.