declaring empty types in go

Question

For a given type Data , I would like to define a set of filters, each processing Data in a certain way. Some filters only need the data to be processed, other may need additional parameters.

type Data struct {
    ...
}

I want to be able to define a list of filters, and apply them sequentially to an instance of Data . To acheive this, I defined a Filter interface :

type Filter interface {
    Apply (d *Data) error
}

To define a filter, all I have to do is create a new type and define the Apply method for it.

Now, let's say I have a filter that does not need any additional information. Is it good practice to define it as an empty struct ?

type MySimpleFilter struct {}

func (f *MySimpleFilter) Apply (d *Data) {
    ...
}

Answer 1

I'd argue this is good practice if you have no use for a Field, especially compared to using another type (ie type MySimpleFilter int ) because an empty struct uses no space:

https://codereview.appspot.com/4634124

and it can still fulfill interface contracts (hence can be more useful than a functional approach in some cases).

This can also be a good idiom when using a map that you have no use for the value (ie map[string]struct{} ). See this discussion for details:

https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/lb4xLHq7wug

Answer 2

This is a question that doesn't have a clear answer since it's a matter of taste. I'd say it is good practice because it makes MySimpleFilter symmetrical to the other filters, which makes it easier to understand the code.