Multiple constructors in Go / optional parameters?

Question

I was wondering if there was a way to implement multiple constructors (with the same function name) in Go, just like you can do in Java. Another option could be to only have one constructor with an optional parameter, but I am not sure how to do that exactly.

This seems similar to what I was trying to do

type Query struct {
    TagsQuery string
    PageQuery string
}

// First Constructor
func NewQuery(TagsQuery string) Query {
    return Query{
        TagsQuery: TagsQuery,
        PageQuery: "0", // default to first page
    }
}

// Second Constructor
func NewQuery(TagsQuery string, PageQuery string) Query {
    return Query{
        TagsQuery: TagsQuery,
        PageQuery: PageQuery,
    }
}

The first constructor takes one argument TagsQuery and defaults PageQuery to 0. The second constructor takes two arguments: TagsQuery and PageQuery.

Answer 1

Dave Cheney offered a solution to this problem with functional options:

https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

So you would accept any number of functions that can modify the struct being created. In your case you could do it just for the optional PageQuery field though as it is a ... parameter it could accept more in the future:

type Query struct {
    TagsQuery string
    PageQuery string
}

// First Constructor
func NewQuery(TagsQuery string, options ...Option) Query {
    query := Query{
        TagsQuery: TagsQuery,
        PageQuery: "0", // default to first page
    }
    // Apply options if there are any, can overwrite default
    for _, option := range options {
        query = option(query)
    }
    return query
}

// Option definition
type Option func(Query) Query

// Function to create Option func to set pageQuery
func WithPageQuery(pageQuery string) Option {
    return func(query Query) Query {
        query.PageQuery = pageQuery
        return query
    }
}

And then to call it:

NewQuery("tags query only")
NewQuery("tags query", WithPageQuery("page query"))