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Composite literals

… The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key.

If you’ve written any Go code, you probably already know about keys in maps:

var x = map[string]float64{
	"key1": 1.0,
	"key2": 2.0,
	"keyπ": 3.14159,
}

What’s maybe slightly less obvious, but not surprising, is that the field names in structs are also called keys, in a composite literal:

var x = struct {
	Key1 string
	Key2 int
}{
	Key1: "neat",
	Key2: 15,
}

What’s less known… and in fact, I didn’t even realize it, or I had forgotten, until I was preparing to write this… is that slice and array literals can include keys as well. In such a case, the keys must be integers, and indicate the index of the value. This allows you to specify slice/array values out of order, if for some reason you wish to, or to sparsely populate a slice or array.

var x = []string{
	1: "foo",
	5: "bar",
	2: "baz",
}

fmt.Println(strings.Join(x,",")) // Prints: ,foo,baz,,,bar

Quotes from The Go Programming Language Specification Version of August 2, 2023