Anonymous struct, difference between struct{}{} and {}

Question

I have string to struct map in golang defined in the following way:

var Foo = map[string]struct{}{
    "foo": struct{}{},
}

Gogland by default marks this declaration as warning, saying "Redundant type declaration".

var Foo = map[string]struct{}{
    "foo": {},
}

Above code solves the warning, but I couldn't find any information about the difference between struct{}{} and {} declaration. Is it kind of a "short notation"?

https://play.golang.org/p/0Akx98XtB4

Answer 1

This:

struct{}{}

is a composite literal which contains the type (struct{}) and the literal's value ({}).

This:

{}

is also a composite literal without the type, just the value.

Generally you have to specify / include the type in the composite literal to let the compiler know what kind of (what "type" of) composite literal you're creating, hence the syntax is:

CompositeLit = LiteralType LiteralValue .

But when you specify a map composite literal, the types of the key and value are known from the map type, and thus may be omitted if you intend to specify values of those types. This is mentioned in Spec: Composite literals:

Within a composite literal of array, slice, or map type T, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of T. Similarly, elements or keys that are addresses of composite literals may elide the &T when the element or key type is *T.

(Note: due to an oversight, this is only valid from Go 1.5.)