When Can You Omit \'struct{}{}\' for Anonymous Structs in Go Maps?

Delving into the Difference Between struct{}{} and {} in Go

Go's anonymous structs offer a convenient way to represent simple data structures without defining a named type. However, the syntax struct{}{} and {} for creating such structs can raise questions about their distinction.

When declaring a map of strings to anonymous structs in Go, as shown below:

<code class="go">var Foo = map[string]struct{}{
    "foo": struct{}{},
}</code>

IntelliJ Idea may raise a warning "Redundant type declaration." To address this, the following code can be used:

<code class="go">var Foo = map[string]struct{}{
    "foo": {},
}</code>

The difference between the two syntaxes lies in how the compiler treats the incomplete anonymous struct literal. In the first syntax, struct{}{}, the type is explicitly specified even though it is anonymous. In the second syntax, {}, the type is omitted altogether.

According to the Go language specification, within a composite literal of a map type, if the types of the keys and values are known from the map type itself, they can be omitted when it is intended to specify values of those types. This explains why the compiler does not require the type specification for {} in this context.

It is important to note that this omission is valid only from Go 1.5 onwards due to an oversight.

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