While declaring a variable with anonymous type in Go, I'm seeing var v versus v:= syntaxes work differently. Imagine we're declaring an empty anonymous struct type instance and assign it to a variable.
This works:
func main() {
var v struct {}
_ = v
But this does not:
func main() {
t := struct{}
_ = t
}
compiling this gives the following error (https://play.golang.org/p/MgbttbBVmYE):
prog.go:8:7: type struct {} is not an expression
Why is this the case?
var v struct{} gives v type struct{} but doesn't explicitly set a value, so it gets the zero value {}.
t := struct{} isn't a complete expression. You'd need t := struct{}{} to create t with type struct {} and give it the value {}.
In other words, struct{} is a type, but creating t with := needs a value, not just a type on the right side. struct{}{} is how you write the literal form of an anonymous empty struct.