Defining bash function body using parenthesis instead of braces

This script demonstrates defining a bash function with parenthesis verses with braces. The parenthesis have the nice effect of making environment variables created in the function "local", I guess because the function body is executed as a sub-shell. The output is:

A=something
A=
B=something
B=something

The question is if this is allowed syntax for defining a function.

#!/bin/bash

foo() (
    export A=something
    echo A=$A
)

bar() {
    export B=something
    echo B=$B
}

foo
echo A=$A
bar
echo B=$B