Return Statement

returnXxx in Flow.apex only specifies what type of result is returned. To actually return something, call doReturn(Object) or doReturnRaw(Object).

Flow f = new Flow()
    .inputAs('a', 'b').returnInteger()
    .doReturn(0);

If no doReturn or equivalent is added, the Flow Func will return null.

The difference between doReturn and doReturnRaw is that doReturn treats String as FlowScript while doReturnRaw does not.

Flow f = new Flow()
    .inputAs('a', 'b').returnInteger()
    .doReturn('"message"');

is equivalent to

Flow f = new Flow()
    .inputAs('a', 'b').returnInteger()
    .doReturnRaw('message');