Re: [boost] [outcome] expected, etc. why are these assignable

Robert Ramey wrote:

And wouldn't making these things immutable, eliminate a large part of the whole controversy?

An immutable result/outcome would have no default constructor (because it's default-constructed only in cases you need to put the value into it later) which would eliminate the motivation for an empty state as something into which to default-construct... but that's only a small part of the whole controversy. Niall's argument for empty state does not rest entirely on default construction. He argues that it makes Outcome a four-state variant instead of a tri-state one and having the fourth state is useful in certain scenarios when the function that returns it has a legitimate need to report four states. That is, he wants to be able to express this: enum class no_difference { _ } expected void return_first_difference( std::path const & p1, std::path const & p2 ) { // ... } (or something along these lines, I may not have gotten the example entirely correct, but that's the spirit of it.) There are different legitimate ways to draw the line; you could say three states are it, for more use expected<>, or you could say, four states cover the majority of the legitimate cases, so let's have that.