[Feature request] Better semantics for try/catch

[SkyyySi]

Note: this is a breaking change!

I would like to see the semantics of try/catch be changed to make it (in my opinion) more useful.

I think that

x = try
    do_something()
catch err
    handle_error(err)

should be changed from compiling to

local x = xpcall(function()
    return do_something()
end, function(err)
    return handle_error(err)
end)

into compiling to this:

local x = select(2, xpcall(function()
    return do_something()
end, function(err)
    return handle_error(err)
end))

The addition of select(2, [...]) causes the boolean that's normally returned by xpcall() to be skipped. This would feel more natural to use in many cases, and allow for patterns like this:

x = (try maybe_failing_operation()) ?? "default"

[pigpigyyy]

The expression:

x = select(2, try maybe_failing_operation!) ?? 'default'

won’t work as expected, because Lua’s pcall / xpcall return two values: (true, result) on success, or (false, errorMessage) on failure.

A more accurate and idiomatic pattern for now would be:

x = res if ok, res := try maybe_failing_operation!
x ??= 'default'

To make this cleaner, I propose introducing a try! syntax:

x, y = try! maybe_failing_operation!

This could expand to:

x, y = res1, res2 if ok, res1, res2 := try maybe_failing_operation!

This keeps the original try semantics intact while offering a more ergonomic opt-in for common use cases.