Safe Navigation

For Member Access

Accessing deeply nested fields which can potentially be nil was problematic, because you'd need an unreasonable amount of guard clauses to prevent an "attempt to index nil" error. Pluto now offers this syntax:

Basic Usage
pluto
local person = {
    name = "Alice"
}
local pet_name = person.pet?.name

In this example, person.pet is nil. However, this does not throw an "attempt to index nil" error. pet_name is simply set to nil.

Practical Usage
pluto
-- Pretend userConfig is parsed from a JSON file, or something.
--- Returning the user's preferred color, or Red if they have no preferred color.
local function get_color()
    return userConfig.colors?.preferred ?? "Red"
end
print(get_color())

In this example, we also use the null coalescing operator ??.

info

These cannot be used for statements, like a?.b?.c = 0. They're only valid as expressions.

Safe Navigation (for member access) was written by Sven Olsen.

For Method Calls

Similarly, if you want to have optional hook functions, you can use safe method calls instead of writing conditionals:

Lua Way
pluto
if self.onEvent then
    self:onEvent(event)
end

Pluto Way
pluto
self:onEvent?(event)

You can also check if the left-hand side of : is not nil:

pluto
object?:onEvent(event) -- 'object' may be nil
object?:onEvent?(event) -- 'object' and 'object.onEvent' may be nil

Note that this feature will truncate multiple return values if they're not used as part of a fixed assignment:

pluto
function myfunc()
    return 1, 2
end
local a, b, c = _G:myfunc?()
print(a, b, c) --> 1   2   nil
print(_G:myfunc?()) --> 1