luau/rfcs/syntax-safe-navigation-operator.md

Safe navigation postfix operator (?)

Summary

Introduce syntax to navigate through nil values, or short-circuit with nil if it was encountered.

Motivation

nil values are very common in Lua, and take care to prevent runtime errors.

Currently, attempting to index dog.name while caring for dog being nil requires some form of the following:

local dogName = nil
if dog ~= nil then
    dogName = dog.name
end

...or the unusual to read...

local dogName = dog and dog.name

...which will return false if dog is false, instead of throwing an error because of the index of false.name.

This trick gets worse in the case of calling methods. For example, let's suppose we wanted to call dog.fetch(), while dog is still potentially nil.

The one-line and trick will no longer work (or will get less readable as you try to shoe-horn this in), so we must:

if dog ~= nil then
    dog.fetch()
end

But this gets even worse when it comes to chained indexing. Let's suppose we wanted to run dog.owner.handshake(), while dog can be nil and owner can be nil.

if dog ~= nil and dog.owner ~= nil then
    dog.owner.handshake()
end

...which gets even worse in the context of calling this function in, say, another function:

-- Oops! dog and dog.owner can be nil
logDogName(getLogger(), dog.name, dog.owner:getDisplayName())

-- In order to preserve this order (assuming argument execution order mattered)...
local logger = getLogger()
local name = dog and dog.name
local displayName

if dog.owner ~= nil then
    displayName = dog.owner:getDisplayName()
end

logDogName(logger, name, displayName)

Design

The safe navigation operator will make all of these smooth, by supporting x?.y and similar indexing operators. dog?.name would resolve to nil if dog was nil, or the name otherwise. owner?.handshake() would only call handshake if owner is not nil.

The long example would turn into:

logDogName(getLogger(), dog?.name, dog?.owner?:getDisplayName())

Failing the nil-safety check early would make the entire expression nil, for instance dog?.body.legs would resolve to nil if dog is nil, rather than resolve dog?.body into nil, then turning into nil.legs.

The list of valid operators to follow the safe navigation operator would be:

dog?.name
dog?.getName()
dog?:getName()

The operator must be used in the context of either a call or an index, and so:

local value = x?

...would be invalid syntax.

This syntax would be based on expressions, and not identifiers, meaning that (x or y)?.call() would be valid syntax.

Type

If the expression is typed as an optional, then the resulting type would be the final expression, also optional. Otherwise, it'll just be the resulting type if ? wasn't used.

local optionalObject: { name: string }?
local optionalObjectName = optionalObject?.name -- resolves to `string?`

local nonOptionalObject: { name: string }
local nonOptionalObjectName = nonOptionalObject?.name -- resolves to `string`

Drawbacks

As with all syntax additions, this adds complexity to the parsing of expressions, and the execution of cancelling the rest of the expression could prove challenging.

Furthermore, with the proposed syntax, it might lock off other uses of ? within code (and not types) for the future as being ambiguous.

Alternatives

Doing nothing is an option, as current standard if-checks already work, as well as the and trick in other use cases, but as shown before this can create some hard to read code, and nil values are common enough that the safe navigation operator is welcome.

Supporting optional calls/indexes, such as x?[1] and x?(), while not out of scope, are likely too fringe to support, while adding on a significant amount of parsing difficulty, especially in the case of shorthand function calls, such as x?{} and x?"".