4.3 KiB
nil-navigation postfix operator (?)
Summary
Introduce syntax to navigate through nil values.
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 nil-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.
Because functions can also be nil, the nil-navigation operator will also support x?(), which will only call x if it is not nil.
The list of valid operators to follow the nil-navigation operator would be:
dog?.name
dog?.getName()
dog?:getName()
dogs?[1]
getDog?()
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.
The type of an expression using the nil-safety operator would simply resolve to the expression, nillable. Meaning if dog.name was a string, then dog?.name would be string?.
Linting
Though not required, lints to prevent superfluous use of the nil-navigation operator would be welcome.
For example, if dog has a property body (which cannot be nil), which has a property legs, then dog?.body.legs is valid, as dog?.body being nil will cancel the rest of the expression. However, a user might instead write dog?.body?.legs, perhaps expecting dog?.body to resolve to nil, but still indexing legs.
It would be nice to lint this if dog.body's type does not support nil.
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. It would need to be respected in several different potential grammars, including function calls and indexes (most notably in the case of parsing difficulty, [1] style indexing).
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 nil-navigation operator is welcome.