SPICE-0020: Deferred, type-safe references

[HT154]

SPICE: apple/pkl-evolution#27

Resolves #912

[native-pkl-cli]

[bioball]

Did a first pass!

[bioball]

This is a binary condition; it's either property or entry access. Do we need two booleans?

[HT154]

This is for ease/fluency of use: if a user is writing e.g. a function that transforms a Reference to a String, they may need to know whether each access is a subscript key or property; it may be necessary to check one way or the other and access.isSubscript is more meaningful than !access.isProperty (and vice versa).

Another alternative here is something like this:

type: "property" | "subscript"

But that makes usage sites pretty verbose!

Curious if you have thoughts or alternative suggestions here.

[bioball]

Is two booleans helpful? I suspect most code will look something like this:

if (access.isProperty) ".\(access.property)"
else "[\(access.key)]"

[HT154]

Yes, in the sense that the conditional may need to check if it's a subscript instead and

if (access.isSubscript)

is more legible than

if (!access.isProperty)

and both are preferable to

if (access.type == "subscript")

or

if (access.property == null)

[bioball]

Did another pass!

[bioball]

Is two booleans helpful? I suspect most code will look something like this:

if (access.isProperty) ".\(access.property)"
else "[\(access.key)]"