Add null-coalescing (??) and optional chaining (?.) operators

[gnodet]

Proposal

Add two new operators for ergonomic null handling in DataSonnet expressions:

?? — Null Coalescing

payload.currency ?? "USD"

Returns the left-hand side if it is not null, otherwise evaluates and returns the right-hand side. Unlike default (which catches any error via try/catch), ?? specifically checks for null values only. The right-hand side is lazily evaluated (short-circuit).

?. / ?[ — Optional Chaining

payload.order?.customer?.email
payload.items?[0]

Safely accesses fields or indices on a value that may be null. If the value is null, returns null instead of throwing an error. Chainable and combinable with ??:

payload.address?.city ?? "Unknown"

Difference from default

	default	??
Missing field	catches error, returns fallback	throws (field not found)
Null value	catches error (if any), returns fallback	returns fallback
Other error	catches, returns fallback	throws
Semantics	try/catch	null check

Precedence

?? has the lowest precedence of all binary operators (lower than ||), matching JavaScript/Kotlin semantics. x ?? 1 + 2 parses as x ?? (1 + 2).

Implementation

A working implementation is available at gnodet/datasonnet-mapper@null-coalescing-optional-chaining with:

• Parser, AST, and evaluator changes (4 files modified)
• 34 tests covering null coalescing, optional chaining, edge cases, precedence, error semantics, and combined usage
• Documentation in language.adoc

Happy to open a PR if the project accepts external contributions.