Improve support for handling undefined inside of queries

[tsandall]

It's often necessary to select a nested field out of a document but fallback to a default value if the referenced field is undefined. In the past users could write a helper function to assist with this, however, over time we ended up adding the object.get built-in function to make it easier.

Programming languages don't typically include this type of feature--it's just implemented in libraries. Looking at query languages for hierarchical data:

• jq uses // for it's alternative operator
• XPath 2.0 allows you to construct a sequence. If an expression is undefined, it's not included in the sequence (roughly). So you can say (foo, bar)[1] and if foo is undefined, the return value is bar (XPath indices are 1-based.)

One issue with object.get is that it only works one level at a time. For example:

a := object.get(input, "foo", {})
b := object.get(input, "bar", {})
c := object.get(input, "baz", 7)

You could improve this using something like walk:

c := object.select(input, "foo.bar.baz", 7)

One option would be to include some new operator for providing alternatives, e.g., following jq:

c := input.foo.bar.baz // 7

Some questions:

• Can alternative operator be applied to non-ref terms or expressions? If it's applied to expressions, it's tempting to make it catch false as well.
• What about allowing default to be used against references (similar to with)?