Default list values

[cueckoo]

Originally opened by @verytable in cuelang/cue#914

General question. Do lists work well with defaults? If they don't I think this should be explicitly mentioned in docs.

My doubts are based on the following quote from docs:

Lists can be thought of as structs:

List: *null | {
Elem: _
Tail: List
}

that shows that List has default (*) in it's definition. Also there are no list examples in the "Default values " section of the docs.

Example.
Evaluation of the following code (with cue 0.3.2)
{ arr: [...string] | [] } & {arr: *[] | [...string]}
results in
arr: incomplete value [] | []

However, this
{ arr: [...string]} & {arr: *[] | [...string]}
results in
{ "arr": [] }
The first example also works with cue 0.2.2.

What is the difference between these two snippets?

[cueckoo]

Original reply by @myitcv in cuelang/cue#914 (comment)

@verytable sorry for the slow response here.

Lists and structs have a default value of the empty list or struct respectively:

exec cue export
-- x.cue --
package x


l: [...string]
s: {...}

gives:

{
    "l": [],
    "s": {}
}

I think the quote from the spec "Lists can be thought of as structs" is meant to illustrate another way of thinking about lists, but I can see how it is misleading in light of the above.

The behaviour of your example:

{ arr: [...string] | [] } & {arr: *[] | [...string]}

has changed with respect to v0.2.2. I will, however, defer to @mpvl on the finer points of that, and how it will change in the future (it relates to elimination of disjuncts, something that is a function of how disjuncts are considered equivalent or not, for some definition of equivalent).