Best practice to overriding default value from a definition in another definition?

[cueckoo]

Originally opened by @leitzler in cuelang/cue#989

Using v0.4.0-rc.1:

I'm trying to provide a "new default value" for a nested definition and the error message is slightly confusing. Is there a more natural way to achieve this?

E.g.: I want a new definition #Foo that has a field bar that must be a #AB, but IF it is a #B it should have a different default value.

package foobar

#AB: {
	"a"
} | {
	b: #B
}

#B: {
	bf: string | *"default"
}

#Foo: {
	// This works:
	//bar: {b: #B & {bf: "newDefault"}}
	//  $ cue eval foo.cue --out yaml
	//  bar:
	//    b:
	//      bf: newDefault

	// This also works:
	//bar: #AB
	//
	//   $ cue eval foo.cue --out yaml
	//   bar:
	//     b:
	//       bf: default
	//

	// Doesn't work:
	bar: #AB | {
		b: #B & {bf: "newDefault"}
	}
	//   $ cue vet -c foo.cue
	//   bar: incomplete value {b:{bf:*"default" | string}} | {b:{bf:"newDefault"}}
	//
}

// I want to enforce a default value for "bf" via "& #Foo"
{
	bar: b: #B
} & #Foo

[cueckoo]

Original reply by @mpvl in cuelang/cue#989 (comment)

In general, a good approach is to not have any defaults in the base template, and then add them later as one desires.

In this case, the issue is that bar introduces a disjunction without a default. To ensure it "reaches" the newDefault value, you could write

	bar: #AB | *{ // <---- NOTE THE ADDED DEFAULT MARKER 
		b: #B & {bf: "newDefault"}
	}