cue: relax rules for closed structs
Closedness constraints of literal structs now don't take
effect until after a field expression is evaluated.
This is only part of the implementatation. Several
aspects are implemented in follow-up CLs.
Change-Id: I588223cd555f8cde537ade7caf1901e6e425291a
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3640
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index cc6ad2c..f07149d 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1330,8 +1330,18 @@
to be concrete when emitting data.
It is illegal to have a regular field and a definition with the same name
within the same struct.
-Literal structs that are part of a definition's value are implicitly closed.
+Literal structs that are part of a definition's value are implicitly closed,
+but may unify unrestricted with other structs within the field's declaration.
This excludes literals structs in embeddings and aliases.
+<!--
+This may be a more intuitive definition:
+ Literal structs that are part of a definition's value are implicitly closed.
+ Implicitly closed literal structs that are unified within
+ a single field declaration are considered to be a single literal struct.
+However, this would make unification non-commutative, unless one imposes an
+ordering where literal structs are unified before unifying them with others.
+Imposing such an ordering is complex and error prone.
+-->
An ellipsis `...` in such literal structs keeps them open,
as it defines `_` for all labels.
<!--
@@ -1359,22 +1369,17 @@
-->
```
-// MyStruct is closed and as there is no expression label or `...`, we know
-// this is the full definition.
MyStruct :: {
- field: string
- enabled?: bool
+ sub field: string
}
-// Without the `...`, this field would not unify with its previous declaration.
MyStruct :: {
- enabled: bool | *false
- ...
+ sub enabled?: bool
}
myValue: MyStruct & {
- feild: 2 // error, feild not defined in MyStruct
- enabled: true // okay
+ sub feild: 2 // error, feild not defined in MyStruct
+ sub enabled: true // okay
}
D :: {
@@ -1441,6 +1446,7 @@
// attr: int @xml(,attr) @xml(a1,attr) @go(Attr)
```
+
#### Aliases
In addition to fields, a struct literal may also define aliases.
