doc/ref/spec.md: introduction of value aliases
Also:
- removes a deprecated example
- simplifies the definition of AliasExpr
Change-Id: I4f6e389146850102c21fcc9fd75f8bd59daa07df
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9684
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index 72c6416..f0e968c 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -1163,7 +1163,7 @@
Declaration = Field | Ellipsis | Embedding | LetClause | attribute .
Ellipsis = "..." [ Expression ] .
Embedding = Comprehension | AliasExpr .
-Field = Label ":" { Label ":" } Expression { attribute } .
+Field = Label ":" { Label ":" } AliasExpr { attribute } .
Label = [ identifier "=" ] LabelExpr .
LabelExpr = LabelName [ "?" ] | "[" AliasExpr "]" .
LabelName = identifier | simple_string_lit .
@@ -1472,7 +1472,7 @@
The name of an alias must be unique within its scope.
```
-AliasExpr = Expression | identifier "=" Expression .
+AliasExpr = [ identifier "=" ] Expression .
```
Aliases can appear in several positions:
@@ -1492,6 +1492,10 @@
- for optional fields (`foo?: bar` and `[foo]: bar`),
the bound identifier is only visible within the field value (`bar`).
+Before a value (`foo: X=x`)
+
+- binds the identifier to the value it precedes within the scope of that value.
+
Inside a bracketed label (`[X=expr]: value`):
- binds the identifier to the the concrete label that matches `expr`
@@ -1508,14 +1512,14 @@
-->
```
-// An alias declaration
-Alias = 3
-a: Alias // 3
-
// A field alias
foo: X // 4
X="not an identifier": 4
+// A value alias
+foo: X={x: X.a}
+bar: foo & {a: 1} // {a: 1, x: 1}
+
// A label alias
[Y=string]: { name: Y }
foo: { value: 1 } // outputs: foo: { name: "foo", value: 1 }
