internal/core/adt: make comprehension value an Expr

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: I070f9b8985762d45e11467e39dbba535f0f2d21f
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/529521
Reviewed-by: Paul Jolly <paul@myitcv.io>

Author: mpvl

internal/core/adt/adt.go

@@ -183,6 +183,13 @@ func (*StructLit) expr()       {}
 func (*ListLit) expr()         {}
 func (*DisjunctionExpr) expr() {}
 
+// TODO: also allow?
+//       a: b: if cond {}
+//
+// It is unclear here, though, whether field `a` should be added
+// unconditionally.
+// func (*Comprehension) expr() {}
+
 // Expr and Value
 
 func (*Bottom) expr()           {}

internal/core/adt/composite.go

@@ -325,6 +325,7 @@ func (v *Vertex) Value() Value {
 	case *StructMarker, *ListMarker:
 		return v
 	case Value:
+		// TODO: recursively descend into Vertex?
 		return x
 	default:
 		panic(fmt.Sprintf("unexpected type %T", v.BaseValue))
@@ -455,8 +456,7 @@ func (v *Vertex) Finalize(c *OpContext) {
 }
 
 func (v *Vertex) AddErr(ctx *OpContext, b *Bottom) {
-	v.BaseValue = CombineErrors(nil, v.Value(), b)
-	v.UpdateStatus(Finalized)
+	v.SetValue(ctx, Finalized, CombineErrors(nil, v.Value(), b))
 }
 
 func (v *Vertex) SetValue(ctx *OpContext, state VertexStatus, value BaseValue) *Bottom {

internal/core/adt/expr.go

@@ -1606,7 +1606,7 @@ func (x *Disjunction) Kind() Kind {
 
 type Comprehension struct {
 	Clauses Yielder
-	Value   *StructLit // TODO: changes this to Expr?
+	Value   Expr
 }
 
 func (x *Comprehension) Source() ast.Node {