internal/core/adt: introduce ArcNotPresent

ArcVoid's use is currently overloaded. The new
evaluator more strictly distinguishes between it.

ArcVoid is renamed to ArcPending, signifying that
it is still unknown which type the arc has and that
more work needs to be done to figure out. This
can be the case for comprehensions.

ArcNonExisting represents an arc that is known to
not exist. These may exist and linger after processing
as it is not always possible to remove an arc from
processing.

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ie090cae5e6bf5c664f863b1be491cb9ed4696d44
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/551885
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/testdata/cycle/compbottom2.txtar

@@ -314,7 +314,7 @@ Disjuncts:    205
         foo: (string){ "" }
       }
       a: (struct){
-        bar: (_|_){
+        bar*: (_|_){
           // [cycle] mutual.mutualCycleFail.a: cycle with field b.foo:
           //     ./mutual.cue:9:10
         }

cue/testdata/cycle/compbottomnofinal.txtar

@@ -375,7 +375,7 @@ Disjuncts:    230
       port: (string){ "" }
     }
     b: (struct){
-      port: (_|_){
+      port*: (_|_){
         // [cycle] minimal.b: cycle with field a.port:
         //     ./in.cue:18:6
       }
@@ -518,7 +518,7 @@ Disjuncts:    230
       Y: (struct){
         userinfo: (string){ "user" }
         host: (string){ "mod.test" }
-        port: (_|_){
+        port*: (_|_){
           // [cycle] large.p2.Y: undefined field: port:
           //     ./in.cue:267:10
         }
@@ -544,7 +544,7 @@ Disjuncts:    230
       Y: (struct){
         userinfo: (string){ "user" }
         host: (string){ "mod.test" }
-        port: (_|_){
+        port*: (_|_){
           // [cycle] large.p3.Y: undefined field: port:
           //     ./in.cue:308:10
         }
@@ -570,7 +570,7 @@ Disjuncts:    230
       Y: (struct){
         userinfo: (string){ "user" }
         host: (string){ "mod.test" }
-        port: (_|_){
+        port*: (_|_){
           // [cycle] large.p4.Y: undefined field: port:
           //     ./in.cue:351:10
         }

internal/core/adt/composite.go

@@ -196,8 +196,8 @@ type Vertex struct {
 	// Used for cycle detection.
 	IsDynamic bool
 
-	// hasVoidArc is set if this Vertex has a void arc (e.g. for comprehensions)
-	hasVoidArc bool
+	// hasPendingArc is set if this Vertex has a void arc (e.g. for comprehensions)
+	hasPendingArc bool
 
 	// ArcType indicates the level of optionality of this arc.
 	ArcType ArcType
@@ -275,17 +275,29 @@ const (
 	// for foo in case it is defined.
 	ArcOptional
 
+	// ArcPending means that it is not known yet whether an arc exists and that
+	// its conjuncts need to be processed to find out. This happens when an arc
+	// is provisionally added as part of a comprehension, but when this
+	// comprehension has not yet yielded any results.
+	ArcPending
+
+	// ArcNotPresent indicates that this arc is not present and, unlike
+	// ArcPending, needs no further processing.
+	ArcNotPresent
+
 	// TODO: define a type for optional arcs. This will be needed for pulling
 	// in optional fields into the Vertex, which, in turn, is needed for
 	// structure sharing, among other things.
 	// We could also define types for required fields and potentially lets.
-
-	// ArcVoid means that an arc does not exist. This happens when an arc
-	// is provisionally added as part of a comprehension, but when this
-	// comprehension has not yet yielded any results.
-	ArcVoid
 )
 
+// definitelyExists reports whether an arc is a constraint or member arc.
+// TODO: we should check that users of this call ensure there are no
+// ArcPendings.
+func (v *Vertex) definitelyExists() bool {
+	return v.ArcType < ArcPending
+}
+
 // ConstraintFromToken converts a given AST constraint token to the
 // corresponding ArcType.
 func ConstraintFromToken(t token.Token) ArcType {
@@ -318,6 +330,12 @@ func (a ArcType) Suffix() string {
 		return "?"
 	case ArcRequired:
 		return "!"
+
+	// For debugging internal state. This is not CUE syntax.
+	case ArcPending:
+		return "*"
+	case ArcNotPresent:
+		return "-"
 	}
 	return ""
 }
@@ -850,8 +868,8 @@ func (v *Vertex) GetArc(c *OpContext, f Feature, t ArcType) (arc *Vertex, isNew
 	}
 	arc = &Vertex{Parent: v, Label: f, ArcType: t}
 	v.Arcs = append(v.Arcs, arc)
-	if t == ArcVoid {
-		v.hasVoidArc = true
+	if t == ArcPending {
+		v.hasPendingArc = true
 	}
 	return arc, true
 }

internal/core/adt/comprehension.go

@@ -182,7 +182,7 @@ func (n *nodeContext) insertComprehension(
 				}
 
 				conjunct := MakeConjunct(env, c, ci)
-				n.node.state.insertFieldUnchecked(f.Label, ArcVoid, conjunct)
+				n.node.state.insertFieldUnchecked(f.Label, ArcPending, conjunct)
 				fields = append(fields, f)
 				// TODO: adjust ci to embed?
 
@@ -203,7 +203,7 @@ func (n *nodeContext) insertComprehension(
 				}
 
 				conjunct := MakeConjunct(env, c, ci)
-				n.node.state.insertFieldUnchecked(f.Label, ArcVoid, conjunct)
+				n.node.state.insertFieldUnchecked(f.Label, ArcPending, conjunct)
 				fields = append(fields, f)
 
 			default:

internal/core/adt/eval.go

@@ -78,9 +78,9 @@ func (c *OpContext) evaluate(v *Vertex, r Resolver, state vertexStatus) Value {
 		// Use node itself to allow for cycle detection.
 		c.unify(v, state)
 
-		if v.ArcType == ArcVoid {
+		if v.ArcType == ArcPending {
 			if v.status == evaluating {
-				for ; v.Parent != nil && v.ArcType == ArcVoid; v = v.Parent {
+				for ; v.Parent != nil && v.ArcType == ArcPending; v = v.Parent {
 				}
 				err := c.Newf("cycle with field %v", r)
 				b := &Bottom{Code: CycleError, Err: err}
@@ -492,7 +492,7 @@ func (n *nodeContext) postDisjunct(state vertexStatus) {
 
 	switch err := n.getErr(); {
 	case err != nil:
-		if err.Code < IncompleteError && n.node.ArcType == ArcVoid {
+		if err.Code < IncompleteError && n.node.ArcType == ArcPending {
 			n.node.ArcType = ArcMember
 		}
 		n.node.BaseValue = err
@@ -754,7 +754,7 @@ func (n *nodeContext) completeArcs(state vertexStatus) {
 	if state <= conjuncts &&
 		// Is allowed to go one step back. See Vertex.UpdateStatus.
 		n.node.status <= state+1 &&
-		(!n.node.hasVoidArc || n.node.ArcType == ArcMember) {
+		(!n.node.hasPendingArc || n.node.ArcType == ArcMember) {
 
 		n.node.updateStatus(conjuncts)
 		return
@@ -772,11 +772,11 @@ func (n *nodeContext) completeArcs(state vertexStatus) {
 			// correctly and that we are not regressing.
 			n.node.updateStatus(evaluatingArcs)
 
-			wasVoid := a.ArcType == ArcVoid
+			wasVoid := a.ArcType == ArcPending
 
 			ctx.unify(a, finalized)
 
-			if a.ArcType == ArcVoid {
+			if a.ArcType == ArcPending {
 				continue
 			}
 
@@ -1744,7 +1744,7 @@ func (n *nodeContext) addValueConjunct(env *Environment, v Value, id CloseInfo)
 		n.node.Structs = append(n.node.Structs, x.Structs...)
 
 		for _, a := range x.Arcs {
-			if a.ArcType == ArcVoid {
+			if !a.definitelyExists() {
 				continue
 			}
 			// TODO(errors): report error when this is a regular field.

internal/core/adt/expr.go

@@ -1882,7 +1882,7 @@ func (x *ForClause) yield(s *compState) {
 		}
 
 		c.unify(a, partial)
-		if a.ArcType == ArcVoid {
+		if !a.definitelyExists() {
 			continue
 		}
 
@@ -1894,13 +1894,15 @@ func (x *ForClause) yield(s *compState) {
 			// processing, eluding the deallocation step.
 			status:    finalized,
 			IsDynamic: true,
+			ArcType:   ArcMember,
 		}
 
 		if x.Value != InvalidLabel {
 			b := &Vertex{
 				Label:     x.Value,
 				BaseValue: a,
 				IsDynamic: true,
+				ArcType:   ArcPending,
 			}
 			n.Arcs = append(n.Arcs, b)
 		}

internal/core/debug/debug.go

@@ -220,6 +220,9 @@ func (w *printer) node(n adt.Node) {
 		}
 
 		for _, a := range x.Arcs {
+			if a.ArcType == adt.ArcNotPresent {
+				continue
+			}
 			if a.Label.IsLet() {
 				w.string("\n")
 				w.string("let ")

internal/core/export/expr.go

@@ -320,7 +320,7 @@ type conjuncts struct {
 func (c *conjuncts) getField(label adt.Feature) field {
 	f, ok := c.fields[label]
 	if !ok {
-		f.arcType = adt.ArcVoid
+		f.arcType = adt.ArcNotPresent
 	}
 	return f
 }
@@ -385,7 +385,7 @@ func (e *conjuncts) addExpr(env *adt.Environment, src *adt.Vertex, x adt.Elem, i
 
 		for _, d := range x.Decls {
 			var label adt.Feature
-			t := adt.ArcVoid
+			t := adt.ArcNotPresent
 			switch f := d.(type) {
 			case *adt.Field:
 				label = f.Label