internal/core/adt: move and rename Indirect

There are now different kinds of dereferencing. We rename
Indirect to DerefValue and move it close to its siblings.
Goals:
- keep all types of dereference together
- keep naming consistent
- use DerefValue, instead of Deref or DerefAll, to make the
  purpose of this particular dereference clear: getting the
  underlying value.

Added notes on which dereferences might need to change
in the future.

Issue #2854

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I3ccde0ac21e248f65d9a462515840c77348f658a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194087
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mpvl

cue/types.go

@@ -235,11 +235,11 @@ func (i *Iterator) Next() bool {
 	p := linkParent(i.val.parent_, i.val.v, arc)
 	i.f = arc.Label
 	i.arcType = arc.ArcType
-	// TODO: this should not indirect disjunctions, as this will cause losing
+	// TODO(deref): this should not indirect disjunctions, as this will cause losing
 	// information, which is now available in the new evaluator. Find a safe
 	// way to preserve disjunction information while keeping backward
 	// compatibility in the API.
-	arc = arc.Indirect()
+	arc = arc.DerefValue()
 	i.cur = makeValue(i.val.idx, arc, p)
 	i.p++
 	return true

internal/core/adt/composite.go

@@ -267,7 +267,7 @@ type Vertex struct {
 }
 
 func deref(v *Vertex) *Vertex {
-	v = v.Indirect()
+	v = v.DerefValue()
 	n := v.state
 	if n != nil {
 		v = n.underlying
@@ -679,7 +679,10 @@ func (v *Vertex) isUndefined() bool {
 
 // isFinal reports whether this node may no longer be modified.
 func (v *Vertex) isFinal() bool {
-	v = v.Indirect()
+	// TODO(deref): the accounting of what is final should be recorded
+	// in the original node. Remove this dereference once the old
+	// evaluator has been removed.
+	v = v.DerefValue()
 	return v.status == finalized
 }
 
@@ -852,7 +855,11 @@ func Unwrap(v Value) Value {
 	if !ok {
 		return v
 	}
-	x = x.Indirect()
+	// TODO(deref): BaseValue is currently overloaded to track cycles as well
+	// as the actual or dereferenced value. Once the old evaluator can be
+	// removed, we should use the new cycle tracking mechanism for cycle
+	// detection and keep BaseValue clean.
+	x = x.DerefValue()
 	if n := x.state; n != nil && isCyclePlaceholder(x.BaseValue) {
 		if n.errs != nil && !n.errs.IsIncomplete() {
 			return n.errs
@@ -864,19 +871,6 @@ func Unwrap(v Value) Value {
 	return x.Value()
 }
 
-// Indirect unrolls indirections of Vertex values. These may be introduced,
-// for instance, by temporary bindings such as comprehension values.
-// It returns v itself if v does not point to another Vertex.
-func (v *Vertex) Indirect() *Vertex {
-	for {
-		arc, ok := v.BaseValue.(*Vertex)
-		if !ok {
-			return v
-		}
-		v = arc
-	}
-}
-
 // OptionalType is a bit field of the type of optional constraints in use by an
 // Acceptor.
 type OptionalType int8
@@ -973,7 +967,14 @@ func (v *Vertex) Accept(ctx *OpContext, f Feature) bool {
 		return true
 	}
 
-	v = v.Indirect()
+	// TODO(deref): right now a dereferenced value holds all the necessary
+	// closedness information. In the future we may want to allow sharing nodes
+	// with different closedness information. In that case, we should reconsider
+	// the use of this dereference. Consider, for instance:
+	//
+	//     #a: b     // this node is currently not shared, but could be.
+	//     b: {c: 1}
+	v = v.DerefValue()
 	if x, ok := v.BaseValue.(*Disjunction); ok {
 		for _, v := range x.Values {
 			if x, ok := v.(*Vertex); ok && x.Accept(ctx, f) {
@@ -1068,12 +1069,12 @@ func (v *Vertex) IsList() bool {
 func (v *Vertex) Lookup(f Feature) *Vertex {
 	for _, a := range v.Arcs {
 		if a.Label == f {
-			// TODO(share): this indirection should ultimately be eliminated:
-			// the original node may have useful information (like original
-			// conjuncts) that are eliminated after indirection.
-			// We should leave it up to the user of Lookup at what point an
+			// TODO(P1)/TODO(deref): this indirection should ultimately be
+			// eliminated: the original node may have useful information (like
+			// original conjuncts) that are eliminated after indirection. We
+			// should leave it up to the user of Lookup at what point an
 			// indirection is necessary.
-			a = a.Indirect()
+			a = a.DerefValue()
 			return a
 		}
 	}

internal/core/adt/context.go

@@ -497,7 +497,7 @@ func (c *OpContext) resolveState(x Conjunct, r Resolver, state combinedFlags) (*
 	}
 
 	if !c.isDevVersion() {
-		arc = arc.Indirect()
+		arc = arc.DerefValue()
 	}
 
 	return arc, err
@@ -512,7 +512,9 @@ func (c *OpContext) Lookup(env *Environment, r Resolver) (*Vertex, *Bottom) {
 	err := c.PopState(s)
 
 	if arc != nil {
-		arc = arc.Indirect()
+		// TODO(P1)/TODO(deref): lookup should probably not use DerefValue, but
+		// rather only dereference disjunctions.
+		arc = arc.DerefValue()
 	}
 
 	return arc, err

internal/core/adt/default.go

@@ -45,7 +45,7 @@ func (d *Disjunction) Default() Value {
 //
 // It also closes a list, representing its default value.
 func (v *Vertex) Default() *Vertex {
-	v = v.Indirect()
+	v = v.DerefValue()
 	switch d := v.BaseValue.(type) {
 	default:
 		return v

internal/core/adt/disjunct2.go

@@ -517,7 +517,7 @@ func appendDisjunct(ctx *OpContext, a []*nodeContext, x *nodeContext) []*nodeCon
 		return a
 	}
 
-	nv := x.node.Indirect()
+	nv := x.node.DerefValue()
 	nx := nv.BaseValue
 	if nx == nil || isCyclePlaceholder(nx) {
 		nx = x.getValidators(finalized)
@@ -528,7 +528,7 @@ func appendDisjunct(ctx *OpContext, a []*nodeContext, x *nodeContext) []*nodeCon
 	// (overlayed) closeContexts are identical.
 outer:
 	for _, xn := range a {
-		xv := xn.node.Indirect()
+		xv := xn.node.DerefValue()
 		if xv.status != finalized || nv.status != finalized {
 			// Partial node
 
@@ -564,7 +564,7 @@ outer:
 			}
 		} else {
 			// Complete nodes.
-			if !Equal(ctx, xn.node.Indirect(), x.node.Indirect(), CheckStructural) {
+			if !Equal(ctx, xn.node.DerefValue(), x.node.DerefValue(), CheckStructural) {
 				continue outer
 			}
 		}

internal/core/adt/equality.go

@@ -53,8 +53,8 @@ func equalVertex(ctx *OpContext, x *Vertex, v Value, flags Flag) bool {
 		return false
 	}
 
-	x = x.Indirect()
-	y = y.Indirect()
+	x = x.DerefValue()
+	y = y.DerefValue()
 
 	if x == y {
 		return true

internal/core/adt/eval.go

@@ -614,7 +614,7 @@ func (n *nodeContext) validateValue(state vertexStatus) {
 	} else if len(n.node.Structs) > 0 {
 		markStruct = n.kind&StructKind != 0 && !n.hasTop
 	}
-	v := n.node.Indirect().Value()
+	v := n.node.DerefValue().Value()
 	if n.node.BaseValue == nil && markStruct {
 		n.node.BaseValue = &StructMarker{}
 		v = n.node

internal/core/adt/expr.go

@@ -1431,7 +1431,7 @@ func (c *OpContext) validate(env *Environment, src ast.Node, x Expr, op Op, flag
 }
 
 func isFinalError(n *Vertex) bool {
-	n = n.Indirect()
+	n = n.DerefValue()
 	if b, ok := Unwrap(n).(*Bottom); ok && b.Code < IncompleteError {
 		return true
 	}

internal/core/adt/share.go

@@ -106,6 +106,31 @@ func (n *nodeContext) shareIfPossible(c Conjunct, arc *Vertex, id CloseInfo) boo
 	return true
 }
 
+// Vertex values that are held in BaseValue will be wrapped in the following
+// order:
+//
+//    disjuncts -> (shared | computed | data)
+//
+// DerefDisjunct
+//   - get the current value under computation
+//
+// DerefValue
+//   - get the value the node ultimately represents.
+//
+
+// DerefValue unrolls indirections of Vertex values. These may be introduced,
+// for instance, by temporary bindings such as comprehension values.
+// It returns v itself if v does not point to another Vertex.
+func (v *Vertex) DerefValue() *Vertex {
+	for {
+		arc, ok := v.BaseValue.(*Vertex)
+		if !ok {
+			return v
+		}
+		v = arc
+	}
+}
+
 // DerefDisjunct indirects a node that points to a disjunction.
 func (v *Vertex) DerefDisjunct() *Vertex {
 	for {

internal/core/adt/unify.go

@@ -273,7 +273,7 @@ func (v *Vertex) unify(c *OpContext, needs condition, mode runMode) bool {
 
 	n.finalizeDisjunctions()
 
-	w = v.Indirect() // Dereference anything, including shared nodes.
+	w = v.DerefValue() // Dereference anything, including shared nodes.
 	if w != v {
 		// Clear value fields that are now referred to in the dereferenced
 		// value (w).
@@ -595,7 +595,7 @@ func (v *Vertex) lookup(c *OpContext, pos token.Pos, f Feature, flags combinedFl
 	needs := flags.conditions()
 	runMode := flags.runMode()
 
-	v = v.Indirect()
+	v = v.DerefValue()
 
 	c.Logf(c.vertex, "LOOKUP %v", f)
 
@@ -731,7 +731,7 @@ func (v *Vertex) accept(ctx *OpContext, f Feature) bool {
 	// 	return true, true
 	// }
 
-	v = v.Indirect()
+	v = v.DerefValue()
 
 	pc := v.PatternConstraints
 	if pc == nil {