internal/core/adt/composite.go - cue - Git at Google

 // Copyright 2020 CUE Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package adt

 import (
 	"fmt"

 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/token"
 )

 // TODO: unanswered questions about structural cycles:
 //
 // 1. When detecting a structural cycle, should we consider this as:
 //    a) an unevaluated value,
 //    b) an incomplete error (which does not affect parent validity), or
 //    c) a special value.
 //
 // Making it an error is the simplest way to ensure reentrancy is disallowed:
 // without an error it would require an additional mechanism to stop reentrancy
 // from continuing to process. Even worse, in some cases it may only partially
 // evaluate, resulting in unexpected results. For this reason, we are taking
 // approach `b` for now.
 //
 // This has some consequences of how disjunctions are treated though. Consider
 //
 //     list: {
 //        head: _
 //        tail: list | null
 //     }
 //
 // When making it an error, evaluating the above will result in
 //
 //     list: {
 //        head: _
 //        tail: null
 //     }
 //
 // because list will result in a structural cycle, and thus an error, it will be
 // stripped from the disjunction. This may or may not be a desirable property. A
 // nice thing is that it is not required to write `list | *null`. A disadvantage
 // is that this is perhaps somewhat inexplicit.
 //
 // When not making it an error (and simply cease evaluating child arcs upon
 // cycle detection), the result would be:
 //
 //     list: {
 //        head: _
 //        tail: list | null
 //     }
 //
 // In other words, an evaluation would result in a cycle and thus an error.
 // Implementations can recognize such cases by having unevaluated arcs. An
 // explicit structure cycle marker would probably be less error prone.
 //
 // Note that in both cases, a reference to list will still use the original
 // conjuncts, so the result will be the same for either method in this case.
 //
 //
 // 2. Structural cycle allowance.
 //
 // Structural cycle detection disallows reentrancy as well. This means one
 // cannot use structs for recursive computation. This will probably preclude
 // evaluation of some configuration. Given that there is no real alternative
 // yet, we could allow structural cycle detection to be optionally disabled.

 // An Environment links the parent scopes for identifier lookup to a composite
 // node. Each conjunct that make up node in the tree can be associated with
 // a different environment (although some conjuncts may share an Environment).
 type Environment struct {
 	Up     *Environment
 	Vertex *Vertex

 	// DynamicLabel is only set when instantiating a field from a pattern
 	// constraint. It is used to resolve label references.
 	DynamicLabel Feature

 	// TODO(perf): make the following public fields a shareable struct as it
 	// mostly is going to be the same for child nodes.

 	// Cyclic indicates a structural cycle was detected for this conjunct or one
 	// of its ancestors.
 	Cyclic bool

 	// Deref keeps track of nodes that should dereference to Vertex. It is used
 	// for detecting structural cycle.
 	//
 	// The detection algorithm is based on Tomabechi's quasi-destructive graph
 	// unification. This detection requires dependencies to be resolved into
 	// fully dereferenced vertices. This is not the case in our algorithm:
 	// the result of evaluating conjuncts is placed into dereferenced vertices
 	// _after_ they are evaluated, but the Environment still points to the
 	// non-dereferenced context.
 	//
 	// In order to be able to detect structural cycles, we need to ensure that
 	// at least one node that is part of a cycle in the context in which
 	// conjunctions are evaluated dereferences correctly.
 	//
 	// The only field necessary to detect a structural cycle, however, is
 	// the Status field of the Vertex. So rather than dereferencing a node
 	// proper, it is sufficient to copy the Status of the dereferenced nodes
 	// to these nodes (will always be EvaluatingArcs).
 	Deref []*Vertex

 	// Cycles contains vertices for which cycles are detected. It is used
 	// for tracking self-references within structural cycles.
 	//
 	// Unlike Deref, Cycles is not incremented with child nodes.
 	// TODO: Cycles is always a tail end of Deref, so this can be optimized.
 	Cycles []*Vertex

 	cache map[Expr]Value
 }

 type ID int32

 // evalCached is used to look up let expressions. Caching let expressions
 // prevents a possible combinatorial explosion.
 func (e *Environment) evalCached(c *OpContext, x Expr) Value {
 	v, ok := e.cache[x]
 	if !ok {
 		if e.cache == nil {
 			e.cache = map[Expr]Value{}
 		}
 		env, src := c.e, c.src
 		c.e, c.src = e, x.Source()
 		v = c.eval(x)
 		c.e, c.src = env, src
 		e.cache[x] = v
 	}
 	return v
 }

 // A Vertex is a node in the value tree. It may be a leaf or internal node.
 // It may have arcs to represent elements of a fully evaluated struct or list.
 //
 // For structs, it only contains definitions and concrete fields.
 // optional fields are dropped.
 //
 // It maintains source information such as a list of conjuncts that contributed
 // to the value.
 type Vertex struct {
 	// Parent links to a parent Vertex. This parent should only be used to
 	// access the parent's Label field to find the relative location within a
 	// tree.
 	Parent *Vertex

 	// Label is the feature leading to this vertex.
 	Label Feature

 	// TODO: move the following status fields to a separate struct.

 	// status indicates the evaluation progress of this vertex.
 	status VertexStatus

 	// isData indicates that this Vertex is to be interepreted as data: pattern
 	// and additional constraints, as well as optional fields, should be
 	// ignored.
 	isData bool

 	// EvalCount keeps track of temporary dereferencing during evaluation.
 	// If EvalCount > 0, status should be considered to be EvaluatingArcs.
 	EvalCount int

 	// SelfCount is used for tracking self-references.
 	SelfCount int

 	// Value is the value associated with this vertex. For lists and structs
 	// this is a sentinel value indicating its kind.
 	Value Value

 	// ChildErrors is the collection of all errors of children.
 	ChildErrors *Bottom

 	// The parent of nodes can be followed to determine the path within the
 	// configuration of this node.
 	// Value  Value
 	Arcs []*Vertex // arcs are sorted in display order.

 	// Conjuncts lists the structs that ultimately formed this Composite value.
 	// This includes all selected disjuncts.
 	//
 	// This value may be nil, in which case the Arcs are considered to define
 	// the final value of this Vertex.
 	Conjuncts []Conjunct

 	// Structs is a slice of struct literals that contributed to this value.
 	// This information is used to compute the topological sort of arcs.
 	Structs []*StructLit

 	// Closed contains information about how to interpret field labels for the
 	// various conjuncts with respect to which fields are allowed in this
 	// Vertex. If allows all fields if it is nil.
 	// The evaluator will first check existing fields before using this. So for
 	// simple cases, an Acceptor can always return false to close the Vertex.
 	Closed Acceptor
 }

 // VertexStatus indicates the evaluation progress of a Vertex.
 type VertexStatus int8

 const (
 	// Unprocessed indicates a Vertex has not been processed before.
 	// Value must be nil.
 	Unprocessed VertexStatus = iota

 	// Evaluating means that the current Vertex is being evaluated. If this is
 	// encountered it indicates a reference cycle. Value must be nil.
 	Evaluating

 	// Partial indicates that the result was only partially evaluated. It will
 	// need to be fully evaluated to get a complete results.
 	//
 	// TODO: this currently requires a renewed computation. Cache the
 	// nodeContext to allow reusing the computations done so far.
 	Partial

 	// EvaluatingArcs indicates that the arcs of the Vertex are currently being
 	// evaluated. If this is encountered it indicates a structural cycle.
 	// Value does not have to be nil
 	EvaluatingArcs

 	// Finalized means that this node is fully evaluated and that the results
 	// are save to use without further consideration.
 	Finalized
 )

 func (v *Vertex) Status() VertexStatus {
 	if v.EvalCount > 0 {
 		return EvaluatingArcs
 	}
 	return v.status
 }

 func (v *Vertex) UpdateStatus(s VertexStatus) {
 	if v.status > s+1 {
 		panic(fmt.Sprintf("attempt to regress status from %d to %d", v.Status(), s))
 	}
 	if s == Finalized && v.Value == nil {
 		// panic("not finalized")
 	}
 	v.status = s
 }

 // IsData reports whether v should be interpreted in data mode. In other words,
 // it tells whether optional field matching and non-regular fields, like
 // definitions and hidden fields, should be ignored.
 func (v *Vertex) IsData() bool {
 	return v.isData || len(v.Conjuncts) == 0
 }

 // ToDataSingle creates a new Vertex that represents just the regular fields
 // of this vertex. Arcs are left untouched.
 // It is used by cue.Eval to convert nodes to data on per-node basis.
 func (v *Vertex) ToDataSingle() *Vertex {
 	w := *v
 	w.isData = true
 	return &w
 }

 // ToDataAll returns a new v where v and all its descendents contain only
 // the regular fields.
 func (v *Vertex) ToDataAll() *Vertex {
 	arcs := make([]*Vertex, len(v.Arcs))
 	for i, a := range v.Arcs {
 		arcs[i] = a.ToDataAll()
 	}
 	w := *v
 	w.Arcs = arcs
 	w.isData = true
 	w.Conjuncts = make([]Conjunct, len(v.Conjuncts))
 	copy(w.Conjuncts, v.Conjuncts)
 	for i := range w.Conjuncts {
 		w.Conjuncts[i].CloseID = 0
 	}
 	w.Closed = nil
 	return &w
 }

 // func (v *Vertex) IsEvaluating() bool {
 // 	return v.Value == cycle
 // }

 func (v *Vertex) IsErr() bool {
 	// if v.Status() > Evaluating {
 	if _, ok := v.Value.(*Bottom); ok {
 		return true
 	}
 	// }
 	return false
 }

 func (v *Vertex) Err(c *OpContext, state VertexStatus) *Bottom {
 	c.Unify(c, v, state)
 	if b, ok := v.Value.(*Bottom); ok {
 		return b
 	}
 	return nil
 }

 // func (v *Vertex) Evaluate()

 func (v *Vertex) Finalize(c *OpContext) {
 	if c == nil {
 		fmt.Println("WOT?")
 	}
 	c.Unify(c, v, Finalized)
 }

 func (v *Vertex) AddErr(ctx *OpContext, b *Bottom) {
 	v.Value = CombineErrors(nil, v.Value, b)
 	v.UpdateStatus(Finalized)
 }

 func (v *Vertex) SetValue(ctx *OpContext, state VertexStatus, value Value) *Bottom {
 	v.Value = value
 	v.UpdateStatus(state)
 	return nil
 }

 // ToVertex wraps v in a new Vertex, if necessary.
 func ToVertex(v Value) *Vertex {
 	switch x := v.(type) {
 	case *Vertex:
 		return x
 	default:
 		n := &Vertex{
 			status: Finalized,
 			Value:  x,
 		}
 		n.AddConjunct(MakeRootConjunct(nil, v))
 		return n
 	}
 }

 // Unwrap returns the possibly non-concrete scalar value of v or nil if v is
 // a list, struct or of undefined type.
 func Unwrap(v Value) Value {
 	x, ok := v.(*Vertex)
 	if !ok {
 		return v
 	}
 	switch x.Value.(type) {
 	case *StructMarker, *ListMarker:
 		return v
 	default:
 		return x.Value
 	}
 }

 // Acceptor is a single interface that reports whether feature f is a valid
 // field label for this vertex.
 //
 // TODO: combine this with the StructMarker functionality?
 type Acceptor interface {
 	// Accept reports whether a given field is accepted as output.
 	// Pass an InvalidLabel to determine whether this is always open.
 	Accept(ctx *OpContext, f Feature) bool

 	// MatchAndInsert finds the conjuncts for optional fields, pattern
 	// constraints, and additional constraints that match f and inserts them in
 	// arc. Use f is 0 to match all additional constraints only.
 	MatchAndInsert(c *OpContext, arc *Vertex)

 	// OptionalTypes returns a bit field with the type of optional constraints
 	// that are represented by this Acceptor.
 	OptionalTypes() OptionalType
 }

 // OptionalType is a bit field of the type of optional constraints in use by an
 // Acceptor.
 type OptionalType int

 const (
 	HasField      OptionalType = 1 << iota // X: T
 	HasDynamic                             // (X): T or "\(X)": T
 	HasPattern                             // [X]: T
 	HasAdditional                          // ...T
 	IsOpen                                 // Defined for all fields
 )

 func (v *Vertex) Kind() Kind {
 	// This is possible when evaluating comprehensions. It is potentially
 	// not known at this time what the type is.
 	if v.Value == nil {
 		return TopKind
 	}
 	return v.Value.Kind()
 }

 func (v *Vertex) OptionalTypes() OptionalType {
 	switch {
 	case v.Closed != nil:
 		return v.Closed.OptionalTypes()
 	case v.IsList():
 		return 0
 	default:
 		return IsOpen
 	}
 }

 func (v *Vertex) IsClosed(ctx *OpContext) bool {
 	switch x := v.Value.(type) {
 	case *ListMarker:
 		// TODO: use one mechanism.
 		if x.IsOpen {
 			return false
 		}
 		if v.Closed == nil {
 			return true
 		}
 		return !v.Closed.Accept(ctx, InvalidLabel)

 	case *StructMarker:
 		if x.NeedClose {
 			return true
 		}
 		if v.Closed == nil {
 			return false
 		}
 		return !v.Closed.Accept(ctx, InvalidLabel)
 	}
 	return false
 }

 func (v *Vertex) Accept(ctx *OpContext, f Feature) bool {
 	if !v.IsClosed(ctx) || v.Lookup(f) != nil {
 		return true
 	}
 	if v.Closed != nil {
 		return v.Closed.Accept(ctx, f)
 	}
 	return false
 }

 func (v *Vertex) MatchAndInsert(ctx *OpContext, arc *Vertex) {
 	if v.Closed == nil {
 		return
 	}
 	if !v.Accept(ctx, arc.Label) {
 		return
 	}
 	v.Closed.MatchAndInsert(ctx, arc)
 }

 func (v *Vertex) IsList() bool {
 	_, ok := v.Value.(*ListMarker)
 	return ok
 }

 // Lookup returns the Arc with label f if it exists or nil otherwise.
 func (v *Vertex) Lookup(f Feature) *Vertex {
 	for _, a := range v.Arcs {
 		if a.Label == f {
 			return a
 		}
 	}
 	return nil
 }

 // Elems returns the regular elements of a list.
 func (v *Vertex) Elems() []*Vertex {
 	// TODO: add bookkeeping for where list arcs start and end.
 	a := make([]*Vertex, 0, len(v.Arcs))
 	for _, x := range v.Arcs {
 		if x.Label.IsInt() {
 			a = append(a, x)
 		}
 	}
 	return a
 }

 // GetArc returns a Vertex for the outgoing arc with label f. It creates and
 // ads one if it doesn't yet exist.
 func (v *Vertex) GetArc(f Feature) (arc *Vertex, isNew bool) {
 	arc = v.Lookup(f)
 	if arc == nil {
 		arc = &Vertex{Parent: v, Label: f}
 		v.Arcs = append(v.Arcs, arc)
 		isNew = true
 	}
 	return arc, isNew
 }

 func (v *Vertex) Source() ast.Node { return nil }

 // AddConjunct adds the given Conjuncts to v if it doesn't already exist.
 func (v *Vertex) AddConjunct(c Conjunct) *Bottom {
 	if v.Value != nil {
 		// This is likely a bug in the evaluator and should not happen.
 		return &Bottom{Err: errors.Newf(token.NoPos, "cannot add conjunct")}
 	}
 	for _, x := range v.Conjuncts {
 		if x == c {
 			return nil
 		}
 	}

 	v.Conjuncts = append(v.Conjuncts, c)
 	return nil
 }

 func (v *Vertex) AddStructs(a ...*StructLit) {
 outer:
 	for _, s := range a {
 		for _, t := range v.Structs {
 			if t == s {
 				continue outer
 			}
 		}
 		v.Structs = append(v.Structs, s)
 	}
 }

 // Path computes the sequence of Features leading from the root to of the
 // instance to this Vertex.
 func (v *Vertex) Path() []Feature {
 	return appendPath(nil, v)
 }

 func appendPath(a []Feature, v *Vertex) []Feature {
 	if v.Parent == nil {
 		return a
 	}
 	a = appendPath(a, v.Parent)
 	return append(a, v.Label)
 }

 func (v *Vertex) appendListArcs(arcs []*Vertex) (err *Bottom) {
 	for _, a := range arcs {
 		// TODO(list): BUG this only works if lists do not have definitions
 		// fields.
 		label, err := MakeLabel(a.Source(), int64(len(v.Arcs)), IntLabel)
 		if err != nil {
 			return &Bottom{Src: a.Source(), Err: err}
 		}
 		v.Arcs = append(v.Arcs, &Vertex{
 			Parent:    v,
 			Label:     label,
 			Conjuncts: a.Conjuncts,
 		})
 	}
 	return nil
 }

 // An Conjunct is an Environment-Expr pair. The Environment is the starting
 // point for reference lookup for any reference contained in X.
 type Conjunct struct {
 	Env *Environment
 	x   Node

 	// CloseID is a unique number that tracks a group of conjuncts that need
 	// belong to a single originating definition.
 	CloseID ID
 }

 func (c *Conjunct) ID() ID {
 	return c.CloseID
 }

 // TODO(perf): replace with composite literal if this helps performance.

 // MakeRootConjunct creates a conjunct from the given environment and node.
 // It panics if x cannot be used as an expression.
 func MakeRootConjunct(env *Environment, x Node) Conjunct {
 	return MakeConjunct(env, x, 0)
 }

 func MakeConjunct(env *Environment, x Node, id ID) Conjunct {
 	if env == nil {
 		// TODO: better is to pass one.
 		env = &Environment{}
 	}
 	switch x.(type) {
 	case Expr, interface{ expr() Expr }:
 	default:
 		panic(fmt.Sprintf("invalid Node type %T", x))
 	}
 	return Conjunct{env, x, id}
 }

 func (c *Conjunct) Source() ast.Node {
 	return c.x.Source()
 }

 func (c *Conjunct) Field() Node {
 	return c.x
 }

 func (c *Conjunct) Expr() Expr {
 	switch x := c.x.(type) {
 	case Expr:
 		return x
 	case interface{ expr() Expr }:
 		return x.expr()
 	default:
 		panic("unreachable")
 	}
 }
	// Copyright 2020 CUE Authors
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package adt

	import (
	"fmt"

	"cuelang.org/go/cue/ast"
	"cuelang.org/go/cue/errors"
	"cuelang.org/go/cue/token"
	)

	// TODO: unanswered questions about structural cycles:
	//
	// 1. When detecting a structural cycle, should we consider this as:
	// a) an unevaluated value,
	// b) an incomplete error (which does not affect parent validity), or
	// c) a special value.
	//
	// Making it an error is the simplest way to ensure reentrancy is disallowed:
	// without an error it would require an additional mechanism to stop reentrancy
	// from continuing to process. Even worse, in some cases it may only partially
	// evaluate, resulting in unexpected results. For this reason, we are taking
	// approach `b` for now.
	//
	// This has some consequences of how disjunctions are treated though. Consider
	//
	// list: {
	// head: _
	// tail: list \| null
	// }
	//
	// When making it an error, evaluating the above will result in
	//
	// list: {
	// head: _
	// tail: null
	// }
	//
	// because list will result in a structural cycle, and thus an error, it will be
	// stripped from the disjunction. This may or may not be a desirable property. A
	// nice thing is that it is not required to write `list \| *null`. A disadvantage
	// is that this is perhaps somewhat inexplicit.
	//
	// When not making it an error (and simply cease evaluating child arcs upon
	// cycle detection), the result would be:
	//
	// list: {
	// head: _
	// tail: list \| null
	// }
	//
	// In other words, an evaluation would result in a cycle and thus an error.
	// Implementations can recognize such cases by having unevaluated arcs. An
	// explicit structure cycle marker would probably be less error prone.
	//
	// Note that in both cases, a reference to list will still use the original
	// conjuncts, so the result will be the same for either method in this case.
	//
	//
	// 2. Structural cycle allowance.
	//
	// Structural cycle detection disallows reentrancy as well. This means one
	// cannot use structs for recursive computation. This will probably preclude
	// evaluation of some configuration. Given that there is no real alternative
	// yet, we could allow structural cycle detection to be optionally disabled.

	// An Environment links the parent scopes for identifier lookup to a composite
	// node. Each conjunct that make up node in the tree can be associated with
	// a different environment (although some conjuncts may share an Environment).
	type Environment struct {
	Up *Environment
	Vertex *Vertex

	// DynamicLabel is only set when instantiating a field from a pattern
	// constraint. It is used to resolve label references.
	DynamicLabel Feature

	// TODO(perf): make the following public fields a shareable struct as it
	// mostly is going to be the same for child nodes.

	// Cyclic indicates a structural cycle was detected for this conjunct or one
	// of its ancestors.
	Cyclic bool

	// Deref keeps track of nodes that should dereference to Vertex. It is used
	// for detecting structural cycle.
	//
	// The detection algorithm is based on Tomabechi's quasi-destructive graph
	// unification. This detection requires dependencies to be resolved into
	// fully dereferenced vertices. This is not the case in our algorithm:
	// the result of evaluating conjuncts is placed into dereferenced vertices
	// _after_ they are evaluated, but the Environment still points to the
	// non-dereferenced context.
	//
	// In order to be able to detect structural cycles, we need to ensure that
	// at least one node that is part of a cycle in the context in which
	// conjunctions are evaluated dereferences correctly.
	//
	// The only field necessary to detect a structural cycle, however, is
	// the Status field of the Vertex. So rather than dereferencing a node
	// proper, it is sufficient to copy the Status of the dereferenced nodes
	// to these nodes (will always be EvaluatingArcs).
	Deref []*Vertex

	// Cycles contains vertices for which cycles are detected. It is used
	// for tracking self-references within structural cycles.
	//
	// Unlike Deref, Cycles is not incremented with child nodes.
	// TODO: Cycles is always a tail end of Deref, so this can be optimized.
	Cycles []*Vertex

	cache map[Expr]Value
	}

	type ID int32

	// evalCached is used to look up let expressions. Caching let expressions
	// prevents a possible combinatorial explosion.
	func (e Environment) evalCached(c OpContext, x Expr) Value {
	v, ok := e.cache[x]
	if !ok {
	if e.cache == nil {
	e.cache = map[Expr]Value{}
	}
	env, src := c.e, c.src
	c.e, c.src = e, x.Source()
	v = c.eval(x)
	c.e, c.src = env, src
	e.cache[x] = v
	}
	return v
	}

	// A Vertex is a node in the value tree. It may be a leaf or internal node.
	// It may have arcs to represent elements of a fully evaluated struct or list.
	//
	// For structs, it only contains definitions and concrete fields.
	// optional fields are dropped.
	//
	// It maintains source information such as a list of conjuncts that contributed
	// to the value.
	type Vertex struct {
	// Parent links to a parent Vertex. This parent should only be used to
	// access the parent's Label field to find the relative location within a
	// tree.
	Parent *Vertex

	// Label is the feature leading to this vertex.
	Label Feature

	// TODO: move the following status fields to a separate struct.

	// status indicates the evaluation progress of this vertex.
	status VertexStatus

	// isData indicates that this Vertex is to be interepreted as data: pattern
	// and additional constraints, as well as optional fields, should be
	// ignored.
	isData bool

	// EvalCount keeps track of temporary dereferencing during evaluation.
	// If EvalCount > 0, status should be considered to be EvaluatingArcs.
	EvalCount int

	// SelfCount is used for tracking self-references.
	SelfCount int

	// Value is the value associated with this vertex. For lists and structs
	// this is a sentinel value indicating its kind.
	Value Value

	// ChildErrors is the collection of all errors of children.
	ChildErrors *Bottom

	// The parent of nodes can be followed to determine the path within the
	// configuration of this node.
	// Value Value
	Arcs []*Vertex // arcs are sorted in display order.

	// Conjuncts lists the structs that ultimately formed this Composite value.
	// This includes all selected disjuncts.
	//
	// This value may be nil, in which case the Arcs are considered to define
	// the final value of this Vertex.
	Conjuncts []Conjunct

	// Structs is a slice of struct literals that contributed to this value.
	// This information is used to compute the topological sort of arcs.
	Structs []*StructLit

	// Closed contains information about how to interpret field labels for the
	// various conjuncts with respect to which fields are allowed in this
	// Vertex. If allows all fields if it is nil.
	// The evaluator will first check existing fields before using this. So for
	// simple cases, an Acceptor can always return false to close the Vertex.
	Closed Acceptor
	}

	// VertexStatus indicates the evaluation progress of a Vertex.
	type VertexStatus int8

	const (
	// Unprocessed indicates a Vertex has not been processed before.
	// Value must be nil.
	Unprocessed VertexStatus = iota

	// Evaluating means that the current Vertex is being evaluated. If this is
	// encountered it indicates a reference cycle. Value must be nil.
	Evaluating

	// Partial indicates that the result was only partially evaluated. It will
	// need to be fully evaluated to get a complete results.
	//
	// TODO: this currently requires a renewed computation. Cache the
	// nodeContext to allow reusing the computations done so far.
	Partial

	// EvaluatingArcs indicates that the arcs of the Vertex are currently being
	// evaluated. If this is encountered it indicates a structural cycle.
	// Value does not have to be nil
	EvaluatingArcs

	// Finalized means that this node is fully evaluated and that the results
	// are save to use without further consideration.
	Finalized
	)

	func (v *Vertex) Status() VertexStatus {
	if v.EvalCount > 0 {
	return EvaluatingArcs
	}
	return v.status
	}

	func (v *Vertex) UpdateStatus(s VertexStatus) {
	if v.status > s+1 {
	panic(fmt.Sprintf("attempt to regress status from %d to %d", v.Status(), s))
	}
	if s == Finalized && v.Value == nil {
	// panic("not finalized")
	}
	v.status = s
	}

	// IsData reports whether v should be interpreted in data mode. In other words,
	// it tells whether optional field matching and non-regular fields, like
	// definitions and hidden fields, should be ignored.
	func (v *Vertex) IsData() bool {
	return v.isData \|\| len(v.Conjuncts) == 0
	}

	// ToDataSingle creates a new Vertex that represents just the regular fields
	// of this vertex. Arcs are left untouched.
	// It is used by cue.Eval to convert nodes to data on per-node basis.
	func (v Vertex) ToDataSingle() Vertex {
	w := *v
	w.isData = true
	return &w
	}

	// ToDataAll returns a new v where v and all its descendents contain only
	// the regular fields.
	func (v Vertex) ToDataAll() Vertex {
	arcs := make([]*Vertex, len(v.Arcs))
	for i, a := range v.Arcs {
	arcs[i] = a.ToDataAll()
	}
	w := *v
	w.Arcs = arcs
	w.isData = true
	w.Conjuncts = make([]Conjunct, len(v.Conjuncts))
	copy(w.Conjuncts, v.Conjuncts)
	for i := range w.Conjuncts {
	w.Conjuncts[i].CloseID = 0
	}
	w.Closed = nil
	return &w
	}

	// func (v *Vertex) IsEvaluating() bool {
	// return v.Value == cycle
	// }

	func (v *Vertex) IsErr() bool {
	// if v.Status() > Evaluating {
	if _, ok := v.Value.(*Bottom); ok {
	return true
	}
	// }
	return false
	}

	func (v Vertex) Err(c OpContext, state VertexStatus) *Bottom {
	c.Unify(c, v, state)
	if b, ok := v.Value.(*Bottom); ok {
	return b
	}
	return nil
	}

	// func (v *Vertex) Evaluate()

	func (v Vertex) Finalize(c OpContext) {
	if c == nil {
	fmt.Println("WOT?")
	}
	c.Unify(c, v, Finalized)
	}

	func (v Vertex) AddErr(ctx OpContext, b *Bottom) {
	v.Value = CombineErrors(nil, v.Value, b)
	v.UpdateStatus(Finalized)
	}

	func (v Vertex) SetValue(ctx OpContext, state VertexStatus, value Value) *Bottom {
	v.Value = value
	v.UpdateStatus(state)
	return nil
	}

	// ToVertex wraps v in a new Vertex, if necessary.
	func ToVertex(v Value) *Vertex {
	switch x := v.(type) {
	case *Vertex:
	return x
	default:
	n := &Vertex{
	status: Finalized,
	Value: x,
	}
	n.AddConjunct(MakeRootConjunct(nil, v))
	return n
	}
	}

	// Unwrap returns the possibly non-concrete scalar value of v or nil if v is
	// a list, struct or of undefined type.
	func Unwrap(v Value) Value {
	x, ok := v.(*Vertex)
	if !ok {
	return v
	}
	switch x.Value.(type) {
	case StructMarker, ListMarker:
	return v
	default:
	return x.Value
	}
	}

	// Acceptor is a single interface that reports whether feature f is a valid
	// field label for this vertex.
	//
	// TODO: combine this with the StructMarker functionality?
	type Acceptor interface {
	// Accept reports whether a given field is accepted as output.
	// Pass an InvalidLabel to determine whether this is always open.
	Accept(ctx *OpContext, f Feature) bool

	// MatchAndInsert finds the conjuncts for optional fields, pattern
	// constraints, and additional constraints that match f and inserts them in
	// arc. Use f is 0 to match all additional constraints only.
	MatchAndInsert(c OpContext, arc Vertex)

	// OptionalTypes returns a bit field with the type of optional constraints
	// that are represented by this Acceptor.
	OptionalTypes() OptionalType
	}

	// OptionalType is a bit field of the type of optional constraints in use by an
	// Acceptor.
	type OptionalType int

	const (
	HasField OptionalType = 1 << iota // X: T
	HasDynamic // (X): T or "\(X)": T
	HasPattern // [X]: T
	HasAdditional // ...T
	IsOpen // Defined for all fields
	)

	func (v *Vertex) Kind() Kind {
	// This is possible when evaluating comprehensions. It is potentially
	// not known at this time what the type is.
	if v.Value == nil {
	return TopKind
	}
	return v.Value.Kind()
	}

	func (v *Vertex) OptionalTypes() OptionalType {
	switch {
	case v.Closed != nil:
	return v.Closed.OptionalTypes()
	case v.IsList():
	return 0
	default:
	return IsOpen
	}
	}

	func (v Vertex) IsClosed(ctx OpContext) bool {
	switch x := v.Value.(type) {
	case *ListMarker:
	// TODO: use one mechanism.
	if x.IsOpen {
	return false
	}
	if v.Closed == nil {
	return true
	}
	return !v.Closed.Accept(ctx, InvalidLabel)

	case *StructMarker:
	if x.NeedClose {
	return true
	}
	if v.Closed == nil {
	return false
	}
	return !v.Closed.Accept(ctx, InvalidLabel)
	}
	return false
	}

	func (v Vertex) Accept(ctx OpContext, f Feature) bool {
	if !v.IsClosed(ctx) \|\| v.Lookup(f) != nil {
	return true
	}
	if v.Closed != nil {
	return v.Closed.Accept(ctx, f)
	}
	return false
	}

	func (v Vertex) MatchAndInsert(ctx OpContext, arc *Vertex) {
	if v.Closed == nil {
	return
	}
	if !v.Accept(ctx, arc.Label) {
	return
	}
	v.Closed.MatchAndInsert(ctx, arc)
	}

	func (v *Vertex) IsList() bool {
	_, ok := v.Value.(*ListMarker)
	return ok
	}

	// Lookup returns the Arc with label f if it exists or nil otherwise.
	func (v Vertex) Lookup(f Feature) Vertex {
	for _, a := range v.Arcs {
	if a.Label == f {
	return a
	}
	}
	return nil
	}

	// Elems returns the regular elements of a list.
	func (v Vertex) Elems() []Vertex {
	// TODO: add bookkeeping for where list arcs start and end.
	a := make([]*Vertex, 0, len(v.Arcs))
	for _, x := range v.Arcs {
	if x.Label.IsInt() {
	a = append(a, x)
	}
	}
	return a
	}

	// GetArc returns a Vertex for the outgoing arc with label f. It creates and
	// ads one if it doesn't yet exist.
	func (v Vertex) GetArc(f Feature) (arc Vertex, isNew bool) {
	arc = v.Lookup(f)
	if arc == nil {
	arc = &Vertex{Parent: v, Label: f}
	v.Arcs = append(v.Arcs, arc)
	isNew = true
	}
	return arc, isNew
	}

	func (v *Vertex) Source() ast.Node { return nil }

	// AddConjunct adds the given Conjuncts to v if it doesn't already exist.
	func (v Vertex) AddConjunct(c Conjunct) Bottom {
	if v.Value != nil {
	// This is likely a bug in the evaluator and should not happen.
	return &Bottom{Err: errors.Newf(token.NoPos, "cannot add conjunct")}
	}
	for _, x := range v.Conjuncts {
	if x == c {
	return nil
	}
	}

	v.Conjuncts = append(v.Conjuncts, c)
	return nil
	}

	func (v Vertex) AddStructs(a ...StructLit) {
	outer:
	for _, s := range a {
	for _, t := range v.Structs {
	if t == s {
	continue outer
	}
	}
	v.Structs = append(v.Structs, s)
	}
	}

	// Path computes the sequence of Features leading from the root to of the
	// instance to this Vertex.
	func (v *Vertex) Path() []Feature {
	return appendPath(nil, v)
	}

	func appendPath(a []Feature, v *Vertex) []Feature {
	if v.Parent == nil {
	return a
	}
	a = appendPath(a, v.Parent)
	return append(a, v.Label)
	}

	func (v Vertex) appendListArcs(arcs []Vertex) (err *Bottom) {
	for _, a := range arcs {
	// TODO(list): BUG this only works if lists do not have definitions
	// fields.
	label, err := MakeLabel(a.Source(), int64(len(v.Arcs)), IntLabel)
	if err != nil {
	return &Bottom{Src: a.Source(), Err: err}
	}
	v.Arcs = append(v.Arcs, &Vertex{
	Parent: v,
	Label: label,
	Conjuncts: a.Conjuncts,
	})
	}
	return nil
	}

	// An Conjunct is an Environment-Expr pair. The Environment is the starting
	// point for reference lookup for any reference contained in X.
	type Conjunct struct {
	Env *Environment
	x Node

	// CloseID is a unique number that tracks a group of conjuncts that need
	// belong to a single originating definition.
	CloseID ID
	}

	func (c *Conjunct) ID() ID {
	return c.CloseID
	}

	// TODO(perf): replace with composite literal if this helps performance.

	// MakeRootConjunct creates a conjunct from the given environment and node.
	// It panics if x cannot be used as an expression.
	func MakeRootConjunct(env *Environment, x Node) Conjunct {
	return MakeConjunct(env, x, 0)
	}

	func MakeConjunct(env *Environment, x Node, id ID) Conjunct {
	if env == nil {
	// TODO: better is to pass one.
	env = &Environment{}
	}
	switch x.(type) {
	case Expr, interface{ expr() Expr }:
	default:
	panic(fmt.Sprintf("invalid Node type %T", x))
	}
	return Conjunct{env, x, id}
	}

	func (c *Conjunct) Source() ast.Node {
	return c.x.Source()
	}

	func (c *Conjunct) Field() Node {
	return c.x
	}

	func (c *Conjunct) Expr() Expr {
	switch x := c.x.(type) {
	case Expr:
	return x
	case interface{ expr() Expr }:
	return x.expr()
	default:
	panic("unreachable")
	}
	}