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// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This file implements Scopes.
package types
import (
// A Scope maintains a set of objects and links to its containing
// (parent) and contained (children) scopes. Objects may be inserted
// and looked up by name. The zero value for Scope is a ready-to-use
// empty scope.
type Scope struct {
parent *Scope
children []*Scope
number int // parent.children[number-1] is this scope; 0 if there is no parent
elems map[string]Object // lazily allocated
pos, end token.Pos // scope extent; may be invalid
comment string // for debugging only
isFunc bool // set if this is a function scope (internal use only)
// NewScope returns a new, empty scope contained in the given parent
// scope, if any. The comment is for debugging only.
func NewScope(parent *Scope, pos, end token.Pos, comment string) *Scope {
s := &Scope{parent, nil, 0, nil, pos, end, comment, false}
// don't add children to Universe scope!
if parent != nil && parent != Universe {
parent.children = append(parent.children, s)
s.number = len(parent.children)
return s
// Parent returns the scope's containing (parent) scope.
func (s *Scope) Parent() *Scope { return s.parent }
// Len returns the number of scope elements.
func (s *Scope) Len() int { return len(s.elems) }
// Names returns the scope's element names in sorted order.
func (s *Scope) Names() []string {
names := make([]string, len(s.elems))
i := 0
for name := range s.elems {
names[i] = name
return names
// NumChildren returns the number of scopes nested in s.
func (s *Scope) NumChildren() int { return len(s.children) }
// Child returns the i'th child scope for 0 <= i < NumChildren().
func (s *Scope) Child(i int) *Scope { return s.children[i] }
// Lookup returns the object in scope s with the given name if such an
// object exists; otherwise the result is nil.
func (s *Scope) Lookup(name string) Object {
return resolve(name, s.elems[name])
// LookupParent follows the parent chain of scopes starting with s until
// it finds a scope where Lookup(name) returns a non-nil object, and then
// returns that scope and object. If a valid position pos is provided,
// only objects that were declared at or before pos are considered.
// If no such scope and object exists, the result is (nil, nil).
// Note that obj.Parent() may be different from the returned scope if the
// object was inserted into the scope and already had a parent at that
// time (see Insert). This can only happen for dot-imported objects
// whose scope is the scope of the package that exported them.
func (s *Scope) LookupParent(name string, pos token.Pos) (*Scope, Object) {
for ; s != nil; s = s.parent {
if obj := s.Lookup(name); obj != nil && (!pos.IsValid() || obj.scopePos() <= pos) {
return s, obj
return nil, nil
// Insert attempts to insert an object obj into scope s.
// If s already contains an alternative object alt with
// the same name, Insert leaves s unchanged and returns alt.
// Otherwise it inserts obj, sets the object's parent scope
// if not already set, and returns nil.
func (s *Scope) Insert(obj Object) Object {
name := obj.Name()
if alt := s.Lookup(name); alt != nil {
return alt
s.insert(name, obj)
if obj.Parent() == nil {
return nil
// _InsertLazy is like Insert, but allows deferring construction of the
// inserted object until it's accessed with Lookup. The Object
// returned by resolve must have the same name as given to _InsertLazy.
// If s already contains an alternative object with the same name,
// _InsertLazy leaves s unchanged and returns false. Otherwise it
// records the binding and returns true. The object's parent scope
// will be set to s after resolve is called.
func (s *Scope) _InsertLazy(name string, resolve func() Object) bool {
if s.elems[name] != nil {
return false
s.insert(name, &lazyObject{parent: s, resolve: resolve})
return true
func (s *Scope) insert(name string, obj Object) {
if s.elems == nil {
s.elems = make(map[string]Object)
s.elems[name] = obj
// squash merges s with its parent scope p by adding all
// objects of s to p, adding all children of s to the
// children of p, and removing s from p's children.
// The function f is called for each object obj in s which
// has an object alt in p. s should be discarded after
// having been squashed.
func (s *Scope) squash(err func(obj, alt Object)) {
p := s.parent
assert(p != nil)
for name, obj := range s.elems {
obj = resolve(name, obj)
if alt := p.Insert(obj); alt != nil {
err(obj, alt)
j := -1 // index of s in p.children
for i, ch := range p.children {
if ch == s {
j = i
assert(j >= 0)
k := len(p.children) - 1
p.children[j] = p.children[k]
p.children = p.children[:k]
p.children = append(p.children, s.children...)
s.children = nil
s.elems = nil
// Pos and End describe the scope's source code extent [pos, end).
// The results are guaranteed to be valid only if the type-checked
// AST has complete position information. The extent is undefined
// for Universe and package scopes.
func (s *Scope) Pos() token.Pos { return s.pos }
func (s *Scope) End() token.Pos { return s.end }
// Contains reports whether pos is within the scope's extent.
// The result is guaranteed to be valid only if the type-checked
// AST has complete position information.
func (s *Scope) Contains(pos token.Pos) bool {
return s.pos <= pos && pos < s.end
// Innermost returns the innermost (child) scope containing
// pos. If pos is not within any scope, the result is nil.
// The result is also nil for the Universe scope.
// The result is guaranteed to be valid only if the type-checked
// AST has complete position information.
func (s *Scope) Innermost(pos token.Pos) *Scope {
// Package scopes do not have extents since they may be
// discontiguous, so iterate over the package's files.
if s.parent == Universe {
for _, s := range s.children {
if inner := s.Innermost(pos); inner != nil {
return inner
if s.Contains(pos) {
for _, s := range s.children {
if s.Contains(pos) {
return s.Innermost(pos)
return s
return nil
// WriteTo writes a string representation of the scope to w,
// with the scope elements sorted by name.
// The level of indentation is controlled by n >= 0, with
// n == 0 for no indentation.
// If recurse is set, it also writes nested (children) scopes.
func (s *Scope) WriteTo(w io.Writer, n int, recurse bool) {
const ind = ". "
indn := strings.Repeat(ind, n)
fmt.Fprintf(w, "%s%s scope %p {\n", indn, s.comment, s)
indn1 := indn + ind
for _, name := range s.Names() {
fmt.Fprintf(w, "%s%s\n", indn1, s.Lookup(name))
if recurse {
for _, s := range s.children {
s.WriteTo(w, n+1, recurse)
fmt.Fprintf(w, "%s}\n", indn)
// String returns a string representation of the scope, for debugging.
func (s *Scope) String() string {
var buf bytes.Buffer
s.WriteTo(&buf, 0, false)
return buf.String()
// A lazyObject represents an imported Object that has not been fully
// resolved yet by its importer.
type lazyObject struct {
parent *Scope
resolve func() Object
obj Object
once sync.Once
// resolve returns the Object represented by obj, resolving lazy
// objects as appropriate.
func resolve(name string, obj Object) Object {
if lazy, ok := obj.(*lazyObject); ok {
lazy.once.Do(func() {
obj := lazy.resolve()
if _, ok := obj.(*lazyObject); ok {
panic("recursive lazy object")
if obj.Name() != name {
panic("lazy object has unexpected name")
if obj.Parent() == nil {
lazy.obj = obj
obj = lazy.obj
return obj
// stub implementations so *lazyObject implements Object and we can
// store them directly into Scope.elems.
func (*lazyObject) Parent() *Scope { panic("unreachable") }
func (*lazyObject) Pos() token.Pos { panic("unreachable") }
func (*lazyObject) Pkg() *Package { panic("unreachable") }
func (*lazyObject) Name() string { panic("unreachable") }
func (*lazyObject) Type() Type { panic("unreachable") }
func (*lazyObject) Exported() bool { panic("unreachable") }
func (*lazyObject) Id() string { panic("unreachable") }
func (*lazyObject) String() string { panic("unreachable") }
func (*lazyObject) order() uint32 { panic("unreachable") }
func (*lazyObject) color() color { panic("unreachable") }
func (*lazyObject) setType(Type) { panic("unreachable") }
func (*lazyObject) setOrder(uint32) { panic("unreachable") }
func (*lazyObject) setColor(color color) { panic("unreachable") }
func (*lazyObject) setParent(*Scope) { panic("unreachable") }
func (*lazyObject) sameId(pkg *Package, name string) bool { panic("unreachable") }
func (*lazyObject) scopePos() token.Pos { panic("unreachable") }
func (*lazyObject) setScopePos(pos token.Pos) { panic("unreachable") }