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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 (
// TODO(gri) Provide scopes with a name or other mechanism so that
// objects can use that information for better printing.
// 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
elems map[string]Object // lazily allocated
pos, end token.Pos // scope extent; may be invalid
comment string // for debugging 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, nil, pos, end, comment}
// don't add children to Universe scope!
if parent != nil && parent != Universe {
parent.children = append(parent.children, s)
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 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, below). 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.elems[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.elems[name]; alt != nil {
return alt
if s.elems == nil {
s.elems = make(map[string]Object)
s.elems[name] = obj
if obj.Parent() == nil {
return 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 returns true if 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 {", indn, s.comment, s)
if len(s.elems) == 0 {
fmt.Fprintf(w, "}\n")
indn1 := indn + ind
for _, name := range s.Names() {
fmt.Fprintf(w, "%s%s\n", indn1, s.elems[name])
if recurse {
for _, s := range s.children {
s.WriteTo(w, n+1, recurse)
fmt.Fprintf(w, "%s}", 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()