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// Copyright 2012 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.
// Package types declares the data types and implements
// the algorithms for type-checking of Go packages. Use
// Config.Check to invoke the type checker for a package.
// Alternatively, create a new type checked with NewChecker
// and invoke it incrementally by calling Checker.Files.
// Type-checking consists of several interdependent phases:
// Name resolution maps each identifier (ast.Ident) in the program to the
// language object (Object) it denotes.
// Use Info.{Defs,Uses,Implicits} for the results of name resolution.
// Constant folding computes the exact constant value (constant.Value)
// for every expression (ast.Expr) that is a compile-time constant.
// Use Info.Types[expr].Value for the results of constant folding.
// Type inference computes the type (Type) of every expression (ast.Expr)
// and checks for compliance with the language specification.
// Use Info.Types[expr].Type for the results of type inference.
package types // import "go/types"
import (
// An Error describes a type-checking error; it implements the error interface.
// A "soft" error is an error that still permits a valid interpretation of a
// package (such as "unused variable"); "hard" errors may lead to unpredictable
// behavior if ignored.
type Error struct {
Fset *token.FileSet // file set for interpretation of Pos
Pos token.Pos // error position
Msg string // error message
Soft bool // if set, error is "soft"
// Error returns an error string formatted as follows:
// filename:line:column: message
func (err Error) Error() string {
return fmt.Sprintf("%s: %s", err.Fset.Position(err.Pos), err.Msg)
// An importer resolves import paths to Packages.
// See go/importer for existing implementations.
type Importer interface {
// Import returns the imported package for the given import
// path, or an error if the package couldn't be imported.
// Import is responsible for returning the same package for
// matching import paths.
Import(path string) (*Package, error)
// A Config specifies the configuration for type checking.
// The zero value for Config is a ready-to-use default configuration.
type Config struct {
// If IgnoreFuncBodies is set, function bodies are not
// type-checked.
IgnoreFuncBodies bool
// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
// declares an empty "C" package and errors are omitted for qualified
// identifiers referring to package C (which won't find an object).
// This feature is intended for the standard library cmd/api tool.
// Caution: Effects may be unpredictable due to follow-up errors.
// Do not use casually!
FakeImportC bool
// If Error != nil, it is called with each error found
// during type checking; err has dynamic type Error.
// Secondary errors (for instance, to enumerate all types
// involved in an invalid recursive type declaration) have
// error strings that start with a '\t' character.
// If Error == nil, type-checking stops with the first
// error found.
Error func(err error)
// Importer is called for each import declaration except when
// importing package "unsafe". An error is reported if an
// importer is needed but none was installed.
Importer Importer
// If Sizes != nil, it provides the sizing functions for package unsafe.
// Otherwise &StdSizes{WordSize: 8, MaxAlign: 8} is used instead.
Sizes Sizes
// If DisableUnusedImportCheck is set, packages are not checked
// for unused imports.
DisableUnusedImportCheck bool
// Info holds result type information for a type-checked package.
// Only the information for which a map is provided is collected.
// If the package has type errors, the collected information may
// be incomplete.
type Info struct {
// Types maps expressions to their types, and for constant
// expressions, their values. Invalid expressions are omitted.
// For (possibly parenthesized) identifiers denoting built-in
// functions, the recorded signatures are call-site specific:
// if the call result is not a constant, the recorded type is
// an argument-specific signature. Otherwise, the recorded type
// is invalid.
// Identifiers on the lhs of declarations (i.e., the identifiers
// which are being declared) are collected in the Defs map.
// Identifiers denoting packages are collected in the Uses maps.
Types map[ast.Expr]TypeAndValue
// Defs maps identifiers to the objects they define (including
// package names, dots "." of dot-imports, and blank "_" identifiers).
// For identifiers that do not denote objects (e.g., the package name
// in package clauses, or symbolic variables t in t := x.(type) of
// type switch headers), the corresponding objects are nil.
// For an anonymous field, Defs returns the field *Var it defines.
// Invariant: Defs[id] == nil || Defs[id].Pos() == id.Pos()
Defs map[*ast.Ident]Object
// Uses maps identifiers to the objects they denote.
// For an anonymous field, Uses returns the *TypeName it denotes.
// Invariant: Uses[id].Pos() != id.Pos()
Uses map[*ast.Ident]Object
// Implicits maps nodes to their implicitly declared objects, if any.
// The following node and object types may appear:
// node declared object
// *ast.ImportSpec *PkgName for dot-imports and imports without renames
// *ast.CaseClause type-specific *Var for each type switch case clause (incl. default)
// *ast.Field anonymous struct field or parameter *Var
Implicits map[ast.Node]Object
// Selections maps selector expressions (excluding qualified identifiers)
// to their corresponding selections.
Selections map[*ast.SelectorExpr]*Selection
// Scopes maps ast.Nodes to the scopes they define. Package scopes are not
// associated with a specific node but with all files belonging to a package.
// Thus, the package scope can be found in the type-checked Package object.
// Scopes nest, with the Universe scope being the outermost scope, enclosing
// the package scope, which contains (one or more) files scopes, which enclose
// function scopes which in turn enclose statement and function literal scopes.
// Note that even though package-level functions are declared in the package
// scope, the function scopes are embedded in the file scope of the file
// containing the function declaration.
// The following node types may appear in Scopes:
// *ast.File
// *ast.FuncType
// *ast.BlockStmt
// *ast.IfStmt
// *ast.SwitchStmt
// *ast.TypeSwitchStmt
// *ast.CaseClause
// *ast.CommClause
// *ast.ForStmt
// *ast.RangeStmt
Scopes map[ast.Node]*Scope
// InitOrder is the list of package-level initializers in the order in which
// they must be executed. Initializers referring to variables related by an
// initialization dependency appear in topological order, the others appear
// in source order. Variables without an initialization expression do not
// appear in this list.
InitOrder []*Initializer
// TypeOf returns the type of expression e, or nil if not found.
// Precondition: the Types, Uses and Defs maps are populated.
func (info *Info) TypeOf(e ast.Expr) Type {
if t, ok := info.Types[e]; ok {
return t.Type
if id, _ := e.(*ast.Ident); id != nil {
if obj := info.ObjectOf(id); obj != nil {
return obj.Type()
return nil
// ObjectOf returns the object denoted by the specified id,
// or nil if not found.
// If id is an anonymous struct field, ObjectOf returns the field (*Var)
// it uses, not the type (*TypeName) it defines.
// Precondition: the Uses and Defs maps are populated.
func (info *Info) ObjectOf(id *ast.Ident) Object {
if obj, _ := info.Defs[id]; obj != nil {
return obj
return info.Uses[id]
// TypeAndValue reports the type and value (for constants)
// of the corresponding expression.
type TypeAndValue struct {
mode operandMode
Type Type
Value constant.Value
// TODO(gri) Consider eliminating the IsVoid predicate. Instead, report
// "void" values as regular values but with the empty tuple type.
// IsVoid reports whether the corresponding expression
// is a function call without results.
func (tv TypeAndValue) IsVoid() bool {
return tv.mode == novalue
// IsType reports whether the corresponding expression specifies a type.
func (tv TypeAndValue) IsType() bool {
return tv.mode == typexpr
// IsBuiltin reports whether the corresponding expression denotes
// a (possibly parenthesized) built-in function.
func (tv TypeAndValue) IsBuiltin() bool {
return tv.mode == builtin
// IsValue reports whether the corresponding expression is a value.
// Builtins are not considered values. Constant values have a non-
// nil Value.
func (tv TypeAndValue) IsValue() bool {
switch tv.mode {
case constant_, variable, mapindex, value, commaok:
return true
return false
// IsNil reports whether the corresponding expression denotes the
// predeclared value nil.
func (tv TypeAndValue) IsNil() bool {
return tv.mode == value && tv.Type == Typ[UntypedNil]
// Addressable reports whether the corresponding expression
// is addressable (
func (tv TypeAndValue) Addressable() bool {
return tv.mode == variable
// Assignable reports whether the corresponding expression
// is assignable to (provided a value of the right type).
func (tv TypeAndValue) Assignable() bool {
return tv.mode == variable || tv.mode == mapindex
// HasOk reports whether the corresponding expression may be
// used on the lhs of a comma-ok assignment.
func (tv TypeAndValue) HasOk() bool {
return tv.mode == commaok || tv.mode == mapindex
// An Initializer describes a package-level variable, or a list of variables in case
// of a multi-valued initialization expression, and the corresponding initialization
// expression.
type Initializer struct {
Lhs []*Var // var Lhs = Rhs
Rhs ast.Expr
func (init *Initializer) String() string {
var buf bytes.Buffer
for i, lhs := range init.Lhs {
if i > 0 {
buf.WriteString(", ")
buf.WriteString(" = ")
WriteExpr(&buf, init.Rhs)
return buf.String()
// Check type-checks a package and returns the resulting package object,
// the first error if any, and if info != nil, additional type information.
// The package is marked as complete if no errors occurred, otherwise it is
// incomplete. See Config.Error for controlling behavior in the presence of
// errors.
// The package is specified by a list of *ast.Files and corresponding
// file set, and the package path the package is identified with.
// The clean path must not be empty or dot (".").
func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, info *Info) (*Package, error) {
pkg := NewPackage(path, "")
return pkg, NewChecker(conf, fset, pkg, info).Files(files)
// AssertableTo reports whether a value of type V can be asserted to have type T.
func AssertableTo(V *Interface, T Type) bool {
m, _ := assertableTo(V, T)
return m == nil
// AssignableTo reports whether a value of type V is assignable to a variable of type T.
func AssignableTo(V, T Type) bool {
x := operand{mode: value, typ: V}
return x.assignableTo(nil, T) // config not needed for non-constant x
// ConvertibleTo reports whether a value of type V is convertible to a value of type T.
func ConvertibleTo(V, T Type) bool {
x := operand{mode: value, typ: V}
return x.convertibleTo(nil, T) // config not needed for non-constant x
// Implements reports whether type V implements interface T.
func Implements(V Type, T *Interface) bool {
f, _ := MissingMethod(V, T, true)
return f == nil