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Source file src/go/types/api.go

Documentation: go/types

  // 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 checker 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.
  // For a tutorial, see https://golang.org/s/types-tutorial.
  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.
  // CAUTION: This interface does not support the import of locally
  // vendored packages. See https://golang.org/s/go15vendor.
  // If possible, external implementations should implement ImporterFrom.
  type Importer interface {
  	// Import returns the imported package for the given import path.
  	// The semantics is like for ImporterFrom.ImportFrom except that
  	// dir and mode are ignored (since they are not present).
  	Import(path string) (*Package, error)
  // ImportMode is reserved for future use.
  type ImportMode int
  // An ImporterFrom resolves import paths to packages; it
  // supports vendoring per https://golang.org/s/go15vendor.
  // Use go/importer to obtain an ImporterFrom implementation.
  type ImporterFrom interface {
  	// Importer is present for backward-compatibility. Calling
  	// Import(path) is the same as calling ImportFrom(path, "", 0);
  	// i.e., locally vendored packages may not be found.
  	// The types package does not call Import if an ImporterFrom
  	// is present.
  	// ImportFrom returns the imported package for the given import
  	// path when imported by a package file located in dir.
  	// If the import failed, besides returning an error, ImportFrom
  	// is encouraged to cache and return a package anyway, if one
  	// was created. This will reduce package inconsistencies and
  	// follow-on type checker errors due to the missing package.
  	// The mode value must be 0; it is reserved for future use.
  	// Two calls to ImportFrom with the same path and dir must
  	// return the same package.
  	ImportFrom(path, dir string, mode ImportMode) (*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-on 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)
  	// An importer is used to import packages referred to from
  	// import declarations.
  	// If the installed importer implements ImporterFrom, the type
  	// checker calls ImportFrom instead of Import.
  	// The type checker reports an error if an importer is needed
  	// but none was installed.
  	Importer Importer
  	// If Sizes != nil, it provides the sizing functions for package unsafe.
  	// Otherwise SizesFor("gc", "amd64") 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, also 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.
  	// The Types map does not record the type of every identifier,
  	// only those that appear where an arbitrary expression is
  	// permitted. For instance, the identifier f in a selector
  	// expression x.f is found only in the Selections map, the
  	// identifier z in a variable declaration 'var z int' is found
  	// only in the Defs map, and identifiers denoting packages in
  	// qualified identifiers are collected in the Uses map.
  	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 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 (https://golang.org/ref/spec#Address_operators).
  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 and
  // the first error if any. Additionally, if info != nil, Check populates each
  // of the non-nil maps in the Info struct.
  // 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, nil) // 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

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