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go/types: The Go Type Checker

This document is maintained by Alan Donovan

October 2015 GothamGo talk on go/types


  1. Introduction
  2. An Example
  3. Objects
  4. Identifier Resolution
  5. Scopes
  6. Initialization Order
  7. Types
    1. Basic types
    2. Simple Composite Types
    3. Struct Types
    4. Tuple Types
    5. Function and Method Types
    6. Named Types
    7. Interface Types
    8. TypeAndValue
  8. Selections
  9. Ids
  10. Method Sets
  11. Constants
  12. Size and Alignment
  13. Imports
  14. Formatting support
  15. Getting from A to B


The go/types package is a type-checker for Go programs, designed by Robert Griesemer. It became part of Go's standard library in Go 1.5. Measured by lines of code and by API surface area, it is one of the most complex packages in Go's standard library, and using it requires a firm grasp of the structure of Go programs. This tutorial will help you find your bearings. It comes with several example programs that you can obtain with go get and play with. We assume you are a proficient Go programmer who wants to build tools to analyze or manipulate Go programs and that you have some knowledge of how a typical compiler works.

The type checker complements several existing standard packages for analyzing Go programs. We've listed them below.

→   go/types

Starting at the bottom, the go/token package defines the lexical tokens of Go. The go/scanner package tokenizes an input stream and records file position information for use in diagnostics or for file surgery in a refactoring tool. The go/ast package defines the data types of the abstract syntax tree (AST). The go/parser package provides a robust recursive-descent parser that constructs the AST. And go/constant provides representations and arithmetic operations for the values of compile-time constant expressions, as we'll see in Constants.

The package from the x/tools repository is a client of the type checker that loads, parses, and type-checks a complete Go program from source code. We use it in some of our examples and you may find it useful too.

The Go type checker does three main things. First, for every name in the program, it determines which declaration the name refers to; this is known as identifier resolution. Second, for every expression in the program, it determines what type that expression has, or reports an error if the expression has no type, or has an inappropriate type for its context; this is known as type deduction. Third, for every constant expression in the program, it determines the value of that constant; this is known as constant evaluation.

Superficially, it appears that these three processes could be done sequentially, in the order above, but perhaps surprisingly, they must be done together. For example, the value of a constant may depend on the type of an expression due to operators like unsafe.Sizeof. Conversely, the type of an expression may depend on the value of a constant, since array types contain constants. As a result, type deduction and constant evaluation must be done together.

As another example, we cannot resolve the identifier k in the composite literal T{k: 0} until we know whether T is a struct type. If it is, then k must be found among T's fields. If not, then k is an ordinary reference to a constant or variable in the lexical environment. Consequently, identifier resolution and type deduction are also inseparable in the general case.

Nonetheless, the three processes of identifier resolution, type deduction, and constant evaluation can be separated for the purpose of explanation.

An Example

The code below shows the most basic use of the type checker to check the hello, world program, supplied as a string. Later examples will be variations on this one, and we'll often omit boilerplate details such as parsing. To check out and build the examples, run go get

// go get
package main

import (

const hello = `package main

import "fmt"

func main() {
        fmt.Println("Hello, world")

func main() {
	fset := token.NewFileSet()

	// Parse the input string, []byte, or io.Reader,
	// recording position information in fset.
	// ParseFile returns an *ast.File, a syntax tree.
	f, err := parser.ParseFile(fset, "hello.go", hello, 0)
	if err != nil {
		log.Fatal(err) // parse error

	// A Config controls various options of the type checker.
	// The defaults work fine except for one setting:
	// we must specify how to deal with imports.
	conf := types.Config{Importer: importer.Default()}

	// Type-check the package containing only file f.
	// Check returns a *types.Package.
	pkg, err := conf.Check("cmd/hello", fset, []*ast.File{f}, nil)
	if err != nil {
		log.Fatal(err) // type error

	fmt.Printf("Package  %q\n", pkg.Path())
	fmt.Printf("Name:    %s\n", pkg.Name())
	fmt.Printf("Imports: %s\n", pkg.Imports())
	fmt.Printf("Scope:   %s\n", pkg.Scope())

First, the program creates a token.FileSet. To avoid the need to store file names and line and column numbers in every node of the syntax tree, the go/token package provides FileSet, a data structure that stores this information compactly for a sequence of files. A FileSet records each file name only once, and records only the byte offsets of each newline, allowing a position within any file to be identified using a small integer called a token.Pos. Many tools create a single FileSet at startup. Any part of the program that needs to convert a token.Pos into an intelligible location---as part of an error message, for instance---must have access to the FileSet.

Second, the program parses the input string. More realistic packages contain several source files, so the parsing step must be repeated for each one, or better, done in parallel. Third, it creates a Config that specifies type-checking options. Since the hello, world program uses imports, we must indicate how to locate the imported packages. Here we use importer.Default(), which loads compiler-generated export data, but we'll explore alternatives in Imports.

Fourth, the program calls Check. This creates a Package whose path is "cmd/hello", and type-checks each of the specified files---just one in this example. The final (nil) argument is a pointer to an optional Info struct that returns additional deductions from the type checker; more on that later. Check returns a Package even when it also returns an error. The type checker is robust to ill-formed input, and goes to great lengths to report accurate partial information even in the vicinity of syntax or type errors. Package has this definition:

type Package struct{ ... }
func (*Package) Path() string
func (*Package) Name() string
func (*Package) Scope() *Scope
func (*Package) Imports() []*Package

Finally, the program prints the attributes of the package, shown below. (The hexadecimal number may vary from one run to the next.)

$ go build
$ ./pkginfo
Package  "cmd/hello"
Name:    main
Imports: [package fmt ("fmt")]
Scope:   package "cmd/hello" scope 0x820533590 {
.  func cmd/hello.main()

A package's Path, such as "encoding/json", is the string by which import declarations identify it. It is unique within a $GOPATH workspace, and for published packages it must be globally unique.

A package's Name is the identifier in the package declaration of each source file within the package, such as json. The type checker reports an error if not all the package declarations in the package agree. The package name determines how the package is known when it is imported into a file (unless a renaming import is used), but is otherwise not visible to a program.

Scope returns the package's lexical block, which provides access to all the named entities or objects declared at package level. Imports returns the set of packages directly imported by this one, and may be useful for computing dependencies (Initialization Order).


The task of identifier resolution is to map every identifier in the syntax tree, that is, every ast.Ident, to an object. For our purposes, an object is a named entity created by a declaration, such as a var, type, or func declaration. (This is different from the everyday meaning of object in object-oriented programming.)

Objects are represented by the Object interface:

type Object interface {
    Name() string   // package-local object name
    Exported() bool // reports whether the name starts with a capital letter
    Type() Type     // object type
    Pos() token.Pos // position of object identifier in declaration

    Parent() *Scope // scope in which this object is declared
    Pkg() *Package  // nil for objects in the Universe scope and labels
    Id() string     // object id (see Ids section below)

The first four methods are straightforward; we'll explain the other three later. Name returns the object's name---an identifier. Exported is a convenience method that reports whether the first letter of Name is a capital, indicating that the object may be visible from outside the package. It's a shorthand for ast.IsExported(obj.Name()). Type returns the object's type; we'll come back to that in Types.

Pos returns the source position of the object's declaring identifier. To make sense of a token.Pos, we need to call the (*token.FileSet).Position method, which returns a struct with individual fields for the file name, line number, column, and byte offset, though usually we just call its String method:

fmt.Println(fset.Position(obj.Pos())) // "hello.go:10:6"

Not all objects carry position information. Since the file format for compiler export data (Imports) does not record position information, calling Pos on an object imported from such a file returns zero, also known as token.NoPos.

There are eight kinds of objects in the Go type checker. Most familiar are the kinds that can be declared at package level: constants, variables, functions, and types. Less familiar are statement labels, imported package names (such as json in a file containing an import "encoding/json" declaration), built-in functions (such as append and len), and the pre-declared nil. The eight types shown below are the only concrete types that satisfy the Object interface. In other words, Object is a discriminated union of 8 possible types, and we commonly use a type switch to distinguish them.

Object = *Func         // function, concrete method, or abstract method
       | *Var          // variable, parameter, result, or struct field
       | *Const        // constant
       | *TypeName     // type name
       | *Label        // statement label
       | *PkgName      // package name, e.g. json after import "encoding/json"
       | *Builtin      // predeclared function such as append or len
       | *Nil          // predeclared nil

Objects are canonical. That is, two Objects x and y denote the same entity if and only if x==y. Object identity is significant, and objects are routinely compared by the addresses of the underlying pointers. Although a package-level object is uniquely identified by its name and enclosing package, for other objects there is no simple way to obtain a string that uniquely identifies it.

The Parent method returns the Scope (lexical block) in which the object was declared; we'll come back to this in Scopes. Fields and methods are not found in the lexical environment, so their objects have no Parent.

The Pkg method returns the Package to which this object belongs, even for objects not declared at package level. Only predeclared objects have no package. The Id method will be explained in Ids.

Not all methods make sense for each kind of object. For instance, the last four kinds above have no meaningful Type method. And some kinds of objects have methods in addition to those required by the Object interface:

func (*Func) Scope() *Scope
func (*Var) Anonymous() bool
func (*Var) IsField() bool
func (*Const) Val() constant.Value
func (*TypeName) IsAlias() bool // expected in Go 1.9
func (*PkgName) Imported() *Package

(*Func).Scope returns the lexical block containing the function's parameters, results, and other local declarations. (*Var).IsField distinguishes struct fields from ordinary variables, and (*Var).Anonymous discriminates named fields like the one in struct{T T} from anonymous fields like the one in struct{T}. (*Const).Val returns the value of a named constant.

(*TypeName).IsAlias, to be introduced in Go 1.9, reports whether the type name is simply an alias for a type (as in type I = int), as opposed to a definition of a Named type, as in type Celsius float64.

(*PkgName).Imported returns the package (for instance, encoding/json) denoted by a given import name such as json. Each time a package is imported, a new PkgName object is created, usually with the same name as the Package it denotes, but not always, as in the case of a renaming import. PkgNames are objects, but Packages are not. We'll look more closely at this in Imports.

All relationships between the syntax trees (ast.Nodes) and type checker data structures such as Objects and Types are stored in mappings outside the syntax tree itself. Be aware that the go/ast package also defines a type called Object that resembles---and predates---the type checker's Object, and that ast.Objects are held directly by identifiers in the AST. They are created by the parser, which has a necessarily limited view of the package, so the information they represent is at best partial and in some cases wrong, as in the T{k: 0} example mentioned above. If you are using the type checker, there is no reason to use the older ast.Object mechanism.

Identifier Resolution

Identifier resolution computes the relationship between identifiers and objects. Its results are recorded in the Info struct optionally passed to Check. The fields related to identifier resolution are shown below.

type Info struct {
	Defs       map[*ast.Ident]Object
	Uses       map[*ast.Ident]Object
	Implicits  map[ast.Node]Object
	Selections map[*ast.SelectorExpr]*Selection
	Scopes     map[ast.Node]*Scope

Since not all facts computed by the type checker are needed by every client, the API lets clients control which components of the result should be recorded and which discarded: only fields that hold a non-nil map will be populated during the call to Check.

The two fields of type map[*ast.Ident]Object are the most important: Defs records declaring identifiers and Uses records referring identifiers. In the example below, the comments indicate which identifiers are of which kind.

var x int        // def of x, use of int
fmt.Println(x)   // uses of fmt, Println, and x
type T struct{U} // def of T, use of U (type), def of U (field)

The final line above illustrates why we don't combine Defs and Uses into one map. In the anonymous field declaration struct{U}, the identifier U is both a use of the type U (a TypeName) and a definition of the anonymous field (a Var).

The function below prints the location of each referring and defining identifier in the input program, and the object it refers to.

// go get
func PrintDefsUses(fset *token.FileSet, files ...*ast.File) error {
	conf := types.Config{Importer: importer.Default()}
	info := &types.Info{
		Defs: make(map[*ast.Ident]types.Object),
		Uses: make(map[*ast.Ident]types.Object),
	_, err := conf.Check("hello", fset, files, info)
	if err != nil {
		return err // type error

	for id, obj := range info.Defs {
		fmt.Printf("%s: %q defines %v\n",
			fset.Position(id.Pos()), id.Name, obj)
	for id, obj := range info.Uses {
		fmt.Printf("%s: %q uses %v\n",
			fset.Position(id.Pos()), id.Name, obj)
	return nil

Let's use the hello, world program again as the input:

// go get
package main

import "fmt"

func main() {
	fmt.Println("Hello, 世界")

This is what it prints:

$ go build
$ ./defsuses
hello.go:1:9: "main" defines <nil>
hello.go:5:6: "main" defines func hello.main()
hello.go:6:9: "fmt" uses package fmt
hello.go:6:13: "Println" uses func fmt.Println(a ...interface{}) (n int, err error)

Notice that the Defs mapping may contain nil entries in a few cases. The first line of output reports that the package identifier main is present in the Defs mapping, but has no associated object.

The Implicits mapping handles two cases of the syntax in which an Object is declared without an ast.Ident, namely type switches and import declarations.

In the type switch below, which declares a local variable y, the type of y is different in each case of the switch:

switch y := x.(type) {
case int:
	fmt.Printf("%d", y)
case string:
	fmt.Printf("%q", y)

To represent this, for each single-type case, the type checker creates a separate Var object for y with the appropriate type, and Implicits maps each ast.CaseClause to the Var for that case. The default case, the nil case, and cases with more than one type all use the regular Var object that is associated with the identifier y, which is found in the Defs mapping.

The import declaration below defines the name json without an ast.Ident:

import "encoding/json"

Implicits maps this ast.ImportSpec to the PkgName object named json that it implicitly declares.

The Selections mapping, of type map[*ast.SelectorExpr]*Selection, records the meaning of each expression of the form expr.f, where expr is an expression or type and f is the name of a field or method. These expressions, called selections, are represented by ast.SelectorExpr nodes in the AST. We'll talk more about the Selection type in Selections.

Not all ast.SelectorExpr nodes represent selections. Expressions like fmt.Println, in which a package name precedes the dot, are qualified identifiers. They do not appear in the Selections mapping, but their constituent identifiers (such as fmt and Println) both appear in Uses.

Referring identifiers that are not part of an ast.SelectorExpr are lexical references. That is, they are resolved to an object by searching for the innermost enclosing lexical declaration of that name. We'll see how that search works in the next section.


The Scope type is a mapping from names to objects.

type Scope struct{ ... }

func (s *Scope) Names() []string
func (s *Scope) Lookup(name string) Object

Names returns the set of names in the mapping, in sorted order. (It is not a simple accessor though, so call it sparingly.) The Lookup method returns the object for a given name, so we can print all the entries or bindings in a scope like this:

for _, name := range scope.Names() {

The scope of a declaration of a name is the region of program source in which a reference to the name resolves to that declaration. That is, scope is a property of a declaration. However, in the go/types API, the Scope type represents a lexical block, which is one component of the lexical environment. Consider the hello, world program again:

package main

import "fmt"

func main() {
	const message = "hello, world"

There are four lexical blocks in this program. The outermost one is the universe block, which maps the pre-declared names like int, true, and append to their objects---a TypeName, a Const, and a Builtin, respectively. The universe block is represented by the global variable Universe, of type *Scope, although it's logically a constant so you shouldn't modify it.

Next is the package block, which maps "main" to the main function. Following that is the file block, which maps "fmt" to the PkgName object for this import of the fmt package. And finally, the innermost block is that of function main, a local block, which contains the declaration of message, a Const. The main function is trivial, but many functions contain several blocks since each if, for, switch, case, or select statement creates at least one additional block. Local blocks nest to arbitrary depths.

The structure of the lexical environment thus forms a tree, with the universe block at the root, the package blocks beneath it, the file blocks beneath them, and then any number of local blocks beneath the files. We can access and navigate this tree structure with the following methods of Scope:

func (s *Scope) Parent() *Scope
func (s *Scope) NumChildren() int
func (s *Scope) Child(i int) *Scope

Parent lets us walk up the tree, and Child lets us walk down it. Note that although the Parent of every package Scope is Universe, Universe has no children. This asymmetry is a consequence of using a global variable to hold Universe.

To obtain the universe block, we use the Universe global variable. To obtain the lexical block of a Package, we call its Scope method. To obtain the scope of a file (*ast.File), or any smaller piece of syntax such as an *ast.IfStmt, we consult the Scopes mapping in the Info struct, which maps each block-creating syntax node to its block. The lexical block of a named function or method can also be obtained by calling its (*Func).Scope method.

To look up a name in the lexical environment, we must search the tree of lexical blocks, starting at a particular Scope and walking up to the root until a declaration of the name is found. For convenience, the LookupParent method does this, returning not just the object, if found, but also the Scope in which it was declared, which may be an ancestor of the initial one:

func (s *Scope) LookupParent(name string, pos token.Pos) (*Scope, Object)

The pos parameter determines the position in the source code at which the name should be resolved. The effective lexical environment is different at each point in the block because it depends on which local declarations appear before or after that point. (We'll see an illustration in a moment.)

Scope has several other methods relating to source positions:

func (s *Scope) Pos() token.Pos
func (s *Scope) End() token.Pos
func (s *Scope) Contains(pos token.Pos) bool
func (s *Scope) Innermost(pos token.Pos) *Scope

Pos and End report the Scope's start and end position which, for explicit blocks, coincide with its curly braces. Contains is a convenience method that reports whether a position lies in this interval. Innermost returns the innermost scope containing the specified position, which may be a child or other descendent of the initial scope.

These features are useful for tools that wish to resolve names or evaluate constant expressions as if they had appeared at a particular point within the program. The next example program finds all the comments in the input, treating the contents of each one as a name. It looks up each name in the environment at the position of the comment, and prints what it finds. Observe that the ParseComments flag directs the parser to preserve comments in the input.

// go get
func main() {
	fset := token.NewFileSet()
	f, err := parser.ParseFile(fset, "hello.go", hello, parser.ParseComments)
	if err != nil {
		log.Fatal(err) // parse error

	conf := types.Config{Importer: importer.Default()}
	pkg, err := conf.Check("cmd/hello", fset, []*ast.File{f}, nil)
	if err != nil {
		log.Fatal(err) // type error

	// Each comment contains a name.
	// Look up that name in the innermost scope enclosing the comment.
	for _, comment := range f.Comments {
		pos := comment.Pos()
		name := strings.TrimSpace(comment.Text())
		fmt.Printf("At %s,\t%q = ", fset.Position(pos), name)
		inner := pkg.Scope().Innermost(pos)
		if _, obj := inner.LookupParent(name, pos); obj != nil {
		} else {
			fmt.Println("not found")

The expression pkg.Scope().Innermost(pos) finds the innermost Scope that encloses the comment, and LookupParent(name, pos) does a name lookup at a specific position in that lexical block.

A typical input is shown below. The first comment causes a lookup of "append" in the file block. The second comment looks up "fmt" in the main function's block, and so on.

const hello = `
package main

import "fmt"

// append
func main() {
        // fmt
        fmt.Println("Hello, world")
        // main
        main, x := 1, 2
        // main
        print(main, x)
        // x
// x

Here's the output:

$ go build
$ ./lookup
At hello.go:6:1,        "append" = builtin append
At hello.go:8:9,        "fmt" = package fmt
At hello.go:10:9,       "main" = func cmd/hello.main()
At hello.go:12:9,       "main" = var main int
At hello.go:14:9,       "x" = var x int
At hello.go:16:1,       "x" = not found

Notice how the two lookups of main return different results, even though they occur in the same block, because one precedes the declaration of the local variable named main and the other follows it. Also notice that there are two lookups of the name x but only the first one, in the function block, succeeds.

Download the program and modify both the input program and the set of comments to get a better feel for how name resolution works.

The table below summarizes which kinds of objects may be declared at each level of the tree of lexical blocks.

            Universe File Package Local
Builtin        ✔
Nil            ✔
Const          ✔            ✔      ✔
TypeName       ✔            ✔      ✔
Func                        ✔
Var                         ✔      ✔
PkgName               ✔
Label                              ✔

Initialization Order

In the course of identifier resolution, the type checker constructs a graph of references among declarations of package-level variables and functions. The type checker reports an error if the initializer expression for a variable refers to that variable, whether directly or indirectly.

The reference graph determines the initialization order of the package-level variables, as required by the Go spec, using a breadth-first algorithm. First, variables in the graph with no successors are removed, sorted into the order in which they appear in the source code, then added to a list. This creates more variables that have no successors. The process repeats until they have all been removed.

The result is available in the InitOrder field of the Info struct, whose type is []Initializer.

type Info struct {
	InitOrder []Initializer

type Initializer struct {
	Lhs []*Var // var Lhs = Rhs
	Rhs ast.Expr

Each element of the list represents a single initializer expression that must be executed, and the variables to which it is assigned. The variables may number zero, one, or more, as in these examples:

var _ io.Writer = new(bytes.Buffer)
var rx = regexp.MustCompile("^b(an)*a$")
var cwd, cwdErr = os.Getwd()

This process governs the initialization order of variables within a package. Across packages, dependencies must be initialized first, although the order among them is not specified. That is, any topological order of the import graph will do. The (*Package).Imports method returns the set of direct dependencies of a package.


The main job of the type checker is, of course, to deduce the type of each expression and to report type errors. Like Object, Type is an interface type used as a discriminated union of several concrete types but, unlike Object, Type has very few methods because types have little in common with each other. Here is the interface:

type Type interface {
	Underlying() Type

And here are the eleven concrete types that satisfy it:

Type = *Basic
     | *Pointer
     | *Array
     | *Slice
     | *Map
     | *Chan
     | *Struct
     | *Tuple
     | *Signature
     | *Named
     | *Interface

With the exception of Named types, instances of Type are not canonical. That is, it is usually a mistake to compare types using t1==t2 since this equivalence is not the same as the type identity relation defined by the Go spec. Use this function instead:

func Identical(t1, t2 Type) bool

For the same reason, you should not use a Type as a key in a map. The package provides a map keyed by types that uses the correct equivalence relation.

The Go spec defines three relations over types. Assignability governs which pairs of types may appear on the left- and right-hand side of an assignment, including implicit assignments such as function calls, map and channel operations, and so on. Comparability determines which types may appear in a comparison x==y or a switch case or may be used as a map key. Convertibility governs which pairs of types are allowed in a conversion operation T(v). You can query these relations with the following predicate functions:

func AssignableTo(V, T Type) bool
func Comparable(T Type) bool
func ConvertibleTo(V, T Type) bool

Let's take a look at each kind of type.

Basic types

Basic represents all types that are not composed from simpler types. This is essentially the set of underlying types that a constant expression is permitted to have--strings, booleans, and numbers---but it also includes unsafe.Pointer and untyped nil.

type Basic struct{...}
func (*Basic) Kind() BasicKind
func (*Basic) Name() string
func (*Basic) Info() BasicInfo

The Kind method returns an "enum" value that indicates which basic type this is. The kinds Bool, String, Int16, and so on, represent the corresponding predeclared boolean, string, or numeric types. There are two synonyms: Byte is equivalent to Uint8 and Rune is equivalent to Int32. The kind UnsafePointer represents unsafe.Pointer. The kinds UntypedBool, UntypedInt and so on represent the six kinds of "untyped" constant types: boolean, integer, rune, float, complex, and string. The kind UntypedNil represents the type of the predeclared nil value. And the kind Invalid indicates the invalid type, which is used for expressions containing errors, or for objects without types, like Label, Builtin, or PkgName.

The Name method returns the name of the type, such as "float64", and the Info method returns a bitfield that encodes information about the type, such as whether it is signed or unsigned, integer or floating point, or real or complex.

Typ is a table of canonical basic types, indexed by kind, so Typ[String] returns the *Basic that represents string, for instance. Like Universe, Typ is logically a constant, so don't modify it.

A few minor subtleties: According to the Go spec, pre-declared types such as int are named types for the purposes of assignability, even though the type checker does not represent them using Named. And unsafe.Pointer is a pointer type for the purpose of determining whether the receiver type of a method is legal, even though the type checker does not represent it using Pointer.

The "untyped" types are usually only ascribed to constant expressions, but there is one exception. A comparison x==y has type "untyped bool", so the result of this expression may be assigned to a variable of type bool or any other named boolean type.

Simple Composite Types

The types Pointer, Array, Slice, Map, and Chan are pretty self-explanatory. All have an Elem method that returns the element type T for a pointer *T, an array [n]T, a slice []T, a map map[K]T, or a channel chan T. This should feel familiar if you've used the reflect.Value API.

In addition, the *Map, *Chan, and *Array types have accessor methods that return their key type, direction, and length, respectively:

func (*Map) Key() Type
func (*Chan) Dir() ChanDir      // = Send | Recv | SendRecv
func (*Array) Len() int64

Struct Types

A struct type has an ordered list of fields and a corresponding ordered list of field tags.

type Struct struct{ ... } 
func (*Struct) NumFields() int
func (*Struct) Field(i int) *Var
func (*Struct) Tag(i int) string

Each field is a Var object whose IsField method returns true. Field objects have no Parent scope, because they are resolved through selections, not through the lexical environment.

Thanks to embedding, the expression new(S).f may be a shorthand for a longer expression such as new(S).d.e.f, but in the representation of Struct types, these field selection operations are explicit. That is, the set of fields of struct type S does not include f. An anonymous field is represented like a regular field, but its Anonymous method returns true.

One subtlety is relevant to tools that generate documentation. When analyzing a declaration such as this,

type T struct{x int}

it may be tempting to consider the Var object for field x as if it had the name "T.x", but beware: field objects do not have canonical names and there is no way to obtain the name "T" from the Var for x. That's because several types may have the same underlying struct type, as in this code:

type T struct{x int}
type U T

Here, the Var for field x belongs equally to T and to U, and short of inspecting source positions or walking the AST---neither of which is possible for objects loaded from compiler export data---it is not possible to ascertain that x was declared as part of T. The type checker builds the exact same data structures given this input:

type T U
type U struct{x int}

A similar issue applies to the methods of named interface types.

Tuple Types

Like a struct, a tuple type has an ordered list of fields, and fields may be named.

type Tuple struct{ ... }
func (*Tuple) Len() int
func (*Tuple) At(i int) *Var

Although tuples are not the type of any variable in Go, they are the type of some expressions, such as the right-hand sides of these assignments:

v, ok = m[key]
v, ok = <-ch
v, ok = x.(T)
f, err = os.Open(filename)

Tuples also represent the types of the parameter list and the result list of a function, as we will see.

Since empty tuples are common, the nil *Tuple pointer is a valid empty tuple.

Function and Method Types

The types of functions and methods are represented by a Signature, which has a tuple of parameter types and a tuple of result types.

type Signature struct{ ... }
func (*Signature) Recv() *Var
func (*Signature) Params() *Tuple
func (*Signature) Results() *Tuple
func (*Signature) Variadic() bool

Variadic functions such as fmt.Println have the Variadic flag set. The final parameter of such functions is always a slice, or in the special case of certain calls to append, a string.

A Signature for a method, whether concrete or abstract, has a non-nil receiver parameter, Recv. The type of the receiver is usually a named type or a pointer to a named type, but it may be an unnamed struct or interface type in some cases. Method types are rather second-class: they are only used for the Func objects created by method declarations, and no Go expression has a method type. When printing a method type, the receiver does not appear, and the Identical predicate ignores the receiver.

The types of Builtin objects like append cannot be expressed as a Signature since those types require parametric polymorphism. Builtin objects are thus ascribed the Invalid basic type. However, the type of each call to a built-in function has a specific and expressible Go type. These types are recorded during type checking for later use (TypeAndValue).

Named Types

Type declarations come in two forms. The simplest kind, to be introduced in Go 1.9, merely declares a (possibly alternative) name for an existing type. Type names used in this way are informally called type aliases. For example, this declaration lets you use the type Dictionary as an alias for map[string]string:

type Dictionary = map[string]string

The declaration creates a TypeName object for Dictionary. The object's IsAlias method returns true, and its Type method returns a Map type that represents map[string]string.

The second form of type declaration, and the only kind prior to Go 1.9, does not use an equals sign:

type Celsius float64

This declaration does more than just give a name to a type. It first defines a new Named type whose underlying type is float64; this Named type is different from any other type, including float64. The declaration binds the TypeName object to the Named type.

Since Go 1.9, the Go language specification has used the term defined types instead of named types; the essential property of a defined type is not that it has a name, but that it is a distinct type with its own method set. However, the type checker API predates that change and instead calls defined types "named" types.

type Named struct{ ... }
func (*Named) NumMethods() int
func (*Named) Method(i int) *Func
func (*Named) Obj() *TypeName
func (*Named) Underlying() Type

The Named type's Obj method returns the TypeName object, which provides the name, position, and other properties of the declaration. Conversely, the TypeName object's Type method returns the Named type.

A Named type may appear as the receiver type in a method declaration. Methods are associated with the Named type, not the name (the TypeName object); it's possible---though cryptic---to declare a method on a Named type using one of its aliases. The NumMethods and Method methods enumerate the declared methods associated with this Named type (or a pointer to it), in the order they were declared. However, due to the subtleties of anonymous fields and the difference between value and pointer receivers, a named type may have more or fewer methods than this list. We'll return to this in Method Sets.

Every Type has an Underlying method, but for all of them except *Named, it is simply the identity function. For a named type, Underlying returns its underlying type, which is always an unnamed type. Thus Underlying returns int for both T and U below.

type T int
type U T

Clients of the type checker often use type assertions or type switches with a Type operand. When doing so, it is often necessary to switch on the type that underlies the type of interest, and failure to do so may be a bug.

This is a common pattern:

// handle types of composite literal
switch u := t.Underlying().(type) {
case *Struct:        // ...
case *Map:           // ...
case *Array, *Slice: // ...

Interface Types

Interface types are represented by Interface.

type Interface struct{ ... }
func (*Interface) Empty() bool
func (*Interface) NumMethods() int
func (*Interface) Method(i int) *Func
func (*Interface) NumEmbeddeds() int
func (*Interface) Embedded(i int) *Named
func (*Interface) NumExplicitMethods() int
func (*Interface) ExplicitMethod(i int) *Func

Syntactically, an interface type has a list of explicitly declared methods (ExplicitMethod), and a list of embedded named interface types (Embedded), but many clients care only about the complete set of methods, which can be enumerated via Method. All three lists are ordered by name. Since the empty interface is an important special case, the Empty predicate provides a shorthand for NumMethods() == 0.

As with the fields of structs (see above), the methods of interfaces may belong equally to more than one interface type. The Func object for method f in the code below is shared by I and J:

type I interface { f() }
type J I

Because the difference between interface (abstract) and non-interface (concrete) types is so important in Go, the IsInterface predicate is provided for convenience.

func IsInterface(Type) bool

The type checker provides three utility methods relating to interface satisfaction:

func Implements(V Type, T *Interface) bool
func AssertableTo(V *Interface, T Type) bool
func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool)

The Implements predicate reports whether a type satisfies an interface type. MissingMethod is like Implements, but instead of returning false, it explains why a type does not satisfy the interface, for use in diagnostics.

AssertableTo reports whether a type assertion v.(T) is legal. If T is a concrete type that doesn't have all the methods of interface v, then the type assertion is not legal, as in this example:

// error: io.Writer is not assertible to int
func f(w io.Writer) int { return w.(int) } 


The type checker records the type of each expression in another field of the Info struct, namely Types:

type Info struct {
	Types map[ast.Expr]TypeAndValue

No entries are recorded for identifiers since the Defs and Uses maps provide more information about them. Also, no entries are recorded for pseudo-expressions like *ast.KeyValuePair or *ast.Ellipsis.

The value of the Types map is a TypeAndValue, which (unsurprisingly) holds the type and value of the expression, and in addition, its mode. The mode is opaque, but has predicates to answer questions such as: Does this expression denote a value or a type? Does this value have an address? Does this expression appear on the left-hand side of an assignment? Does this expression appear in a context that expects two results? The comments in the code below give examples of expressions that satisfy each predicate.

type TypeAndValue struct {
	Type  Type
	Value constant.Value // for constant expressions only

func (TypeAndValue) IsVoid() bool      // e.g. "main()"
func (TypeAndValue) IsType() bool      // e.g. "*os.File"
func (TypeAndValue) IsBuiltin() bool   // e.g. "len(x)"
func (TypeAndValue) IsValue() bool     // e.g. "*os.Stdout"
func (TypeAndValue) IsNil() bool       // e.g. "nil"
func (TypeAndValue) Addressable() bool // e.g. "a[i]" but not "f()", "m[key]"
func (TypeAndValue) Assignable() bool  // e.g. "a[i]", "m[key]"
func (TypeAndValue) HasOk() bool       // e.g. "<-ch", "m[key]"

The statement below inspects every expression within the AST of a single type-checked file and prints its type, value, and mode:

// go get
// f is a parsed, type-checked *ast.File.
ast.Inspect(f, func(n ast.Node) bool {
	if expr, ok := n.(ast.Expr); ok {
		if tv, ok := info.Types[expr]; ok {
			fmt.Printf("%-24s\tmode:  %s\n", nodeString(expr), mode(tv))
			fmt.Printf("\t\t\t\ttype:  %v\n", tv.Type)
			if tv.Value != nil {
				fmt.Printf("\t\t\t\tvalue: %v\n", tv.Value)
	return true

It makes use of these two helper functions, which are not shown:

// nodeString formats a syntax tree in the style of gofmt.
func nodeString(n ast.Node) string

// mode returns a string describing the mode of an expression.
func mode(tv types.TypeAndValue) string

Given this input:

const input = `
package main

var m = make(map[string]int)

func main() {
	v, ok := m["hello, " + "world"]
	print(rune(v), ok)

the program prints:

$ go build
$ ./typeandvalue
make(map[string]int)            mode:  value
                                type:  map[string]int
make                            mode:  builtin
                                type:  func(map[string]int) map[string]int
map[string]int                  mode:  type
                                type:  map[string]int
string                          mode:  type
                                type:  string
int                             mode:  type
                                type:  int
m["hello, "+"world"]            mode:  value,assignable,ok
                                type:  (int, bool)
m                               mode:  value,addressable,assignable
                                type:  map[string]int
"hello, " + "world"             mode:  value
                                type:  string
                                value: "hello, world"
"hello, "                       mode:  value
                                type:  untyped string
                                value: "hello, "
"world"                         mode:  value
                                type:  untyped string
                                value: "world"
print(rune(v), ok)              mode:  void
                                type:  ()
print                           mode:  builtin
                                type:  func(rune, bool)
rune(v)                         mode:  value
                                type:  rune
rune                            mode:  type
                                type:  rune
...more not shown...

Notice that the identifiers for the built-ins make and print have types that are specific to the particular calls in which they appear. Also notice m["hello"] has a 2-tuple type (int, bool) and that it is assignable, but unlike the variable m, it is not addressable.

Download the example and vary the inputs and see what the program prints.

Here's another example, adapted from the govet static checking tool. It checks for accidental uses of a method value x.f when a call x.f() was intended; comparing a method x.f against nil is a common mistake.

// go get
// CheckNilFuncComparison reports unintended comparisons
// of functions against nil, e.g., "if x.Method == nil {".
func CheckNilFuncComparison(info *types.Info, n ast.Node) {
	e, ok := n.(*ast.BinaryExpr)
	if !ok {
		return // not a binary operation
	if e.Op != token.EQL && e.Op != token.NEQ {
		return // not a comparison

	// If this is a comparison against nil, find the other operand.
	var other ast.Expr
	if info.Types[e.X].IsNil() {
		other = e.Y
	} else if info.Types[e.Y].IsNil() {
		other = e.X
	} else {
		return // not a comparison against nil

	// Find the object.
	var obj types.Object
	switch v := other.(type) {
	case *ast.Ident:
		obj = info.Uses[v]
	case *ast.SelectorExpr:
		obj = info.Uses[v.Sel]
		return // not an identifier or selection

	if _, ok := obj.(*types.Func); !ok {
		return // not a function or method

	fmt.Printf("%s: comparison of function %v %v nil is always %v\n",
		fset.Position(e.Pos()), obj.Name(), e.Op, e.Op == token.NEQ)

Given this input,

const input = `package main

import "bytes"

func main() {
	var buf bytes.Buffer
	if buf.Bytes == nil && bytes.Repeat != nil && main == nil {
		// ...

the program reports these errors:

$ go build
$ ./nilfunc
input.go:7:5: comparison of function Bytes == nil is always false
input.go:7:25: comparison of function Repeat != nil is always true
input.go:7:48: comparison of function main == nil is always false


A selection is an expression expr.f in which f denotes either a struct field or a method. A selection is resolved not by looking for a name in the lexical environment, but by looking within a type. The type checker ascertains the meaning of each selection in the package---a surprisingly tricky business---and records it in the Selections mapping of the Info struct, whose values are of type Selection:

type Selection struct{ ... }
func (s *Selection) Kind() SelectionKind // = FieldVal | MethodVal | MethodExpr
func (s *Selection) Recv() Type
func (s *Selection) Obj() Object
func (s *Selection) Type() Type
func (s *Selection) Index() []int
func (s *Selection) Indirect() bool

The Kind method discriminates between the three (legal) kinds of selections, as indicated by the comments below.

type T struct{Field int}
func (T) Method() {}
var v T

                     // Kind            Type
    var _ = v.Field  // FieldVal        int
    var _ = v.Method // MethodVal       func()
    var _ = T.Method // MethodExpr      func(T)

Because of embedding, a selection may denote more than one field or method, in which case it is ambiguous, and no Selection is recorded for it.

The Obj method returns the Object for the selected field (*Var) or method (*Func). Due to embedding, the object may belong to a different type than that of the receiver expression expr. The Type method returns the type of the selection. For a field selection, this is the type of the field, but for method selections, the result is a function type that is not the same as the type of the method. For a MethodVal, the receiver parameter is dropped, and for a MethodExpr, the receiver parameter becomes a regular parameter, as shown in the example above.

The Index and Indirect methods report information about implicit operations occurring during the selection that a compiler would need to know about. Because of embedding, a selection expr.f may be shorthand for a sequence containing several implicit field selections, expr.d.e.f, and Index reports the complete sequence. And because of automatic pointer dereferencing during struct field accesses and method calls, a selection may imply one or more indirect loads from memory; Indirect reports whether this occurs.

Clients of the type checker can call LookupFieldOrMethod to look up a name within a type, as if by a selection. This function has an intimidating signature, but conceptually it accepts just a Type and a name, and returns a Selection:

func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) \
    (obj Object, index []int, indirect bool)

The result is not actually a Selection, but it contains the three main components of one: Obj, Index, and Indirect.

The addressable flag should be set if the receiver is a variable of type T, since in a method selection on a variable, an implicit address-of operation (&) may occur. The flag indicates whether the methods of type *T should be considered during the lookup. (You may wonder why this parameter is necessary. Couldn't clients instead call LookupFieldOrMethod on the pointer type *T if the receiver is a T variable? The answer is that if T is an interface type, the type *T has no methods at all.)

The final two parameters of LookupFieldOrMethod are (pkg *Package, name string).
Together they specify the name of the field or method to look up. This brings us to Ids.


LookupFieldOrMethod's need for a Package parameter is a subtle consequence of the Uniqueness of identifiers section in the Go spec: "Two identifiers are different if they are spelled differently, or if they appear in different packages and are not exported." In practical terms, this means that a type may have two methods (or two fields, or one of each) both named f so long as those methods are defined in different packages, as in this example:

package a
type A int
func (A) f()

package b
type B int
func (B) f()

package c
import ( "a"; "b" )
type C struct{a.A; b.B} // C has two methods called f

The type c.C has two methods named f, but there is no ambiguity because the two fs are distinct identifiers---think of them as fᵃ and fᵇ. For an exported method, this situation would be ambiguous because there is no distinction between Fᵃ and Fᵇ; there is only F.

Despite having two methods called f, neither of them can be called from within package c because c has no way to identify them. Within c, f is the identifier fᶜ, and type C has no method of that name. But if we pass an instance of C to code in package a and call its f method via an interface, fᵃ is called.

The practical consequence for tool builders is that any time you need to look up a field or method by name, or construct a map of fields and/or methods keyed by name, it is not sufficient to use the object's name as a key. Instead, you must call the Object.Id method, which returns a string that incorporates the object name, and for unexported objects, the package path too. There is also a standalone function Id that combines a name and the package path in the same way:

func Id(pkg *Package, name string) string

This distinction applies to selections expr.f, but not to lexical references x because for unexported identifiers, declarations and references always appear in the same package.

Fun fact: the reflect.StructField type records both the Name and the PkgPath strings for the same reason. The FieldByName methods of reflect.Value and reflect.Type match field names without regard to the package. If there is more than one match, they return an invalid value.

Method Sets

The method set of a type is the set of methods that can be called on any value of that type. (A variable of type T has access to all the methods of type *T as well, due to the implicit address-of operation during method calls, but those extra methods are not part of the method set of T.)

Clients can request the method set of a type T by calling NewMethodSet(T):

type MethodSet struct{ ... }
func NewMethodSet(T Type) *MethodSet
func (s *MethodSet) Len() int
func (s *MethodSet) At(i int) *Selection
func (s *MethodSet) Lookup(pkg *Package, name string) *Selection

The Len and At methods access a list of Selections, all of kind MethodVal, ordered by Id. The Lookup function allows lookup of a single method by name (and package path, as explained in the previous section).

NewMethodSet can be expensive, so for applications that compute method sets repeatedly, provides a MethodSetCache type that records previous results. If you only need a single method, don't construct the MethodSet at all; it's cheaper to use LookupFieldOrMethod.

The next program generates a boilerplate declaration of a new concrete type that satisfies an existing interface. Here's an example:

$ ./skeleton io ReadWriteCloser buffer
// *buffer implements io.ReadWriteCloser.
type buffer struct{}
func (b *buffer) Close() error {
func (b *buffer) Read(p []byte) (n int, err error) {
func (b *buffer) Write(p []byte) (n int, err error) {

The three arguments are the package and the name of the existing interface type, and the name of the new concrete type. The main function (not shown) loads the specified package and calls PrintSkeleton with the remaining two arguments:

// go get
func PrintSkeleton(pkg *types.Package, ifacename, concname string) error {
	obj := pkg.Scope().Lookup(ifacename)
	if obj == nil {
		return fmt.Errorf("%s.%s not found", pkg.Path(), ifacename)
	if _, ok := obj.(*types.TypeName); !ok {
		return fmt.Errorf("%v is not a named type", obj)
	iface, ok := obj.Type().Underlying().(*types.Interface)
	if !ok {
		return fmt.Errorf("type %v is a %T, not an interface",
			obj, obj.Type().Underlying())
	// Use first letter of type name as receiver parameter.
	if !isValidIdentifier(concname) {
		return fmt.Errorf("invalid concrete type name: %q", concname)
	r, _ := utf8.DecodeRuneInString(concname)

	fmt.Printf("// *%s implements %s.%s.\n", concname, pkg.Path(), ifacename)
	fmt.Printf("type %s struct{}\n", concname)
	mset := types.NewMethodSet(iface)
	for i := 0; i < mset.Len(); i++ {
		meth := mset.At(i).Obj()
		sig := types.TypeString(meth.Type(), (*types.Package).Name)
		fmt.Printf("func (%c *%s) %s%s {\n\tpanic(\"unimplemented\")\n}\n",
			r, concname, meth.Name(),
			strings.TrimPrefix(sig, "func"))
	return nil

First, PrintSkeleton locates the package-level named interface type, handling various error cases. Then it chooses the name for the receiver of the new methods: the first letter of the concrete type. Finally, it iterates over the method set of the interface, printing the corresponding concrete method declarations.

There's a subtlety in the declaration of sig, which is the string form of the method signature. We could have obtained this string from meth.Type().String(), but this would cause any named types within it to be formatted with the complete package path, for instance net/http.ResponseWriter, which is informative in diagnostics but not legal Go syntax. The TypeString function (explained in Formatting Values) allows the caller to control how packages are printed. Passing (*types.Package).Name causes only the package name http to be printed, not the complete path. Here's another example that illustrates it:

$ ./skeleton net/http Handler myHandler
// *myHandler implements net/http.Handler.
type myHandler struct{}
func (m *myHandler) ServeHTTP(http.ResponseWriter, *http.Request) {

The following program inspects all pairs of package-level named types in pkg, and reports the types that satisfy each interface type.

// go get
// Find all named types at package level.
var allNamed []*types.Named
for _, name := range pkg.Scope().Names() {
	if obj, ok := pkg.Scope().Lookup(name).(*types.TypeName); ok {
		allNamed = append(allNamed, obj.Type().(*types.Named))

// Test assignability of all distinct pairs of
// named types (T, U) where U is an interface.
for _, T := range allNamed {
	for _, U := range allNamed {
		if T == U || !types.IsInterface(U) {
		if types.AssignableTo(T, U) {
			fmt.Printf("%s satisfies %s\n", T, U)
		} else if !types.IsInterface(T) &&
			types.AssignableTo(types.NewPointer(T), U) {
			fmt.Printf("%s satisfies %s\n", types.NewPointer(T), U)

Given this input,

// go get
const input = `package main

type A struct{}
func (*A) f()

type B int
func (B) f()
func (*B) g()

type I interface { f() }
type J interface { g() }

the program prints:

$ go build
$ ./implements
*hello.A satisfies hello.I
hello.B satisfies hello.I
*hello.B satisfies hello.J

Notice that the method set of B does not include g, but the method set of *B does. That's why we needed the second assignability check, using the pointer type types.NewPointer(T).


A constant expression is one whose value is guaranteed to be computed at compile time. Constant expressions may appear in types, specifically as the length of an array type such as [16]byte, so one of the jobs of the type checker is to compute the value of each constant expression.

As we saw in the typeandvalue example, the type checker records the value of each constant expression like "Hello, " + "world", storing it in the Value field of the TypeAndValue struct. Constants are represented using the Value interface from the go/constant package.

package constant // go/constant

type Value interface {
	Kind() Kind 

type Kind int // one of Unknown, Bool, String, Int, Float, Complex

The interface has only one method, for discriminating the various kinds of constants, but the package provides many functions for inspecting a value of a known kind,

// Accessors
func BoolVal(x Value) bool
func Float32Val(x Value) (float32, bool)
func Float64Val(x Value) (float64, bool)
func Int64Val(x Value) (int64, bool)
func StringVal(x Value) string
func Uint64Val(x Value) (uint64, bool)
func Bytes(x Value) []byte
func BitLen(x Value) int
func Sign(x Value) int

for performing arithmetic on values,

// Operations
func Compare(x Value, op token.Token, y Value) bool
func UnaryOp(op token.Token, y Value, prec uint) Value
func BinaryOp(x Value, op token.Token, y Value) Value
func Shift(x Value, op token.Token, s uint) Value
func Denom(x Value) Value
func Num(x Value) Value
func Real(x Value) Value
func Imag(x Value) Value

and for constructing new values:

// Constructors
func MakeBool(b bool) Value
func MakeFloat64(x float64) Value
func MakeFromBytes(bytes []byte) Value
func MakeFromLiteral(lit string, tok token.Token, prec uint) Value
func MakeImag(x Value) Value
func MakeInt64(x int64) Value
func MakeString(s string) Value
func MakeUint64(x uint64) Value
func MakeUnknown() Value

All numeric Values, whether integer or floating-point, signed or unsigned, or real or complex, are represented more precisely than ordinary Go types like int64 and float64. Internally, the go/constant package uses multi-precision data types like Int, Rat, and Float from the math/big package so that Values and their arithmetic operations are accurate to at least 256 bits, as required by the Go specification.

Size and Alignment

Because the calls unsafe.Sizeof(v), unsafe.Alignof(v), and unsafe.Offsetof(v.f) are all constant expressions, the type checker must be able to compute the memory layout of any value v.

By default, the type checker uses the same layout algorithm as the Go 1.5 gc compiler targeting amd64. Clients can configure the type checker to use a different algorithm by providing an instance of the types.Sizes interface in the types.Config struct:

package types

type Sizes interface {
	Alignof(T Type) int64
	Offsetsof(fields []*Var) []int64
	Sizeof(T Type) int64

For common changes, like reducing the word size to 32 bits, clients can use an instance of StdSizes:

type StdSizes struct {
	WordSize int64
	MaxAlign int64

This type has two basic size and alignment parameters from which it derives all the other values using common assumptions. For example, pointers, functions, maps, and channels fit in one word, strings and interfaces require two words, and slices need three. The default behaviour is equivalent to StdSizes{8, 8}. For more esoteric layout changes, you'll need to write a new implementation of the Sizes interface.

The hugeparam program below prints all function parameters and results whose size exceeds a threshold. By default, the threshold is 48 bytes, but you can set it via the -bytes command-line flag. Such a tool could help identify inefficient parameter passing in your programs.

// go get
var bytesFlag = flag.Int("bytes", 48, "maximum parameter size in bytes")

var sizeof = (&types.StdSizes{8, 8}).Sizeof // the sizeof function

func PrintHugeParams(fset *token.FileSet, info *types.Info, files []*ast.File) {
	checkTuple := func(descr string, tuple *types.Tuple) {
		for i := 0; i < tuple.Len(); i++ {
			v := tuple.At(i)
			if sz := sizeof(v.Type()); sz > int64(*bytesFlag) {
				fmt.Printf("%s: %q %s: %s = %d bytes\n",
					v.Name(), descr, v.Type(), sz)
	checkSig := func(sig *types.Signature) {
		checkTuple("parameter", sig.Params())
		checkTuple("result", sig.Results())
	for _, file := range files {
		ast.Inspect(file, func(n ast.Node) bool {
			switch n := n.(type) {
			case *ast.FuncDecl:
			case *ast.FuncLit:
			return true

As before, Inspect applies a function to every node in the AST. The function cares about two kinds of nodes: declarations of named functions and methods (*ast.FuncDecl) and function literals (*ast.FuncLit). Observe the two cases' different logic to obtain the type of each function.

Here's a typical invocation on the standard encoding/xml package. It reports a number of places where the 7-word StartElement type is copied.

% ./hugeparam encoding/xml
/go/src/encoding/xml/marshal.go:167:50: "start" parameter: encoding/xml.StartElement = 56 bytes
/go/src/encoding/xml/marshal.go:734:97: "" result: encoding/xml.StartElement = 56 bytes
/go/src/encoding/xml/marshal.go:761:51: "start" parameter: encoding/xml.StartElement = 56 bytes
/go/src/encoding/xml/marshal.go:781:68: "start" parameter: encoding/xml.StartElement = 56 bytes
/go/src/encoding/xml/xml.go:72:30: "" result: encoding/xml.StartElement = 56 bytes


The type checker's Check function processes a slice of parsed files ([]*ast.File) that make up one package. When the type checker encounters an import declaration, it needs the type information for the objects in the imported package. It gets it by calling the Import method of the Importer interface shown below, an instance of which must be provided by the Config. This separation of concerns relieves the type checker from having to know any of the details of Go workspace organization, GOPATH, compiler file formats, and so on.

type Importer interface {
	Import(path string) (*Package, error)

Most of our examples used the simplest Importer implementation, importer.Default(), provided by the go/importer package. This importer looks in $GOROOT and $GOPATH for .a files written by the compiler (gc or gccgo) that was used to build the program. In addition to object code, these files contain export data, that is, a description of all the objects declared by the package, and also of any objects from other packages that were referred to indirectly. Because export data includes information about dependencies, the type checker need load at most one file per import, instead of one per transitive dependency.

Compiler export data is compact and efficient to locate, load, and parse, but it has several shortcomings. First, it does not contain position information for imported objects, reducing the quality of certain diagnostic messages. Second, it does not contain complete syntax trees nor semantic information about the contents of function bodies, so it is not suitable for interprocedural analyses. Third, compiler object data may be stale. Nothing detects or ensures that the object files are more recent than the source files from which they were derived. Generally, object data for standard packages is likely to be up-to-date, but for user packages, it depends on how recently the user ran a go install or go build -i command.

The package provides an alternative Importer that addresses some of these problems. It loads a complete program from source, performing cgo preprocessing if necessary, followed by parsing and type-checking. It loads independent packages in parallel to hide I/O latency, and detects and reports import cycles. For each package, it provides the types.Package containing the package's lexical environment, the list of ast.File syntax trees for each file in the package, the types.Info containing type information for each syntax node, and a list of type errors associated with that package. (Please be aware that the go/loader package's API is likely to change before it finally stabilizes.)

The doc program below demonstrates a simple use of the loader. It is a rudimentary implementation of go doc that prints the type, methods, and documentation of the package-level object specified on the command line. Here's an example:

$ ./doc net/http File
type net/http.File interface{Readdir(count int) ([]os.FileInfo, error); Seek(offset int64, whence int) (int64, error); Stat() (os.FileInfo, error); io.Closer; io.Reader}
/go/src/io/io.go:92:2: method (net/http.File) Close() error
/go/src/io/io.go:71:2: method (net/http.File) Read(p []byte) (n int, err error)
/go/src/net/http/fs.go:65:2: method (net/http.File) Readdir(count int) ([]os.FileInfo, error)
/go/src/net/http/fs.go:66:2: method (net/http.File) Seek(offset int64, whence int) (int64, error)
/go/src/net/http/fs.go:67:2: method (net/http.File) Stat() (os.FileInfo, error)

 A File is returned by a FileSystem's Open method and can be
served by the FileServer implementation.

The methods should behave the same as those on an *os.File.

Observe that it prints the correct location of each method declaration, even though, due to embedding, some of http.File's methods were declared in another package. Here's the first part of the program, showing how to load an entire program starting from the single package, pkgpath:

// go get
pkgpath, name := os.Args[1], os.Args[2]

// The loader loads a complete Go program from source code.
conf := loader.Config{ParserMode: parser.ParseComments}
lprog, err := conf.Load()
if err != nil {
	log.Fatal(err) // load error

// Find the package and package-level object.
pkg := lprog.Package(pkgpath).Pkg
obj := pkg.Scope().Lookup(name)
if obj == nil {
	log.Fatalf("%s.%s not found", pkg.Path(), name)

Notice that we instructed the parser to retain comments during parsing. The rest of the program prints the output:

// go get
// Print the object and its methods (incl. location of definition).
for _, sel := range typeutil.IntuitiveMethodSet(obj.Type(), nil) {
	fmt.Printf("%s: %s\n", lprog.Fset.Position(sel.Obj().Pos()), sel)

// Find the path from the root of the AST to the object's position.
// Walk up to the enclosing ast.Decl for the doc comment.
_, path, _ := lprog.PathEnclosingInterval(obj.Pos(), obj.Pos())
for _, n := range path {
	switch n := n.(type) {
	case *ast.GenDecl:
		fmt.Println("\n", n.Doc.Text())
	case *ast.FuncDecl:
		fmt.Println("\n", n.Doc.Text())

We used IntuitiveMethodSet to compute the method set, instead of NewMethodSet. The result of this convenience function, which is intended for use in user interfaces, includes methods of *T as well as those of T, since that matches most users' intuition about the method set of a type. (Our example, http.File, didn't illustrate the difference, but try running it on a type with both value and pointer methods.)

Also notice PathEnclosingInterval, which finds the set of AST nodes that enclose a particular point, in this case, the object's declaring identifier. By walking up with path, we find the enclosing declaration, to which the documentation is attached.

Formatting support

All types that satisfy Type or Object define a String method that formats the type or object in a readable notation. Selection also provides a String method. All package-level objects within these data structures are printed with the complete package path, as in these examples:

[]encoding/json.Marshaler                                     // a *Slice type
encoding/json.Marshal                                         // a *Func object
(*encoding/json.Encoder).Encode                               // a *Func object (method)
func (enc *encoding/json.Encoder) Encode(v interface{}) error // a method *Signature
func NewEncoder(w io.Writer) *encoding/json.Encoder           // a function *Signature

This notation is unambiguous, but it is not legal Go syntax. Also, package paths may be long, and the same package path may appear many times in a single string, for instance, when formatting a function of several parameters. Because these strings often form part of a tool's user interface---as with the diagnostic messages of hugeparam or the code generated by skeleton---many clients want more control over the formatting of package names.

The go/types package provides these alternatives to the String methods:

func ObjectString(obj Object, qf Qualifier) string
func TypeString(typ Type, qf Qualifier) string
func SelectionString(s *Selection, qf Qualifier) string

type Qualifier func(*Package) string

The TypeString, ObjectString, and SelectionString functions are like the String methods of the respective types, but they accept an additional argument, a Qualifier.

A Qualifier is a client-provided function that determines how a package name is rendered as a string. If it is nil, the default behavior is to print the package's path, just like the String methods do. If a caller passes (*Package).Name as the qualifier, that is, a function that accepts a package and returns its Name, then objects are qualified only by the package name. The above examples would look like this:

func (enc *json.Encoder) Encode(v interface{}) error
func NewEncoder(w io.Writer) *json.Encoder

Often when a tool prints some output, it is implicitly in the context of a particular package, perhaps one specified by the command line or HTTP request. In that case, it is more natural to omit the package qualification altogether for objects belonging to that package, but to qualify all other objects by their package's path. That's what the RelativeTo(pkg) qualifier does:

func RelativeTo(pkg *Package) Qualifier

The examples below show how json.NewEncoder would be printed using three qualifiers, each relative to a different package:

// RelativeTo "encoding/json":
func NewEncoder(w io.Writer) *Encoder

// RelativeTo "io":
func NewEncoder(w Writer) *encoding/json.Encoder

// RelativeTo any other package:
func NewEncoder(w io.Writer) *encoding/json.Encoder

Another qualifier that may be relevant to refactoring tools (but is not currently provided by the type checker) is one that renders each package name using the locally appropriate name within a given source file. Its behavior would depend on the set of import declarations, including renaming imports, within that source file.

Getting from A to B

The type checker and its related packages represent many aspects of a Go program in many different ways, and analysis tools must often map between them. For instance, a named entity may be identified by its Object; by its declaring identifier (ast.Ident) or by any referring identifier; by its declaring ast.Node; by the position (token.Pos) of any those nodes; or by the filename and line/column number (or byte offset) of those token.Pos values.

In this section, we'll list solutions to a number of common problems of the form "I have an A; I need the corresponding B".

To map from a token.Pos to an ast.Node, call the helper function astutil.PathEnclosingInterval. It returns the enclosing ast.Node, and all its ancestors up to the root of the file. You must know which file *ast.File the token.Pos belongs to. Alternatively, you can search an entire program loaded by the loader package, using (*loader.Program).PathEnclosingInterval.

To map from an Object to its declaring syntax, call Pos to get its position, then use PathEnclosingInterval as before. This approach is suitable for a one-off query. For repeated use, it may be more efficient to visit the syntax tree and construct the mapping between declarations and objects.

To map from an ast.Ident to the Object it refers to (or declares), consult the Uses or Defs map for the package, as shown in Identifier Resolution.

To map from an Object to its documentation, find the object's declaration, and look at the attached Doc field. You must have set the parser's ParseComments flag. See the doc example in Imports.