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Source file src/go/types/api.go

Documentation: go/types

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package types declares the data types and implements
     6  // the algorithms for type-checking of Go packages. Use
     7  // Config.Check to invoke the type checker for a package.
     8  // Alternatively, create a new type checker with NewChecker
     9  // and invoke it incrementally by calling Checker.Files.
    10  //
    11  // Type-checking consists of several interdependent phases:
    12  //
    13  // Name resolution maps each identifier (ast.Ident) in the program to the
    14  // language object (Object) it denotes.
    15  // Use Info.{Defs,Uses,Implicits} for the results of name resolution.
    16  //
    17  // Constant folding computes the exact constant value (constant.Value)
    18  // for every expression (ast.Expr) that is a compile-time constant.
    19  // Use Info.Types[expr].Value for the results of constant folding.
    20  //
    21  // Type inference computes the type (Type) of every expression (ast.Expr)
    22  // and checks for compliance with the language specification.
    23  // Use Info.Types[expr].Type for the results of type inference.
    24  //
    25  // For a tutorial, see https://golang.org/s/types-tutorial.
    26  //
    27  package types
    28  
    29  import (
    30  	"bytes"
    31  	"fmt"
    32  	"go/ast"
    33  	"go/constant"
    34  	"go/token"
    35  )
    36  
    37  // An Error describes a type-checking error; it implements the error interface.
    38  // A "soft" error is an error that still permits a valid interpretation of a
    39  // package (such as "unused variable"); "hard" errors may lead to unpredictable
    40  // behavior if ignored.
    41  type Error struct {
    42  	Fset *token.FileSet // file set for interpretation of Pos
    43  	Pos  token.Pos      // error position
    44  	Msg  string         // error message
    45  	Soft bool           // if set, error is "soft"
    46  }
    47  
    48  // Error returns an error string formatted as follows:
    49  // filename:line:column: message
    50  func (err Error) Error() string {
    51  	return fmt.Sprintf("%s: %s", err.Fset.Position(err.Pos), err.Msg)
    52  }
    53  
    54  // An Importer resolves import paths to Packages.
    55  //
    56  // CAUTION: This interface does not support the import of locally
    57  // vendored packages. See https://golang.org/s/go15vendor.
    58  // If possible, external implementations should implement ImporterFrom.
    59  type Importer interface {
    60  	// Import returns the imported package for the given import path.
    61  	// The semantics is like for ImporterFrom.ImportFrom except that
    62  	// dir and mode are ignored (since they are not present).
    63  	Import(path string) (*Package, error)
    64  }
    65  
    66  // ImportMode is reserved for future use.
    67  type ImportMode int
    68  
    69  // An ImporterFrom resolves import paths to packages; it
    70  // supports vendoring per https://golang.org/s/go15vendor.
    71  // Use go/importer to obtain an ImporterFrom implementation.
    72  type ImporterFrom interface {
    73  	// Importer is present for backward-compatibility. Calling
    74  	// Import(path) is the same as calling ImportFrom(path, "", 0);
    75  	// i.e., locally vendored packages may not be found.
    76  	// The types package does not call Import if an ImporterFrom
    77  	// is present.
    78  	Importer
    79  
    80  	// ImportFrom returns the imported package for the given import
    81  	// path when imported by a package file located in dir.
    82  	// If the import failed, besides returning an error, ImportFrom
    83  	// is encouraged to cache and return a package anyway, if one
    84  	// was created. This will reduce package inconsistencies and
    85  	// follow-on type checker errors due to the missing package.
    86  	// The mode value must be 0; it is reserved for future use.
    87  	// Two calls to ImportFrom with the same path and dir must
    88  	// return the same package.
    89  	ImportFrom(path, dir string, mode ImportMode) (*Package, error)
    90  }
    91  
    92  // A Config specifies the configuration for type checking.
    93  // The zero value for Config is a ready-to-use default configuration.
    94  type Config struct {
    95  	// If IgnoreFuncBodies is set, function bodies are not
    96  	// type-checked.
    97  	IgnoreFuncBodies bool
    98  
    99  	// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
   100  	// declares an empty "C" package and errors are omitted for qualified
   101  	// identifiers referring to package C (which won't find an object).
   102  	// This feature is intended for the standard library cmd/api tool.
   103  	//
   104  	// Caution: Effects may be unpredictable due to follow-on errors.
   105  	//          Do not use casually!
   106  	FakeImportC bool
   107  
   108  	// If Error != nil, it is called with each error found
   109  	// during type checking; err has dynamic type Error.
   110  	// Secondary errors (for instance, to enumerate all types
   111  	// involved in an invalid recursive type declaration) have
   112  	// error strings that start with a '\t' character.
   113  	// If Error == nil, type-checking stops with the first
   114  	// error found.
   115  	Error func(err error)
   116  
   117  	// An importer is used to import packages referred to from
   118  	// import declarations.
   119  	// If the installed importer implements ImporterFrom, the type
   120  	// checker calls ImportFrom instead of Import.
   121  	// The type checker reports an error if an importer is needed
   122  	// but none was installed.
   123  	Importer Importer
   124  
   125  	// If Sizes != nil, it provides the sizing functions for package unsafe.
   126  	// Otherwise SizesFor("gc", "amd64") is used instead.
   127  	Sizes Sizes
   128  
   129  	// If DisableUnusedImportCheck is set, packages are not checked
   130  	// for unused imports.
   131  	DisableUnusedImportCheck bool
   132  }
   133  
   134  // Info holds result type information for a type-checked package.
   135  // Only the information for which a map is provided is collected.
   136  // If the package has type errors, the collected information may
   137  // be incomplete.
   138  type Info struct {
   139  	// Types maps expressions to their types, and for constant
   140  	// expressions, also their values. Invalid expressions are
   141  	// omitted.
   142  	//
   143  	// For (possibly parenthesized) identifiers denoting built-in
   144  	// functions, the recorded signatures are call-site specific:
   145  	// if the call result is not a constant, the recorded type is
   146  	// an argument-specific signature. Otherwise, the recorded type
   147  	// is invalid.
   148  	//
   149  	// The Types map does not record the type of every identifier,
   150  	// only those that appear where an arbitrary expression is
   151  	// permitted. For instance, the identifier f in a selector
   152  	// expression x.f is found only in the Selections map, the
   153  	// identifier z in a variable declaration 'var z int' is found
   154  	// only in the Defs map, and identifiers denoting packages in
   155  	// qualified identifiers are collected in the Uses map.
   156  	Types map[ast.Expr]TypeAndValue
   157  
   158  	// Defs maps identifiers to the objects they define (including
   159  	// package names, dots "." of dot-imports, and blank "_" identifiers).
   160  	// For identifiers that do not denote objects (e.g., the package name
   161  	// in package clauses, or symbolic variables t in t := x.(type) of
   162  	// type switch headers), the corresponding objects are nil.
   163  	//
   164  	// For an embedded field, Defs returns the field *Var it defines.
   165  	//
   166  	// Invariant: Defs[id] == nil || Defs[id].Pos() == id.Pos()
   167  	Defs map[*ast.Ident]Object
   168  
   169  	// Uses maps identifiers to the objects they denote.
   170  	//
   171  	// For an embedded field, Uses returns the *TypeName it denotes.
   172  	//
   173  	// Invariant: Uses[id].Pos() != id.Pos()
   174  	Uses map[*ast.Ident]Object
   175  
   176  	// Implicits maps nodes to their implicitly declared objects, if any.
   177  	// The following node and object types may appear:
   178  	//
   179  	//     node               declared object
   180  	//
   181  	//     *ast.ImportSpec    *PkgName for imports without renames
   182  	//     *ast.CaseClause    type-specific *Var for each type switch case clause (incl. default)
   183  	//     *ast.Field         anonymous parameter *Var
   184  	//
   185  	Implicits map[ast.Node]Object
   186  
   187  	// Selections maps selector expressions (excluding qualified identifiers)
   188  	// to their corresponding selections.
   189  	Selections map[*ast.SelectorExpr]*Selection
   190  
   191  	// Scopes maps ast.Nodes to the scopes they define. Package scopes are not
   192  	// associated with a specific node but with all files belonging to a package.
   193  	// Thus, the package scope can be found in the type-checked Package object.
   194  	// Scopes nest, with the Universe scope being the outermost scope, enclosing
   195  	// the package scope, which contains (one or more) files scopes, which enclose
   196  	// function scopes which in turn enclose statement and function literal scopes.
   197  	// Note that even though package-level functions are declared in the package
   198  	// scope, the function scopes are embedded in the file scope of the file
   199  	// containing the function declaration.
   200  	//
   201  	// The following node types may appear in Scopes:
   202  	//
   203  	//     *ast.File
   204  	//     *ast.FuncType
   205  	//     *ast.BlockStmt
   206  	//     *ast.IfStmt
   207  	//     *ast.SwitchStmt
   208  	//     *ast.TypeSwitchStmt
   209  	//     *ast.CaseClause
   210  	//     *ast.CommClause
   211  	//     *ast.ForStmt
   212  	//     *ast.RangeStmt
   213  	//
   214  	Scopes map[ast.Node]*Scope
   215  
   216  	// InitOrder is the list of package-level initializers in the order in which
   217  	// they must be executed. Initializers referring to variables related by an
   218  	// initialization dependency appear in topological order, the others appear
   219  	// in source order. Variables without an initialization expression do not
   220  	// appear in this list.
   221  	InitOrder []*Initializer
   222  }
   223  
   224  // TypeOf returns the type of expression e, or nil if not found.
   225  // Precondition: the Types, Uses and Defs maps are populated.
   226  //
   227  func (info *Info) TypeOf(e ast.Expr) Type {
   228  	if t, ok := info.Types[e]; ok {
   229  		return t.Type
   230  	}
   231  	if id, _ := e.(*ast.Ident); id != nil {
   232  		if obj := info.ObjectOf(id); obj != nil {
   233  			return obj.Type()
   234  		}
   235  	}
   236  	return nil
   237  }
   238  
   239  // ObjectOf returns the object denoted by the specified id,
   240  // or nil if not found.
   241  //
   242  // If id is an embedded struct field, ObjectOf returns the field (*Var)
   243  // it uses, not the type (*TypeName) it defines.
   244  //
   245  // Precondition: the Uses and Defs maps are populated.
   246  //
   247  func (info *Info) ObjectOf(id *ast.Ident) Object {
   248  	if obj := info.Defs[id]; obj != nil {
   249  		return obj
   250  	}
   251  	return info.Uses[id]
   252  }
   253  
   254  // TypeAndValue reports the type and value (for constants)
   255  // of the corresponding expression.
   256  type TypeAndValue struct {
   257  	mode  operandMode
   258  	Type  Type
   259  	Value constant.Value
   260  }
   261  
   262  // TODO(gri) Consider eliminating the IsVoid predicate. Instead, report
   263  // "void" values as regular values but with the empty tuple type.
   264  
   265  // IsVoid reports whether the corresponding expression
   266  // is a function call without results.
   267  func (tv TypeAndValue) IsVoid() bool {
   268  	return tv.mode == novalue
   269  }
   270  
   271  // IsType reports whether the corresponding expression specifies a type.
   272  func (tv TypeAndValue) IsType() bool {
   273  	return tv.mode == typexpr
   274  }
   275  
   276  // IsBuiltin reports whether the corresponding expression denotes
   277  // a (possibly parenthesized) built-in function.
   278  func (tv TypeAndValue) IsBuiltin() bool {
   279  	return tv.mode == builtin
   280  }
   281  
   282  // IsValue reports whether the corresponding expression is a value.
   283  // Builtins are not considered values. Constant values have a non-
   284  // nil Value.
   285  func (tv TypeAndValue) IsValue() bool {
   286  	switch tv.mode {
   287  	case constant_, variable, mapindex, value, commaok:
   288  		return true
   289  	}
   290  	return false
   291  }
   292  
   293  // IsNil reports whether the corresponding expression denotes the
   294  // predeclared value nil.
   295  func (tv TypeAndValue) IsNil() bool {
   296  	return tv.mode == value && tv.Type == Typ[UntypedNil]
   297  }
   298  
   299  // Addressable reports whether the corresponding expression
   300  // is addressable (https://golang.org/ref/spec#Address_operators).
   301  func (tv TypeAndValue) Addressable() bool {
   302  	return tv.mode == variable
   303  }
   304  
   305  // Assignable reports whether the corresponding expression
   306  // is assignable to (provided a value of the right type).
   307  func (tv TypeAndValue) Assignable() bool {
   308  	return tv.mode == variable || tv.mode == mapindex
   309  }
   310  
   311  // HasOk reports whether the corresponding expression may be
   312  // used on the rhs of a comma-ok assignment.
   313  func (tv TypeAndValue) HasOk() bool {
   314  	return tv.mode == commaok || tv.mode == mapindex
   315  }
   316  
   317  // An Initializer describes a package-level variable, or a list of variables in case
   318  // of a multi-valued initialization expression, and the corresponding initialization
   319  // expression.
   320  type Initializer struct {
   321  	Lhs []*Var // var Lhs = Rhs
   322  	Rhs ast.Expr
   323  }
   324  
   325  func (init *Initializer) String() string {
   326  	var buf bytes.Buffer
   327  	for i, lhs := range init.Lhs {
   328  		if i > 0 {
   329  			buf.WriteString(", ")
   330  		}
   331  		buf.WriteString(lhs.Name())
   332  	}
   333  	buf.WriteString(" = ")
   334  	WriteExpr(&buf, init.Rhs)
   335  	return buf.String()
   336  }
   337  
   338  // Check type-checks a package and returns the resulting package object and
   339  // the first error if any. Additionally, if info != nil, Check populates each
   340  // of the non-nil maps in the Info struct.
   341  //
   342  // The package is marked as complete if no errors occurred, otherwise it is
   343  // incomplete. See Config.Error for controlling behavior in the presence of
   344  // errors.
   345  //
   346  // The package is specified by a list of *ast.Files and corresponding
   347  // file set, and the package path the package is identified with.
   348  // The clean path must not be empty or dot (".").
   349  func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, info *Info) (*Package, error) {
   350  	pkg := NewPackage(path, "")
   351  	return pkg, NewChecker(conf, fset, pkg, info).Files(files)
   352  }
   353  
   354  // AssertableTo reports whether a value of type V can be asserted to have type T.
   355  func AssertableTo(V *Interface, T Type) bool {
   356  	m, _ := assertableTo(V, T)
   357  	return m == nil
   358  }
   359  
   360  // AssignableTo reports whether a value of type V is assignable to a variable of type T.
   361  func AssignableTo(V, T Type) bool {
   362  	x := operand{mode: value, typ: V}
   363  	return x.assignableTo(nil, T, nil) // config not needed for non-constant x
   364  }
   365  
   366  // ConvertibleTo reports whether a value of type V is convertible to a value of type T.
   367  func ConvertibleTo(V, T Type) bool {
   368  	x := operand{mode: value, typ: V}
   369  	return x.convertibleTo(nil, T) // config not needed for non-constant x
   370  }
   371  
   372  // Implements reports whether type V implements interface T.
   373  func Implements(V Type, T *Interface) bool {
   374  	f, _ := MissingMethod(V, T, true)
   375  	return f == nil
   376  }
   377  

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