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// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package types
import (
// Eval returns the type and, if constant, the value for the
// expression expr, evaluated at position pos of package pkg,
// which must have been derived from type-checking an AST with
// complete position information relative to the provided file
// set.
// The meaning of the parameters fset, pkg, and pos is the
// same as in CheckExpr. An error is returned if expr cannot
// be parsed successfully, or the resulting expr AST cannot be
// type-checked.
func Eval(fset *token.FileSet, pkg *Package, pos token.Pos, expr string) (_ TypeAndValue, err error) {
// parse expressions
node, err := parser.ParseExprFrom(fset, "eval", expr, 0)
if err != nil {
return TypeAndValue{}, err
info := &Info{
Types: make(map[ast.Expr]TypeAndValue),
err = CheckExpr(fset, pkg, pos, node, info)
return info.Types[node], err
// CheckExpr type checks the expression expr as if it had appeared at
// position pos of package pkg. Type information about the expression
// is recorded in info.
// If pkg == nil, the Universe scope is used and the provided
// position pos is ignored. If pkg != nil, and pos is invalid,
// the package scope is used. Otherwise, pos must belong to the
// package.
// An error is returned if pos is not within the package or
// if the node cannot be type-checked.
// Note: Eval and CheckExpr should not be used instead of running Check
// to compute types and values, but in addition to Check, as these
// functions ignore the context in which an expression is used (e.g., an
// assignment). Thus, top-level untyped constants will return an
// untyped type rather then the respective context-specific type.
func CheckExpr(fset *token.FileSet, pkg *Package, pos token.Pos, expr ast.Expr, info *Info) (err error) {
// determine scope
var scope *Scope
if pkg == nil {
scope = Universe
pos = token.NoPos
} else if !pos.IsValid() {
scope = pkg.scope
} else {
// The package scope extent (position information) may be
// incorrect (files spread across a wide range of fset
// positions) - ignore it and just consider its children
// (file scopes).
for _, fscope := range pkg.scope.children {
if scope = fscope.Innermost(pos); scope != nil {
if scope == nil || debug {
s := scope
for s != nil && s != pkg.scope {
s = s.parent
// s == nil || s == pkg.scope
if s == nil {
return fmt.Errorf("no position %s found in package %s", fset.Position(pos),
// initialize checker
check := NewChecker(nil, fset, pkg, info)
check.scope = scope
check.pos = pos
defer check.handleBailout(&err)
// evaluate node
var x operand
check.rawExpr(&x, expr, nil)
check.processDelayed(0) // incl. all functions
return nil