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Source file src/text/template/exec.go

  // Copyright 2011 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 template
  
  import (
  	"bytes"
  	"fmt"
  	"io"
  	"reflect"
  	"runtime"
  	"sort"
  	"strings"
  	"text/template/parse"
  )
  
  // maxExecDepth specifies the maximum stack depth of templates within
  // templates. This limit is only practically reached by accidentally
  // recursive template invocations. This limit allows us to return
  // an error instead of triggering a stack overflow.
  const maxExecDepth = 100000
  
  // state represents the state of an execution. It's not part of the
  // template so that multiple executions of the same template
  // can execute in parallel.
  type state struct {
  	tmpl  *Template
  	wr    io.Writer
  	node  parse.Node // current node, for errors
  	vars  []variable // push-down stack of variable values.
  	depth int        // the height of the stack of executing templates.
  }
  
  // variable holds the dynamic value of a variable such as $, $x etc.
  type variable struct {
  	name  string
  	value reflect.Value
  }
  
  // push pushes a new variable on the stack.
  func (s *state) push(name string, value reflect.Value) {
  	s.vars = append(s.vars, variable{name, value})
  }
  
  // mark returns the length of the variable stack.
  func (s *state) mark() int {
  	return len(s.vars)
  }
  
  // pop pops the variable stack up to the mark.
  func (s *state) pop(mark int) {
  	s.vars = s.vars[0:mark]
  }
  
  // setVar overwrites the top-nth variable on the stack. Used by range iterations.
  func (s *state) setVar(n int, value reflect.Value) {
  	s.vars[len(s.vars)-n].value = value
  }
  
  // varValue returns the value of the named variable.
  func (s *state) varValue(name string) reflect.Value {
  	for i := s.mark() - 1; i >= 0; i-- {
  		if s.vars[i].name == name {
  			return s.vars[i].value
  		}
  	}
  	s.errorf("undefined variable: %s", name)
  	return zero
  }
  
  var zero reflect.Value
  
  // at marks the state to be on node n, for error reporting.
  func (s *state) at(node parse.Node) {
  	s.node = node
  }
  
  // doublePercent returns the string with %'s replaced by %%, if necessary,
  // so it can be used safely inside a Printf format string.
  func doublePercent(str string) string {
  	if strings.Contains(str, "%") {
  		str = strings.Replace(str, "%", "%%", -1)
  	}
  	return str
  }
  
  // TODO: It would be nice if ExecError was more broken down, but
  // the way ErrorContext embeds the template name makes the
  // processing too clumsy.
  
  // ExecError is the custom error type returned when Execute has an
  // error evaluating its template. (If a write error occurs, the actual
  // error is returned; it will not be of type ExecError.)
  type ExecError struct {
  	Name string // Name of template.
  	Err  error  // Pre-formatted error.
  }
  
  func (e ExecError) Error() string {
  	return e.Err.Error()
  }
  
  // errorf records an ExecError and terminates processing.
  func (s *state) errorf(format string, args ...interface{}) {
  	name := doublePercent(s.tmpl.Name())
  	if s.node == nil {
  		format = fmt.Sprintf("template: %s: %s", name, format)
  	} else {
  		location, context := s.tmpl.ErrorContext(s.node)
  		format = fmt.Sprintf("template: %s: executing %q at <%s>: %s", location, name, doublePercent(context), format)
  	}
  	panic(ExecError{
  		Name: s.tmpl.Name(),
  		Err:  fmt.Errorf(format, args...),
  	})
  }
  
  // writeError is the wrapper type used internally when Execute has an
  // error writing to its output. We strip the wrapper in errRecover.
  // Note that this is not an implementation of error, so it cannot escape
  // from the package as an error value.
  type writeError struct {
  	Err error // Original error.
  }
  
  func (s *state) writeError(err error) {
  	panic(writeError{
  		Err: err,
  	})
  }
  
  // errRecover is the handler that turns panics into returns from the top
  // level of Parse.
  func errRecover(errp *error) {
  	e := recover()
  	if e != nil {
  		switch err := e.(type) {
  		case runtime.Error:
  			panic(e)
  		case writeError:
  			*errp = err.Err // Strip the wrapper.
  		case ExecError:
  			*errp = err // Keep the wrapper.
  		default:
  			panic(e)
  		}
  	}
  }
  
  // ExecuteTemplate applies the template associated with t that has the given name
  // to the specified data object and writes the output to wr.
  // If an error occurs executing the template or writing its output,
  // execution stops, but partial results may already have been written to
  // the output writer.
  // A template may be executed safely in parallel.
  func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
  	var tmpl *Template
  	if t.common != nil {
  		tmpl = t.tmpl[name]
  	}
  	if tmpl == nil {
  		return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
  	}
  	return tmpl.Execute(wr, data)
  }
  
  // Execute applies a parsed template to the specified data object,
  // and writes the output to wr.
  // If an error occurs executing the template or writing its output,
  // execution stops, but partial results may already have been written to
  // the output writer.
  // A template may be executed safely in parallel.
  //
  // If data is a reflect.Value, the template applies to the concrete
  // value that the reflect.Value holds, as in fmt.Print.
  func (t *Template) Execute(wr io.Writer, data interface{}) error {
  	return t.execute(wr, data)
  }
  
  func (t *Template) execute(wr io.Writer, data interface{}) (err error) {
  	defer errRecover(&err)
  	value, ok := data.(reflect.Value)
  	if !ok {
  		value = reflect.ValueOf(data)
  	}
  	state := &state{
  		tmpl: t,
  		wr:   wr,
  		vars: []variable{{"$", value}},
  	}
  	if t.Tree == nil || t.Root == nil {
  		state.errorf("%q is an incomplete or empty template", t.Name())
  	}
  	state.walk(value, t.Root)
  	return
  }
  
  // DefinedTemplates returns a string listing the defined templates,
  // prefixed by the string "; defined templates are: ". If there are none,
  // it returns the empty string. For generating an error message here
  // and in html/template.
  func (t *Template) DefinedTemplates() string {
  	if t.common == nil {
  		return ""
  	}
  	var b bytes.Buffer
  	for name, tmpl := range t.tmpl {
  		if tmpl.Tree == nil || tmpl.Root == nil {
  			continue
  		}
  		if b.Len() > 0 {
  			b.WriteString(", ")
  		}
  		fmt.Fprintf(&b, "%q", name)
  	}
  	var s string
  	if b.Len() > 0 {
  		s = "; defined templates are: " + b.String()
  	}
  	return s
  }
  
  // Walk functions step through the major pieces of the template structure,
  // generating output as they go.
  func (s *state) walk(dot reflect.Value, node parse.Node) {
  	s.at(node)
  	switch node := node.(type) {
  	case *parse.ActionNode:
  		// Do not pop variables so they persist until next end.
  		// Also, if the action declares variables, don't print the result.
  		val := s.evalPipeline(dot, node.Pipe)
  		if len(node.Pipe.Decl) == 0 {
  			s.printValue(node, val)
  		}
  	case *parse.IfNode:
  		s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
  	case *parse.ListNode:
  		for _, node := range node.Nodes {
  			s.walk(dot, node)
  		}
  	case *parse.RangeNode:
  		s.walkRange(dot, node)
  	case *parse.TemplateNode:
  		s.walkTemplate(dot, node)
  	case *parse.TextNode:
  		if _, err := s.wr.Write(node.Text); err != nil {
  			s.writeError(err)
  		}
  	case *parse.WithNode:
  		s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
  	default:
  		s.errorf("unknown node: %s", node)
  	}
  }
  
  // walkIfOrWith walks an 'if' or 'with' node. The two control structures
  // are identical in behavior except that 'with' sets dot.
  func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
  	defer s.pop(s.mark())
  	val := s.evalPipeline(dot, pipe)
  	truth, ok := isTrue(val)
  	if !ok {
  		s.errorf("if/with can't use %v", val)
  	}
  	if truth {
  		if typ == parse.NodeWith {
  			s.walk(val, list)
  		} else {
  			s.walk(dot, list)
  		}
  	} else if elseList != nil {
  		s.walk(dot, elseList)
  	}
  }
  
  // IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
  // and whether the value has a meaningful truth value. This is the definition of
  // truth used by if and other such actions.
  func IsTrue(val interface{}) (truth, ok bool) {
  	return isTrue(reflect.ValueOf(val))
  }
  
  func isTrue(val reflect.Value) (truth, ok bool) {
  	if !val.IsValid() {
  		// Something like var x interface{}, never set. It's a form of nil.
  		return false, true
  	}
  	switch val.Kind() {
  	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
  		truth = val.Len() > 0
  	case reflect.Bool:
  		truth = val.Bool()
  	case reflect.Complex64, reflect.Complex128:
  		truth = val.Complex() != 0
  	case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Interface:
  		truth = !val.IsNil()
  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  		truth = val.Int() != 0
  	case reflect.Float32, reflect.Float64:
  		truth = val.Float() != 0
  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  		truth = val.Uint() != 0
  	case reflect.Struct:
  		truth = true // Struct values are always true.
  	default:
  		return
  	}
  	return truth, true
  }
  
  func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
  	s.at(r)
  	defer s.pop(s.mark())
  	val, _ := indirect(s.evalPipeline(dot, r.Pipe))
  	// mark top of stack before any variables in the body are pushed.
  	mark := s.mark()
  	oneIteration := func(index, elem reflect.Value) {
  		// Set top var (lexically the second if there are two) to the element.
  		if len(r.Pipe.Decl) > 0 {
  			s.setVar(1, elem)
  		}
  		// Set next var (lexically the first if there are two) to the index.
  		if len(r.Pipe.Decl) > 1 {
  			s.setVar(2, index)
  		}
  		s.walk(elem, r.List)
  		s.pop(mark)
  	}
  	switch val.Kind() {
  	case reflect.Array, reflect.Slice:
  		if val.Len() == 0 {
  			break
  		}
  		for i := 0; i < val.Len(); i++ {
  			oneIteration(reflect.ValueOf(i), val.Index(i))
  		}
  		return
  	case reflect.Map:
  		if val.Len() == 0 {
  			break
  		}
  		for _, key := range sortKeys(val.MapKeys()) {
  			oneIteration(key, val.MapIndex(key))
  		}
  		return
  	case reflect.Chan:
  		if val.IsNil() {
  			break
  		}
  		i := 0
  		for ; ; i++ {
  			elem, ok := val.Recv()
  			if !ok {
  				break
  			}
  			oneIteration(reflect.ValueOf(i), elem)
  		}
  		if i == 0 {
  			break
  		}
  		return
  	case reflect.Invalid:
  		break // An invalid value is likely a nil map, etc. and acts like an empty map.
  	default:
  		s.errorf("range can't iterate over %v", val)
  	}
  	if r.ElseList != nil {
  		s.walk(dot, r.ElseList)
  	}
  }
  
  func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
  	s.at(t)
  	tmpl := s.tmpl.tmpl[t.Name]
  	if tmpl == nil {
  		s.errorf("template %q not defined", t.Name)
  	}
  	if s.depth == maxExecDepth {
  		s.errorf("exceeded maximum template depth (%v)", maxExecDepth)
  	}
  	// Variables declared by the pipeline persist.
  	dot = s.evalPipeline(dot, t.Pipe)
  	newState := *s
  	newState.depth++
  	newState.tmpl = tmpl
  	// No dynamic scoping: template invocations inherit no variables.
  	newState.vars = []variable{{"$", dot}}
  	newState.walk(dot, tmpl.Root)
  }
  
  // Eval functions evaluate pipelines, commands, and their elements and extract
  // values from the data structure by examining fields, calling methods, and so on.
  // The printing of those values happens only through walk functions.
  
  // evalPipeline returns the value acquired by evaluating a pipeline. If the
  // pipeline has a variable declaration, the variable will be pushed on the
  // stack. Callers should therefore pop the stack after they are finished
  // executing commands depending on the pipeline value.
  func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
  	if pipe == nil {
  		return
  	}
  	s.at(pipe)
  	for _, cmd := range pipe.Cmds {
  		value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
  		// If the object has type interface{}, dig down one level to the thing inside.
  		if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
  			value = reflect.ValueOf(value.Interface()) // lovely!
  		}
  	}
  	for _, variable := range pipe.Decl {
  		s.push(variable.Ident[0], value)
  	}
  	return value
  }
  
  func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
  	if len(args) > 1 || final.IsValid() {
  		s.errorf("can't give argument to non-function %s", args[0])
  	}
  }
  
  func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
  	firstWord := cmd.Args[0]
  	switch n := firstWord.(type) {
  	case *parse.FieldNode:
  		return s.evalFieldNode(dot, n, cmd.Args, final)
  	case *parse.ChainNode:
  		return s.evalChainNode(dot, n, cmd.Args, final)
  	case *parse.IdentifierNode:
  		// Must be a function.
  		return s.evalFunction(dot, n, cmd, cmd.Args, final)
  	case *parse.PipeNode:
  		// Parenthesized pipeline. The arguments are all inside the pipeline; final is ignored.
  		return s.evalPipeline(dot, n)
  	case *parse.VariableNode:
  		return s.evalVariableNode(dot, n, cmd.Args, final)
  	}
  	s.at(firstWord)
  	s.notAFunction(cmd.Args, final)
  	switch word := firstWord.(type) {
  	case *parse.BoolNode:
  		return reflect.ValueOf(word.True)
  	case *parse.DotNode:
  		return dot
  	case *parse.NilNode:
  		s.errorf("nil is not a command")
  	case *parse.NumberNode:
  		return s.idealConstant(word)
  	case *parse.StringNode:
  		return reflect.ValueOf(word.Text)
  	}
  	s.errorf("can't evaluate command %q", firstWord)
  	panic("not reached")
  }
  
  // idealConstant is called to return the value of a number in a context where
  // we don't know the type. In that case, the syntax of the number tells us
  // its type, and we use Go rules to resolve. Note there is no such thing as
  // a uint ideal constant in this situation - the value must be of int type.
  func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
  	// These are ideal constants but we don't know the type
  	// and we have no context.  (If it was a method argument,
  	// we'd know what we need.) The syntax guides us to some extent.
  	s.at(constant)
  	switch {
  	case constant.IsComplex:
  		return reflect.ValueOf(constant.Complex128) // incontrovertible.
  	case constant.IsFloat && !isHexConstant(constant.Text) && strings.ContainsAny(constant.Text, ".eE"):
  		return reflect.ValueOf(constant.Float64)
  	case constant.IsInt:
  		n := int(constant.Int64)
  		if int64(n) != constant.Int64 {
  			s.errorf("%s overflows int", constant.Text)
  		}
  		return reflect.ValueOf(n)
  	case constant.IsUint:
  		s.errorf("%s overflows int", constant.Text)
  	}
  	return zero
  }
  
  func isHexConstant(s string) bool {
  	return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X')
  }
  
  func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
  	s.at(field)
  	return s.evalFieldChain(dot, dot, field, field.Ident, args, final)
  }
  
  func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
  	s.at(chain)
  	if len(chain.Field) == 0 {
  		s.errorf("internal error: no fields in evalChainNode")
  	}
  	if chain.Node.Type() == parse.NodeNil {
  		s.errorf("indirection through explicit nil in %s", chain)
  	}
  	// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
  	pipe := s.evalArg(dot, nil, chain.Node)
  	return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
  }
  
  func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
  	// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
  	s.at(variable)
  	value := s.varValue(variable.Ident[0])
  	if len(variable.Ident) == 1 {
  		s.notAFunction(args, final)
  		return value
  	}
  	return s.evalFieldChain(dot, value, variable, variable.Ident[1:], args, final)
  }
  
  // evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
  // dot is the environment in which to evaluate arguments, while
  // receiver is the value being walked along the chain.
  func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
  	n := len(ident)
  	for i := 0; i < n-1; i++ {
  		receiver = s.evalField(dot, ident[i], node, nil, zero, receiver)
  	}
  	// Now if it's a method, it gets the arguments.
  	return s.evalField(dot, ident[n-1], node, args, final, receiver)
  }
  
  func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
  	s.at(node)
  	name := node.Ident
  	function, ok := findFunction(name, s.tmpl)
  	if !ok {
  		s.errorf("%q is not a defined function", name)
  	}
  	return s.evalCall(dot, function, cmd, name, args, final)
  }
  
  // evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
  // The 'final' argument represents the return value from the preceding
  // value of the pipeline, if any.
  func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
  	if !receiver.IsValid() {
  		if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
  			s.errorf("nil data; no entry for key %q", fieldName)
  		}
  		return zero
  	}
  	typ := receiver.Type()
  	receiver, isNil := indirect(receiver)
  	// Unless it's an interface, need to get to a value of type *T to guarantee
  	// we see all methods of T and *T.
  	ptr := receiver
  	if ptr.Kind() != reflect.Interface && ptr.CanAddr() {
  		ptr = ptr.Addr()
  	}
  	if method := ptr.MethodByName(fieldName); method.IsValid() {
  		return s.evalCall(dot, method, node, fieldName, args, final)
  	}
  	hasArgs := len(args) > 1 || final.IsValid()
  	// It's not a method; must be a field of a struct or an element of a map.
  	switch receiver.Kind() {
  	case reflect.Struct:
  		tField, ok := receiver.Type().FieldByName(fieldName)
  		if ok {
  			if isNil {
  				s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
  			}
  			field := receiver.FieldByIndex(tField.Index)
  			if tField.PkgPath != "" { // field is unexported
  				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
  			}
  			// If it's a function, we must call it.
  			if hasArgs {
  				s.errorf("%s has arguments but cannot be invoked as function", fieldName)
  			}
  			return field
  		}
  	case reflect.Map:
  		if isNil {
  			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
  		}
  		// If it's a map, attempt to use the field name as a key.
  		nameVal := reflect.ValueOf(fieldName)
  		if nameVal.Type().AssignableTo(receiver.Type().Key()) {
  			if hasArgs {
  				s.errorf("%s is not a method but has arguments", fieldName)
  			}
  			result := receiver.MapIndex(nameVal)
  			if !result.IsValid() {
  				switch s.tmpl.option.missingKey {
  				case mapInvalid:
  					// Just use the invalid value.
  				case mapZeroValue:
  					result = reflect.Zero(receiver.Type().Elem())
  				case mapError:
  					s.errorf("map has no entry for key %q", fieldName)
  				}
  			}
  			return result
  		}
  	}
  	s.errorf("can't evaluate field %s in type %s", fieldName, typ)
  	panic("not reached")
  }
  
  var (
  	errorType        = reflect.TypeOf((*error)(nil)).Elem()
  	fmtStringerType  = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
  	reflectValueType = reflect.TypeOf((*reflect.Value)(nil)).Elem()
  )
  
  // evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
  // it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
  // as the function itself.
  func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
  	if args != nil {
  		args = args[1:] // Zeroth arg is function name/node; not passed to function.
  	}
  	typ := fun.Type()
  	numIn := len(args)
  	if final.IsValid() {
  		numIn++
  	}
  	numFixed := len(args)
  	if typ.IsVariadic() {
  		numFixed = typ.NumIn() - 1 // last arg is the variadic one.
  		if numIn < numFixed {
  			s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
  		}
  	} else if numIn < typ.NumIn()-1 || !typ.IsVariadic() && numIn != typ.NumIn() {
  		s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), len(args))
  	}
  	if !goodFunc(typ) {
  		// TODO: This could still be a confusing error; maybe goodFunc should provide info.
  		s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
  	}
  	// Build the arg list.
  	argv := make([]reflect.Value, numIn)
  	// Args must be evaluated. Fixed args first.
  	i := 0
  	for ; i < numFixed && i < len(args); i++ {
  		argv[i] = s.evalArg(dot, typ.In(i), args[i])
  	}
  	// Now the ... args.
  	if typ.IsVariadic() {
  		argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
  		for ; i < len(args); i++ {
  			argv[i] = s.evalArg(dot, argType, args[i])
  		}
  	}
  	// Add final value if necessary.
  	if final.IsValid() {
  		t := typ.In(typ.NumIn() - 1)
  		if typ.IsVariadic() {
  			if numIn-1 < numFixed {
  				// The added final argument corresponds to a fixed parameter of the function.
  				// Validate against the type of the actual parameter.
  				t = typ.In(numIn - 1)
  			} else {
  				// The added final argument corresponds to the variadic part.
  				// Validate against the type of the elements of the variadic slice.
  				t = t.Elem()
  			}
  		}
  		argv[i] = s.validateType(final, t)
  	}
  	result := fun.Call(argv)
  	// If we have an error that is not nil, stop execution and return that error to the caller.
  	if len(result) == 2 && !result[1].IsNil() {
  		s.at(node)
  		s.errorf("error calling %s: %s", name, result[1].Interface().(error))
  	}
  	v := result[0]
  	if v.Type() == reflectValueType {
  		v = v.Interface().(reflect.Value)
  	}
  	return v
  }
  
  // canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
  func canBeNil(typ reflect.Type) bool {
  	switch typ.Kind() {
  	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
  		return true
  	case reflect.Struct:
  		return typ == reflectValueType
  	}
  	return false
  }
  
  // validateType guarantees that the value is valid and assignable to the type.
  func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
  	if !value.IsValid() {
  		if typ == nil || canBeNil(typ) {
  			// An untyped nil interface{}. Accept as a proper nil value.
  			return reflect.Zero(typ)
  		}
  		s.errorf("invalid value; expected %s", typ)
  	}
  	if typ == reflectValueType && value.Type() != typ {
  		return reflect.ValueOf(value)
  	}
  	if typ != nil && !value.Type().AssignableTo(typ) {
  		if value.Kind() == reflect.Interface && !value.IsNil() {
  			value = value.Elem()
  			if value.Type().AssignableTo(typ) {
  				return value
  			}
  			// fallthrough
  		}
  		// Does one dereference or indirection work? We could do more, as we
  		// do with method receivers, but that gets messy and method receivers
  		// are much more constrained, so it makes more sense there than here.
  		// Besides, one is almost always all you need.
  		switch {
  		case value.Kind() == reflect.Ptr && value.Type().Elem().AssignableTo(typ):
  			value = value.Elem()
  			if !value.IsValid() {
  				s.errorf("dereference of nil pointer of type %s", typ)
  			}
  		case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
  			value = value.Addr()
  		default:
  			s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
  		}
  	}
  	return value
  }
  
  func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
  	s.at(n)
  	switch arg := n.(type) {
  	case *parse.DotNode:
  		return s.validateType(dot, typ)
  	case *parse.NilNode:
  		if canBeNil(typ) {
  			return reflect.Zero(typ)
  		}
  		s.errorf("cannot assign nil to %s", typ)
  	case *parse.FieldNode:
  		return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, zero), typ)
  	case *parse.VariableNode:
  		return s.validateType(s.evalVariableNode(dot, arg, nil, zero), typ)
  	case *parse.PipeNode:
  		return s.validateType(s.evalPipeline(dot, arg), typ)
  	case *parse.IdentifierNode:
  		return s.validateType(s.evalFunction(dot, arg, arg, nil, zero), typ)
  	case *parse.ChainNode:
  		return s.validateType(s.evalChainNode(dot, arg, nil, zero), typ)
  	}
  	switch typ.Kind() {
  	case reflect.Bool:
  		return s.evalBool(typ, n)
  	case reflect.Complex64, reflect.Complex128:
  		return s.evalComplex(typ, n)
  	case reflect.Float32, reflect.Float64:
  		return s.evalFloat(typ, n)
  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  		return s.evalInteger(typ, n)
  	case reflect.Interface:
  		if typ.NumMethod() == 0 {
  			return s.evalEmptyInterface(dot, n)
  		}
  	case reflect.Struct:
  		if typ == reflectValueType {
  			return reflect.ValueOf(s.evalEmptyInterface(dot, n))
  		}
  	case reflect.String:
  		return s.evalString(typ, n)
  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  		return s.evalUnsignedInteger(typ, n)
  	}
  	s.errorf("can't handle %s for arg of type %s", n, typ)
  	panic("not reached")
  }
  
  func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
  	s.at(n)
  	if n, ok := n.(*parse.BoolNode); ok {
  		value := reflect.New(typ).Elem()
  		value.SetBool(n.True)
  		return value
  	}
  	s.errorf("expected bool; found %s", n)
  	panic("not reached")
  }
  
  func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
  	s.at(n)
  	if n, ok := n.(*parse.StringNode); ok {
  		value := reflect.New(typ).Elem()
  		value.SetString(n.Text)
  		return value
  	}
  	s.errorf("expected string; found %s", n)
  	panic("not reached")
  }
  
  func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
  	s.at(n)
  	if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
  		value := reflect.New(typ).Elem()
  		value.SetInt(n.Int64)
  		return value
  	}
  	s.errorf("expected integer; found %s", n)
  	panic("not reached")
  }
  
  func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
  	s.at(n)
  	if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
  		value := reflect.New(typ).Elem()
  		value.SetUint(n.Uint64)
  		return value
  	}
  	s.errorf("expected unsigned integer; found %s", n)
  	panic("not reached")
  }
  
  func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
  	s.at(n)
  	if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
  		value := reflect.New(typ).Elem()
  		value.SetFloat(n.Float64)
  		return value
  	}
  	s.errorf("expected float; found %s", n)
  	panic("not reached")
  }
  
  func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
  	if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
  		value := reflect.New(typ).Elem()
  		value.SetComplex(n.Complex128)
  		return value
  	}
  	s.errorf("expected complex; found %s", n)
  	panic("not reached")
  }
  
  func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
  	s.at(n)
  	switch n := n.(type) {
  	case *parse.BoolNode:
  		return reflect.ValueOf(n.True)
  	case *parse.DotNode:
  		return dot
  	case *parse.FieldNode:
  		return s.evalFieldNode(dot, n, nil, zero)
  	case *parse.IdentifierNode:
  		return s.evalFunction(dot, n, n, nil, zero)
  	case *parse.NilNode:
  		// NilNode is handled in evalArg, the only place that calls here.
  		s.errorf("evalEmptyInterface: nil (can't happen)")
  	case *parse.NumberNode:
  		return s.idealConstant(n)
  	case *parse.StringNode:
  		return reflect.ValueOf(n.Text)
  	case *parse.VariableNode:
  		return s.evalVariableNode(dot, n, nil, zero)
  	case *parse.PipeNode:
  		return s.evalPipeline(dot, n)
  	}
  	s.errorf("can't handle assignment of %s to empty interface argument", n)
  	panic("not reached")
  }
  
  // indirect returns the item at the end of indirection, and a bool to indicate if it's nil.
  func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
  	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
  		if v.IsNil() {
  			return v, true
  		}
  	}
  	return v, false
  }
  
  // indirectInterface returns the concrete value in an interface value,
  // or else the zero reflect.Value.
  // That is, if v represents the interface value x, the result is the same as reflect.ValueOf(x):
  // the fact that x was an interface value is forgotten.
  func indirectInterface(v reflect.Value) reflect.Value {
  	if v.Kind() != reflect.Interface {
  		return v
  	}
  	if v.IsNil() {
  		return reflect.Value{}
  	}
  	return v.Elem()
  }
  
  // printValue writes the textual representation of the value to the output of
  // the template.
  func (s *state) printValue(n parse.Node, v reflect.Value) {
  	s.at(n)
  	iface, ok := printableValue(v)
  	if !ok {
  		s.errorf("can't print %s of type %s", n, v.Type())
  	}
  	_, err := fmt.Fprint(s.wr, iface)
  	if err != nil {
  		s.writeError(err)
  	}
  }
  
  // printableValue returns the, possibly indirected, interface value inside v that
  // is best for a call to formatted printer.
  func printableValue(v reflect.Value) (interface{}, bool) {
  	if v.Kind() == reflect.Ptr {
  		v, _ = indirect(v) // fmt.Fprint handles nil.
  	}
  	if !v.IsValid() {
  		return "<no value>", true
  	}
  
  	if !v.Type().Implements(errorType) && !v.Type().Implements(fmtStringerType) {
  		if v.CanAddr() && (reflect.PtrTo(v.Type()).Implements(errorType) || reflect.PtrTo(v.Type()).Implements(fmtStringerType)) {
  			v = v.Addr()
  		} else {
  			switch v.Kind() {
  			case reflect.Chan, reflect.Func:
  				return nil, false
  			}
  		}
  	}
  	return v.Interface(), true
  }
  
  // Types to help sort the keys in a map for reproducible output.
  
  type rvs []reflect.Value
  
  func (x rvs) Len() int      { return len(x) }
  func (x rvs) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
  
  type rvInts struct{ rvs }
  
  func (x rvInts) Less(i, j int) bool { return x.rvs[i].Int() < x.rvs[j].Int() }
  
  type rvUints struct{ rvs }
  
  func (x rvUints) Less(i, j int) bool { return x.rvs[i].Uint() < x.rvs[j].Uint() }
  
  type rvFloats struct{ rvs }
  
  func (x rvFloats) Less(i, j int) bool { return x.rvs[i].Float() < x.rvs[j].Float() }
  
  type rvStrings struct{ rvs }
  
  func (x rvStrings) Less(i, j int) bool { return x.rvs[i].String() < x.rvs[j].String() }
  
  // sortKeys sorts (if it can) the slice of reflect.Values, which is a slice of map keys.
  func sortKeys(v []reflect.Value) []reflect.Value {
  	if len(v) <= 1 {
  		return v
  	}
  	switch v[0].Kind() {
  	case reflect.Float32, reflect.Float64:
  		sort.Sort(rvFloats{v})
  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  		sort.Sort(rvInts{v})
  	case reflect.String:
  		sort.Sort(rvStrings{v})
  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  		sort.Sort(rvUints{v})
  	}
  	return v
  }
  

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