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Source file src/pkg/text/template/exec.go

     1	// Copyright 2011 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 template
     6	
     7	import (
     8		"bytes"
     9		"fmt"
    10		"io"
    11		"reflect"
    12		"runtime"
    13		"sort"
    14		"strings"
    15		"text/template/parse"
    16	)
    17	
    18	// state represents the state of an execution. It's not part of the
    19	// template so that multiple executions of the same template
    20	// can execute in parallel.
    21	type state struct {
    22		tmpl *Template
    23		wr   io.Writer
    24		node parse.Node // current node, for errors
    25		vars []variable // push-down stack of variable values.
    26	}
    27	
    28	// variable holds the dynamic value of a variable such as $, $x etc.
    29	type variable struct {
    30		name  string
    31		value reflect.Value
    32	}
    33	
    34	// push pushes a new variable on the stack.
    35	func (s *state) push(name string, value reflect.Value) {
    36		s.vars = append(s.vars, variable{name, value})
    37	}
    38	
    39	// mark returns the length of the variable stack.
    40	func (s *state) mark() int {
    41		return len(s.vars)
    42	}
    43	
    44	// pop pops the variable stack up to the mark.
    45	func (s *state) pop(mark int) {
    46		s.vars = s.vars[0:mark]
    47	}
    48	
    49	// setVar overwrites the top-nth variable on the stack. Used by range iterations.
    50	func (s *state) setVar(n int, value reflect.Value) {
    51		s.vars[len(s.vars)-n].value = value
    52	}
    53	
    54	// varValue returns the value of the named variable.
    55	func (s *state) varValue(name string) reflect.Value {
    56		for i := s.mark() - 1; i >= 0; i-- {
    57			if s.vars[i].name == name {
    58				return s.vars[i].value
    59			}
    60		}
    61		s.errorf("undefined variable: %s", name)
    62		return zero
    63	}
    64	
    65	var zero reflect.Value
    66	
    67	// at marks the state to be on node n, for error reporting.
    68	func (s *state) at(node parse.Node) {
    69		s.node = node
    70	}
    71	
    72	// doublePercent returns the string with %'s replaced by %%, if necessary,
    73	// so it can be used safely inside a Printf format string.
    74	func doublePercent(str string) string {
    75		if strings.Contains(str, "%") {
    76			str = strings.Replace(str, "%", "%%", -1)
    77		}
    78		return str
    79	}
    80	
    81	// errorf formats the error and terminates processing.
    82	func (s *state) errorf(format string, args ...interface{}) {
    83		name := doublePercent(s.tmpl.Name())
    84		if s.node == nil {
    85			format = fmt.Sprintf("template: %s: %s", name, format)
    86		} else {
    87			location, context := s.tmpl.ErrorContext(s.node)
    88			format = fmt.Sprintf("template: %s: executing %q at <%s>: %s", location, name, doublePercent(context), format)
    89		}
    90		panic(fmt.Errorf(format, args...))
    91	}
    92	
    93	// errRecover is the handler that turns panics into returns from the top
    94	// level of Parse.
    95	func errRecover(errp *error) {
    96		e := recover()
    97		if e != nil {
    98			switch err := e.(type) {
    99			case runtime.Error:
   100				panic(e)
   101			case error:
   102				*errp = err
   103			default:
   104				panic(e)
   105			}
   106		}
   107	}
   108	
   109	// ExecuteTemplate applies the template associated with t that has the given name
   110	// to the specified data object and writes the output to wr.
   111	// If an error occurs executing the template or writing its output,
   112	// execution stops, but partial results may already have been written to
   113	// the output writer.
   114	// A template may be executed safely in parallel.
   115	func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
   116		tmpl := t.tmpl[name]
   117		if tmpl == nil {
   118			return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
   119		}
   120		return tmpl.Execute(wr, data)
   121	}
   122	
   123	// Execute applies a parsed template to the specified data object,
   124	// and writes the output to wr.
   125	// If an error occurs executing the template or writing its output,
   126	// execution stops, but partial results may already have been written to
   127	// the output writer.
   128	// A template may be executed safely in parallel.
   129	func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
   130		defer errRecover(&err)
   131		value := reflect.ValueOf(data)
   132		state := &state{
   133			tmpl: t,
   134			wr:   wr,
   135			vars: []variable{{"$", value}},
   136		}
   137		t.init()
   138		if t.Tree == nil || t.Root == nil {
   139			var b bytes.Buffer
   140			for name, tmpl := range t.tmpl {
   141				if tmpl.Tree == nil || tmpl.Root == nil {
   142					continue
   143				}
   144				if b.Len() > 0 {
   145					b.WriteString(", ")
   146				}
   147				fmt.Fprintf(&b, "%q", name)
   148			}
   149			var s string
   150			if b.Len() > 0 {
   151				s = "; defined templates are: " + b.String()
   152			}
   153			state.errorf("%q is an incomplete or empty template%s", t.Name(), s)
   154		}
   155		state.walk(value, t.Root)
   156		return
   157	}
   158	
   159	// Walk functions step through the major pieces of the template structure,
   160	// generating output as they go.
   161	func (s *state) walk(dot reflect.Value, node parse.Node) {
   162		s.at(node)
   163		switch node := node.(type) {
   164		case *parse.ActionNode:
   165			// Do not pop variables so they persist until next end.
   166			// Also, if the action declares variables, don't print the result.
   167			val := s.evalPipeline(dot, node.Pipe)
   168			if len(node.Pipe.Decl) == 0 {
   169				s.printValue(node, val)
   170			}
   171		case *parse.IfNode:
   172			s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
   173		case *parse.ListNode:
   174			for _, node := range node.Nodes {
   175				s.walk(dot, node)
   176			}
   177		case *parse.RangeNode:
   178			s.walkRange(dot, node)
   179		case *parse.TemplateNode:
   180			s.walkTemplate(dot, node)
   181		case *parse.TextNode:
   182			if _, err := s.wr.Write(node.Text); err != nil {
   183				s.errorf("%s", err)
   184			}
   185		case *parse.WithNode:
   186			s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
   187		default:
   188			s.errorf("unknown node: %s", node)
   189		}
   190	}
   191	
   192	// walkIfOrWith walks an 'if' or 'with' node. The two control structures
   193	// are identical in behavior except that 'with' sets dot.
   194	func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
   195		defer s.pop(s.mark())
   196		val := s.evalPipeline(dot, pipe)
   197		truth, ok := isTrue(val)
   198		if !ok {
   199			s.errorf("if/with can't use %v", val)
   200		}
   201		if truth {
   202			if typ == parse.NodeWith {
   203				s.walk(val, list)
   204			} else {
   205				s.walk(dot, list)
   206			}
   207		} else if elseList != nil {
   208			s.walk(dot, elseList)
   209		}
   210	}
   211	
   212	// isTrue reports whether the value is 'true', in the sense of not the zero of its type,
   213	// and whether the value has a meaningful truth value.
   214	func isTrue(val reflect.Value) (truth, ok bool) {
   215		if !val.IsValid() {
   216			// Something like var x interface{}, never set. It's a form of nil.
   217			return false, true
   218		}
   219		switch val.Kind() {
   220		case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
   221			truth = val.Len() > 0
   222		case reflect.Bool:
   223			truth = val.Bool()
   224		case reflect.Complex64, reflect.Complex128:
   225			truth = val.Complex() != 0
   226		case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Interface:
   227			truth = !val.IsNil()
   228		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   229			truth = val.Int() != 0
   230		case reflect.Float32, reflect.Float64:
   231			truth = val.Float() != 0
   232		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   233			truth = val.Uint() != 0
   234		case reflect.Struct:
   235			truth = true // Struct values are always true.
   236		default:
   237			return
   238		}
   239		return truth, true
   240	}
   241	
   242	func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
   243		s.at(r)
   244		defer s.pop(s.mark())
   245		val, _ := indirect(s.evalPipeline(dot, r.Pipe))
   246		// mark top of stack before any variables in the body are pushed.
   247		mark := s.mark()
   248		oneIteration := func(index, elem reflect.Value) {
   249			// Set top var (lexically the second if there are two) to the element.
   250			if len(r.Pipe.Decl) > 0 {
   251				s.setVar(1, elem)
   252			}
   253			// Set next var (lexically the first if there are two) to the index.
   254			if len(r.Pipe.Decl) > 1 {
   255				s.setVar(2, index)
   256			}
   257			s.walk(elem, r.List)
   258			s.pop(mark)
   259		}
   260		switch val.Kind() {
   261		case reflect.Array, reflect.Slice:
   262			if val.Len() == 0 {
   263				break
   264			}
   265			for i := 0; i < val.Len(); i++ {
   266				oneIteration(reflect.ValueOf(i), val.Index(i))
   267			}
   268			return
   269		case reflect.Map:
   270			if val.Len() == 0 {
   271				break
   272			}
   273			for _, key := range sortKeys(val.MapKeys()) {
   274				oneIteration(key, val.MapIndex(key))
   275			}
   276			return
   277		case reflect.Chan:
   278			if val.IsNil() {
   279				break
   280			}
   281			i := 0
   282			for ; ; i++ {
   283				elem, ok := val.Recv()
   284				if !ok {
   285					break
   286				}
   287				oneIteration(reflect.ValueOf(i), elem)
   288			}
   289			if i == 0 {
   290				break
   291			}
   292			return
   293		case reflect.Invalid:
   294			break // An invalid value is likely a nil map, etc. and acts like an empty map.
   295		default:
   296			s.errorf("range can't iterate over %v", val)
   297		}
   298		if r.ElseList != nil {
   299			s.walk(dot, r.ElseList)
   300		}
   301	}
   302	
   303	func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
   304		s.at(t)
   305		tmpl := s.tmpl.tmpl[t.Name]
   306		if tmpl == nil {
   307			s.errorf("template %q not defined", t.Name)
   308		}
   309		// Variables declared by the pipeline persist.
   310		dot = s.evalPipeline(dot, t.Pipe)
   311		newState := *s
   312		newState.tmpl = tmpl
   313		// No dynamic scoping: template invocations inherit no variables.
   314		newState.vars = []variable{{"$", dot}}
   315		newState.walk(dot, tmpl.Root)
   316	}
   317	
   318	// Eval functions evaluate pipelines, commands, and their elements and extract
   319	// values from the data structure by examining fields, calling methods, and so on.
   320	// The printing of those values happens only through walk functions.
   321	
   322	// evalPipeline returns the value acquired by evaluating a pipeline. If the
   323	// pipeline has a variable declaration, the variable will be pushed on the
   324	// stack. Callers should therefore pop the stack after they are finished
   325	// executing commands depending on the pipeline value.
   326	func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
   327		if pipe == nil {
   328			return
   329		}
   330		s.at(pipe)
   331		for _, cmd := range pipe.Cmds {
   332			value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
   333			// If the object has type interface{}, dig down one level to the thing inside.
   334			if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
   335				value = reflect.ValueOf(value.Interface()) // lovely!
   336			}
   337		}
   338		for _, variable := range pipe.Decl {
   339			s.push(variable.Ident[0], value)
   340		}
   341		return value
   342	}
   343	
   344	func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
   345		if len(args) > 1 || final.IsValid() {
   346			s.errorf("can't give argument to non-function %s", args[0])
   347		}
   348	}
   349	
   350	func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
   351		firstWord := cmd.Args[0]
   352		switch n := firstWord.(type) {
   353		case *parse.FieldNode:
   354			return s.evalFieldNode(dot, n, cmd.Args, final)
   355		case *parse.ChainNode:
   356			return s.evalChainNode(dot, n, cmd.Args, final)
   357		case *parse.IdentifierNode:
   358			// Must be a function.
   359			return s.evalFunction(dot, n, cmd, cmd.Args, final)
   360		case *parse.PipeNode:
   361			// Parenthesized pipeline. The arguments are all inside the pipeline; final is ignored.
   362			return s.evalPipeline(dot, n)
   363		case *parse.VariableNode:
   364			return s.evalVariableNode(dot, n, cmd.Args, final)
   365		}
   366		s.at(firstWord)
   367		s.notAFunction(cmd.Args, final)
   368		switch word := firstWord.(type) {
   369		case *parse.BoolNode:
   370			return reflect.ValueOf(word.True)
   371		case *parse.DotNode:
   372			return dot
   373		case *parse.NilNode:
   374			s.errorf("nil is not a command")
   375		case *parse.NumberNode:
   376			return s.idealConstant(word)
   377		case *parse.StringNode:
   378			return reflect.ValueOf(word.Text)
   379		}
   380		s.errorf("can't evaluate command %q", firstWord)
   381		panic("not reached")
   382	}
   383	
   384	// idealConstant is called to return the value of a number in a context where
   385	// we don't know the type. In that case, the syntax of the number tells us
   386	// its type, and we use Go rules to resolve.  Note there is no such thing as
   387	// a uint ideal constant in this situation - the value must be of int type.
   388	func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
   389		// These are ideal constants but we don't know the type
   390		// and we have no context.  (If it was a method argument,
   391		// we'd know what we need.) The syntax guides us to some extent.
   392		s.at(constant)
   393		switch {
   394		case constant.IsComplex:
   395			return reflect.ValueOf(constant.Complex128) // incontrovertible.
   396		case constant.IsFloat && strings.IndexAny(constant.Text, ".eE") >= 0:
   397			return reflect.ValueOf(constant.Float64)
   398		case constant.IsInt:
   399			n := int(constant.Int64)
   400			if int64(n) != constant.Int64 {
   401				s.errorf("%s overflows int", constant.Text)
   402			}
   403			return reflect.ValueOf(n)
   404		case constant.IsUint:
   405			s.errorf("%s overflows int", constant.Text)
   406		}
   407		return zero
   408	}
   409	
   410	func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
   411		s.at(field)
   412		return s.evalFieldChain(dot, dot, field, field.Ident, args, final)
   413	}
   414	
   415	func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
   416		s.at(chain)
   417		// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
   418		pipe := s.evalArg(dot, nil, chain.Node)
   419		if len(chain.Field) == 0 {
   420			s.errorf("internal error: no fields in evalChainNode")
   421		}
   422		return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
   423	}
   424	
   425	func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
   426		// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
   427		s.at(variable)
   428		value := s.varValue(variable.Ident[0])
   429		if len(variable.Ident) == 1 {
   430			s.notAFunction(args, final)
   431			return value
   432		}
   433		return s.evalFieldChain(dot, value, variable, variable.Ident[1:], args, final)
   434	}
   435	
   436	// evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
   437	// dot is the environment in which to evaluate arguments, while
   438	// receiver is the value being walked along the chain.
   439	func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
   440		n := len(ident)
   441		for i := 0; i < n-1; i++ {
   442			receiver = s.evalField(dot, ident[i], node, nil, zero, receiver)
   443		}
   444		// Now if it's a method, it gets the arguments.
   445		return s.evalField(dot, ident[n-1], node, args, final, receiver)
   446	}
   447	
   448	func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
   449		s.at(node)
   450		name := node.Ident
   451		function, ok := findFunction(name, s.tmpl)
   452		if !ok {
   453			s.errorf("%q is not a defined function", name)
   454		}
   455		return s.evalCall(dot, function, cmd, name, args, final)
   456	}
   457	
   458	// evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
   459	// The 'final' argument represents the return value from the preceding
   460	// value of the pipeline, if any.
   461	func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
   462		if !receiver.IsValid() {
   463			return zero
   464		}
   465		typ := receiver.Type()
   466		receiver, _ = indirect(receiver)
   467		// Unless it's an interface, need to get to a value of type *T to guarantee
   468		// we see all methods of T and *T.
   469		ptr := receiver
   470		if ptr.Kind() != reflect.Interface && ptr.CanAddr() {
   471			ptr = ptr.Addr()
   472		}
   473		if method := ptr.MethodByName(fieldName); method.IsValid() {
   474			return s.evalCall(dot, method, node, fieldName, args, final)
   475		}
   476		hasArgs := len(args) > 1 || final.IsValid()
   477		// It's not a method; must be a field of a struct or an element of a map. The receiver must not be nil.
   478		receiver, isNil := indirect(receiver)
   479		if isNil {
   480			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
   481		}
   482		switch receiver.Kind() {
   483		case reflect.Struct:
   484			tField, ok := receiver.Type().FieldByName(fieldName)
   485			if ok {
   486				field := receiver.FieldByIndex(tField.Index)
   487				if tField.PkgPath != "" { // field is unexported
   488					s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
   489				}
   490				// If it's a function, we must call it.
   491				if hasArgs {
   492					s.errorf("%s has arguments but cannot be invoked as function", fieldName)
   493				}
   494				return field
   495			}
   496			s.errorf("%s is not a field of struct type %s", fieldName, typ)
   497		case reflect.Map:
   498			// If it's a map, attempt to use the field name as a key.
   499			nameVal := reflect.ValueOf(fieldName)
   500			if nameVal.Type().AssignableTo(receiver.Type().Key()) {
   501				if hasArgs {
   502					s.errorf("%s is not a method but has arguments", fieldName)
   503				}
   504				return receiver.MapIndex(nameVal)
   505			}
   506		}
   507		s.errorf("can't evaluate field %s in type %s", fieldName, typ)
   508		panic("not reached")
   509	}
   510	
   511	var (
   512		errorType       = reflect.TypeOf((*error)(nil)).Elem()
   513		fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
   514	)
   515	
   516	// evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
   517	// it looks just like a function call.  The arg list, if non-nil, includes (in the manner of the shell), arg[0]
   518	// as the function itself.
   519	func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
   520		if args != nil {
   521			args = args[1:] // Zeroth arg is function name/node; not passed to function.
   522		}
   523		typ := fun.Type()
   524		numIn := len(args)
   525		if final.IsValid() {
   526			numIn++
   527		}
   528		numFixed := len(args)
   529		if typ.IsVariadic() {
   530			numFixed = typ.NumIn() - 1 // last arg is the variadic one.
   531			if numIn < numFixed {
   532				s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
   533			}
   534		} else if numIn < typ.NumIn()-1 || !typ.IsVariadic() && numIn != typ.NumIn() {
   535			s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), len(args))
   536		}
   537		if !goodFunc(typ) {
   538			// TODO: This could still be a confusing error; maybe goodFunc should provide info.
   539			s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
   540		}
   541		// Build the arg list.
   542		argv := make([]reflect.Value, numIn)
   543		// Args must be evaluated. Fixed args first.
   544		i := 0
   545		for ; i < numFixed; i++ {
   546			argv[i] = s.evalArg(dot, typ.In(i), args[i])
   547		}
   548		// Now the ... args.
   549		if typ.IsVariadic() {
   550			argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
   551			for ; i < len(args); i++ {
   552				argv[i] = s.evalArg(dot, argType, args[i])
   553			}
   554		}
   555		// Add final value if necessary.
   556		if final.IsValid() {
   557			t := typ.In(typ.NumIn() - 1)
   558			if typ.IsVariadic() {
   559				t = t.Elem()
   560			}
   561			argv[i] = s.validateType(final, t)
   562		}
   563		result := fun.Call(argv)
   564		// If we have an error that is not nil, stop execution and return that error to the caller.
   565		if len(result) == 2 && !result[1].IsNil() {
   566			s.at(node)
   567			s.errorf("error calling %s: %s", name, result[1].Interface().(error))
   568		}
   569		return result[0]
   570	}
   571	
   572	// canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
   573	func canBeNil(typ reflect.Type) bool {
   574		switch typ.Kind() {
   575		case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
   576			return true
   577		}
   578		return false
   579	}
   580	
   581	// validateType guarantees that the value is valid and assignable to the type.
   582	func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
   583		if !value.IsValid() {
   584			if typ == nil || canBeNil(typ) {
   585				// An untyped nil interface{}. Accept as a proper nil value.
   586				return reflect.Zero(typ)
   587			}
   588			s.errorf("invalid value; expected %s", typ)
   589		}
   590		if typ != nil && !value.Type().AssignableTo(typ) {
   591			if value.Kind() == reflect.Interface && !value.IsNil() {
   592				value = value.Elem()
   593				if value.Type().AssignableTo(typ) {
   594					return value
   595				}
   596				// fallthrough
   597			}
   598			// Does one dereference or indirection work? We could do more, as we
   599			// do with method receivers, but that gets messy and method receivers
   600			// are much more constrained, so it makes more sense there than here.
   601			// Besides, one is almost always all you need.
   602			switch {
   603			case value.Kind() == reflect.Ptr && value.Type().Elem().AssignableTo(typ):
   604				value = value.Elem()
   605				if !value.IsValid() {
   606					s.errorf("dereference of nil pointer of type %s", typ)
   607				}
   608			case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
   609				value = value.Addr()
   610			default:
   611				s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
   612			}
   613		}
   614		return value
   615	}
   616	
   617	func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
   618		s.at(n)
   619		switch arg := n.(type) {
   620		case *parse.DotNode:
   621			return s.validateType(dot, typ)
   622		case *parse.NilNode:
   623			if canBeNil(typ) {
   624				return reflect.Zero(typ)
   625			}
   626			s.errorf("cannot assign nil to %s", typ)
   627		case *parse.FieldNode:
   628			return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, zero), typ)
   629		case *parse.VariableNode:
   630			return s.validateType(s.evalVariableNode(dot, arg, nil, zero), typ)
   631		case *parse.PipeNode:
   632			return s.validateType(s.evalPipeline(dot, arg), typ)
   633		case *parse.IdentifierNode:
   634			return s.evalFunction(dot, arg, arg, nil, zero)
   635		}
   636		switch typ.Kind() {
   637		case reflect.Bool:
   638			return s.evalBool(typ, n)
   639		case reflect.Complex64, reflect.Complex128:
   640			return s.evalComplex(typ, n)
   641		case reflect.Float32, reflect.Float64:
   642			return s.evalFloat(typ, n)
   643		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   644			return s.evalInteger(typ, n)
   645		case reflect.Interface:
   646			if typ.NumMethod() == 0 {
   647				return s.evalEmptyInterface(dot, n)
   648			}
   649		case reflect.String:
   650			return s.evalString(typ, n)
   651		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   652			return s.evalUnsignedInteger(typ, n)
   653		}
   654		s.errorf("can't handle %s for arg of type %s", n, typ)
   655		panic("not reached")
   656	}
   657	
   658	func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
   659		s.at(n)
   660		if n, ok := n.(*parse.BoolNode); ok {
   661			value := reflect.New(typ).Elem()
   662			value.SetBool(n.True)
   663			return value
   664		}
   665		s.errorf("expected bool; found %s", n)
   666		panic("not reached")
   667	}
   668	
   669	func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
   670		s.at(n)
   671		if n, ok := n.(*parse.StringNode); ok {
   672			value := reflect.New(typ).Elem()
   673			value.SetString(n.Text)
   674			return value
   675		}
   676		s.errorf("expected string; found %s", n)
   677		panic("not reached")
   678	}
   679	
   680	func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
   681		s.at(n)
   682		if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
   683			value := reflect.New(typ).Elem()
   684			value.SetInt(n.Int64)
   685			return value
   686		}
   687		s.errorf("expected integer; found %s", n)
   688		panic("not reached")
   689	}
   690	
   691	func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
   692		s.at(n)
   693		if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
   694			value := reflect.New(typ).Elem()
   695			value.SetUint(n.Uint64)
   696			return value
   697		}
   698		s.errorf("expected unsigned integer; found %s", n)
   699		panic("not reached")
   700	}
   701	
   702	func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
   703		s.at(n)
   704		if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
   705			value := reflect.New(typ).Elem()
   706			value.SetFloat(n.Float64)
   707			return value
   708		}
   709		s.errorf("expected float; found %s", n)
   710		panic("not reached")
   711	}
   712	
   713	func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
   714		if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
   715			value := reflect.New(typ).Elem()
   716			value.SetComplex(n.Complex128)
   717			return value
   718		}
   719		s.errorf("expected complex; found %s", n)
   720		panic("not reached")
   721	}
   722	
   723	func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
   724		s.at(n)
   725		switch n := n.(type) {
   726		case *parse.BoolNode:
   727			return reflect.ValueOf(n.True)
   728		case *parse.DotNode:
   729			return dot
   730		case *parse.FieldNode:
   731			return s.evalFieldNode(dot, n, nil, zero)
   732		case *parse.IdentifierNode:
   733			return s.evalFunction(dot, n, n, nil, zero)
   734		case *parse.NilNode:
   735			// NilNode is handled in evalArg, the only place that calls here.
   736			s.errorf("evalEmptyInterface: nil (can't happen)")
   737		case *parse.NumberNode:
   738			return s.idealConstant(n)
   739		case *parse.StringNode:
   740			return reflect.ValueOf(n.Text)
   741		case *parse.VariableNode:
   742			return s.evalVariableNode(dot, n, nil, zero)
   743		case *parse.PipeNode:
   744			return s.evalPipeline(dot, n)
   745		}
   746		s.errorf("can't handle assignment of %s to empty interface argument", n)
   747		panic("not reached")
   748	}
   749	
   750	// indirect returns the item at the end of indirection, and a bool to indicate if it's nil.
   751	// We indirect through pointers and empty interfaces (only) because
   752	// non-empty interfaces have methods we might need.
   753	func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
   754		for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
   755			if v.IsNil() {
   756				return v, true
   757			}
   758			if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
   759				break
   760			}
   761		}
   762		return v, false
   763	}
   764	
   765	// printValue writes the textual representation of the value to the output of
   766	// the template.
   767	func (s *state) printValue(n parse.Node, v reflect.Value) {
   768		s.at(n)
   769		iface, ok := printableValue(v)
   770		if !ok {
   771			s.errorf("can't print %s of type %s", n, v.Type())
   772		}
   773		fmt.Fprint(s.wr, iface)
   774	}
   775	
   776	// printableValue returns the, possibly indirected, interface value inside v that
   777	// is best for a call to formatted printer.
   778	func printableValue(v reflect.Value) (interface{}, bool) {
   779		if v.Kind() == reflect.Ptr {
   780			v, _ = indirect(v) // fmt.Fprint handles nil.
   781		}
   782		if !v.IsValid() {
   783			return "<no value>", true
   784		}
   785	
   786		if !v.Type().Implements(errorType) && !v.Type().Implements(fmtStringerType) {
   787			if v.CanAddr() && (reflect.PtrTo(v.Type()).Implements(errorType) || reflect.PtrTo(v.Type()).Implements(fmtStringerType)) {
   788				v = v.Addr()
   789			} else {
   790				switch v.Kind() {
   791				case reflect.Chan, reflect.Func:
   792					return nil, false
   793				}
   794			}
   795		}
   796		return v.Interface(), true
   797	}
   798	
   799	// Types to help sort the keys in a map for reproducible output.
   800	
   801	type rvs []reflect.Value
   802	
   803	func (x rvs) Len() int      { return len(x) }
   804	func (x rvs) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
   805	
   806	type rvInts struct{ rvs }
   807	
   808	func (x rvInts) Less(i, j int) bool { return x.rvs[i].Int() < x.rvs[j].Int() }
   809	
   810	type rvUints struct{ rvs }
   811	
   812	func (x rvUints) Less(i, j int) bool { return x.rvs[i].Uint() < x.rvs[j].Uint() }
   813	
   814	type rvFloats struct{ rvs }
   815	
   816	func (x rvFloats) Less(i, j int) bool { return x.rvs[i].Float() < x.rvs[j].Float() }
   817	
   818	type rvStrings struct{ rvs }
   819	
   820	func (x rvStrings) Less(i, j int) bool { return x.rvs[i].String() < x.rvs[j].String() }
   821	
   822	// sortKeys sorts (if it can) the slice of reflect.Values, which is a slice of map keys.
   823	func sortKeys(v []reflect.Value) []reflect.Value {
   824		if len(v) <= 1 {
   825			return v
   826		}
   827		switch v[0].Kind() {
   828		case reflect.Float32, reflect.Float64:
   829			sort.Sort(rvFloats{v})
   830		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   831			sort.Sort(rvInts{v})
   832		case reflect.String:
   833			sort.Sort(rvStrings{v})
   834		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   835			sort.Sort(rvUints{v})
   836		}
   837		return v
   838	}
   839	

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