Source file src/text/template/exec.go

Documentation: text/template

     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  	"internal/fmtsort"
    11  	"io"
    12  	"reflect"
    13  	"runtime"
    14  	"strings"
    15  	"text/template/parse"
    16  )
    17  
    18  // maxExecDepth specifies the maximum stack depth of templates within
    19  // templates. This limit is only practically reached by accidentally
    20  // recursive template invocations. This limit allows us to return
    21  // an error instead of triggering a stack overflow.
    22  var maxExecDepth = initMaxExecDepth()
    23  
    24  func initMaxExecDepth() int {
    25  	if runtime.GOARCH == "wasm" {
    26  		return 1000
    27  	}
    28  	return 100000
    29  }
    30  
    31  // state represents the state of an execution. It's not part of the
    32  // template so that multiple executions of the same template
    33  // can execute in parallel.
    34  type state struct {
    35  	tmpl  *Template
    36  	wr    io.Writer
    37  	node  parse.Node // current node, for errors
    38  	vars  []variable // push-down stack of variable values.
    39  	depth int        // the height of the stack of executing templates.
    40  }
    41  
    42  // variable holds the dynamic value of a variable such as $, $x etc.
    43  type variable struct {
    44  	name  string
    45  	value reflect.Value
    46  }
    47  
    48  // push pushes a new variable on the stack.
    49  func (s *state) push(name string, value reflect.Value) {
    50  	s.vars = append(s.vars, variable{name, value})
    51  }
    52  
    53  // mark returns the length of the variable stack.
    54  func (s *state) mark() int {
    55  	return len(s.vars)
    56  }
    57  
    58  // pop pops the variable stack up to the mark.
    59  func (s *state) pop(mark int) {
    60  	s.vars = s.vars[0:mark]
    61  }
    62  
    63  // setVar overwrites the last declared variable with the given name.
    64  // Used by variable assignments.
    65  func (s *state) setVar(name string, value reflect.Value) {
    66  	for i := s.mark() - 1; i >= 0; i-- {
    67  		if s.vars[i].name == name {
    68  			s.vars[i].value = value
    69  			return
    70  		}
    71  	}
    72  	s.errorf("undefined variable: %s", name)
    73  }
    74  
    75  // setTopVar overwrites the top-nth variable on the stack. Used by range iterations.
    76  func (s *state) setTopVar(n int, value reflect.Value) {
    77  	s.vars[len(s.vars)-n].value = value
    78  }
    79  
    80  // varValue returns the value of the named variable.
    81  func (s *state) varValue(name string) reflect.Value {
    82  	for i := s.mark() - 1; i >= 0; i-- {
    83  		if s.vars[i].name == name {
    84  			return s.vars[i].value
    85  		}
    86  	}
    87  	s.errorf("undefined variable: %s", name)
    88  	return zero
    89  }
    90  
    91  var zero reflect.Value
    92  
    93  type missingValType struct{}
    94  
    95  var missingVal = reflect.ValueOf(missingValType{})
    96  
    97  // at marks the state to be on node n, for error reporting.
    98  func (s *state) at(node parse.Node) {
    99  	s.node = node
   100  }
   101  
   102  // doublePercent returns the string with %'s replaced by %%, if necessary,
   103  // so it can be used safely inside a Printf format string.
   104  func doublePercent(str string) string {
   105  	return strings.ReplaceAll(str, "%", "%%")
   106  }
   107  
   108  // TODO: It would be nice if ExecError was more broken down, but
   109  // the way ErrorContext embeds the template name makes the
   110  // processing too clumsy.
   111  
   112  // ExecError is the custom error type returned when Execute has an
   113  // error evaluating its template. (If a write error occurs, the actual
   114  // error is returned; it will not be of type ExecError.)
   115  type ExecError struct {
   116  	Name string // Name of template.
   117  	Err  error  // Pre-formatted error.
   118  }
   119  
   120  func (e ExecError) Error() string {
   121  	return e.Err.Error()
   122  }
   123  
   124  func (e ExecError) Unwrap() error {
   125  	return e.Err
   126  }
   127  
   128  // errorf records an ExecError and terminates processing.
   129  func (s *state) errorf(format string, args ...interface{}) {
   130  	name := doublePercent(s.tmpl.Name())
   131  	if s.node == nil {
   132  		format = fmt.Sprintf("template: %s: %s", name, format)
   133  	} else {
   134  		location, context := s.tmpl.ErrorContext(s.node)
   135  		format = fmt.Sprintf("template: %s: executing %q at <%s>: %s", location, name, doublePercent(context), format)
   136  	}
   137  	panic(ExecError{
   138  		Name: s.tmpl.Name(),
   139  		Err:  fmt.Errorf(format, args...),
   140  	})
   141  }
   142  
   143  // writeError is the wrapper type used internally when Execute has an
   144  // error writing to its output. We strip the wrapper in errRecover.
   145  // Note that this is not an implementation of error, so it cannot escape
   146  // from the package as an error value.
   147  type writeError struct {
   148  	Err error // Original error.
   149  }
   150  
   151  func (s *state) writeError(err error) {
   152  	panic(writeError{
   153  		Err: err,
   154  	})
   155  }
   156  
   157  // errRecover is the handler that turns panics into returns from the top
   158  // level of Parse.
   159  func errRecover(errp *error) {
   160  	e := recover()
   161  	if e != nil {
   162  		switch err := e.(type) {
   163  		case runtime.Error:
   164  			panic(e)
   165  		case writeError:
   166  			*errp = err.Err // Strip the wrapper.
   167  		case ExecError:
   168  			*errp = err // Keep the wrapper.
   169  		default:
   170  			panic(e)
   171  		}
   172  	}
   173  }
   174  
   175  // ExecuteTemplate applies the template associated with t that has the given name
   176  // to the specified data object and writes the output to wr.
   177  // If an error occurs executing the template or writing its output,
   178  // execution stops, but partial results may already have been written to
   179  // the output writer.
   180  // A template may be executed safely in parallel, although if parallel
   181  // executions share a Writer the output may be interleaved.
   182  func (t *Template) ExecuteTemplate(wr io.Writer, name string, data interface{}) error {
   183  	var tmpl *Template
   184  	if t.common != nil {
   185  		tmpl = t.tmpl[name]
   186  	}
   187  	if tmpl == nil {
   188  		return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
   189  	}
   190  	return tmpl.Execute(wr, data)
   191  }
   192  
   193  // Execute applies a parsed template to the specified data object,
   194  // and writes the output to wr.
   195  // If an error occurs executing the template or writing its output,
   196  // execution stops, but partial results may already have been written to
   197  // the output writer.
   198  // A template may be executed safely in parallel, although if parallel
   199  // executions share a Writer the output may be interleaved.
   200  //
   201  // If data is a reflect.Value, the template applies to the concrete
   202  // value that the reflect.Value holds, as in fmt.Print.
   203  func (t *Template) Execute(wr io.Writer, data interface{}) error {
   204  	return t.execute(wr, data)
   205  }
   206  
   207  func (t *Template) execute(wr io.Writer, data interface{}) (err error) {
   208  	defer errRecover(&err)
   209  	value, ok := data.(reflect.Value)
   210  	if !ok {
   211  		value = reflect.ValueOf(data)
   212  	}
   213  	state := &state{
   214  		tmpl: t,
   215  		wr:   wr,
   216  		vars: []variable{{"$", value}},
   217  	}
   218  	if t.Tree == nil || t.Root == nil {
   219  		state.errorf("%q is an incomplete or empty template", t.Name())
   220  	}
   221  	state.walk(value, t.Root)
   222  	return
   223  }
   224  
   225  // DefinedTemplates returns a string listing the defined templates,
   226  // prefixed by the string "; defined templates are: ". If there are none,
   227  // it returns the empty string. For generating an error message here
   228  // and in html/template.
   229  func (t *Template) DefinedTemplates() string {
   230  	if t.common == nil {
   231  		return ""
   232  	}
   233  	var b bytes.Buffer
   234  	for name, tmpl := range t.tmpl {
   235  		if tmpl.Tree == nil || tmpl.Root == nil {
   236  			continue
   237  		}
   238  		if b.Len() > 0 {
   239  			b.WriteString(", ")
   240  		}
   241  		fmt.Fprintf(&b, "%q", name)
   242  	}
   243  	var s string
   244  	if b.Len() > 0 {
   245  		s = "; defined templates are: " + b.String()
   246  	}
   247  	return s
   248  }
   249  
   250  // Walk functions step through the major pieces of the template structure,
   251  // generating output as they go.
   252  func (s *state) walk(dot reflect.Value, node parse.Node) {
   253  	s.at(node)
   254  	switch node := node.(type) {
   255  	case *parse.ActionNode:
   256  		// Do not pop variables so they persist until next end.
   257  		// Also, if the action declares variables, don't print the result.
   258  		val := s.evalPipeline(dot, node.Pipe)
   259  		if len(node.Pipe.Decl) == 0 {
   260  			s.printValue(node, val)
   261  		}
   262  	case *parse.IfNode:
   263  		s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
   264  	case *parse.ListNode:
   265  		for _, node := range node.Nodes {
   266  			s.walk(dot, node)
   267  		}
   268  	case *parse.RangeNode:
   269  		s.walkRange(dot, node)
   270  	case *parse.TemplateNode:
   271  		s.walkTemplate(dot, node)
   272  	case *parse.TextNode:
   273  		if _, err := s.wr.Write(node.Text); err != nil {
   274  			s.writeError(err)
   275  		}
   276  	case *parse.WithNode:
   277  		s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
   278  	default:
   279  		s.errorf("unknown node: %s", node)
   280  	}
   281  }
   282  
   283  // walkIfOrWith walks an 'if' or 'with' node. The two control structures
   284  // are identical in behavior except that 'with' sets dot.
   285  func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
   286  	defer s.pop(s.mark())
   287  	val := s.evalPipeline(dot, pipe)
   288  	truth, ok := isTrue(indirectInterface(val))
   289  	if !ok {
   290  		s.errorf("if/with can't use %v", val)
   291  	}
   292  	if truth {
   293  		if typ == parse.NodeWith {
   294  			s.walk(val, list)
   295  		} else {
   296  			s.walk(dot, list)
   297  		}
   298  	} else if elseList != nil {
   299  		s.walk(dot, elseList)
   300  	}
   301  }
   302  
   303  // IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
   304  // and whether the value has a meaningful truth value. This is the definition of
   305  // truth used by if and other such actions.
   306  func IsTrue(val interface{}) (truth, ok bool) {
   307  	return isTrue(reflect.ValueOf(val))
   308  }
   309  
   310  func isTrue(val reflect.Value) (truth, ok bool) {
   311  	if !val.IsValid() {
   312  		// Something like var x interface{}, never set. It's a form of nil.
   313  		return false, true
   314  	}
   315  	switch val.Kind() {
   316  	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
   317  		truth = val.Len() > 0
   318  	case reflect.Bool:
   319  		truth = val.Bool()
   320  	case reflect.Complex64, reflect.Complex128:
   321  		truth = val.Complex() != 0
   322  	case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Interface:
   323  		truth = !val.IsNil()
   324  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   325  		truth = val.Int() != 0
   326  	case reflect.Float32, reflect.Float64:
   327  		truth = val.Float() != 0
   328  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   329  		truth = val.Uint() != 0
   330  	case reflect.Struct:
   331  		truth = true // Struct values are always true.
   332  	default:
   333  		return
   334  	}
   335  	return truth, true
   336  }
   337  
   338  func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
   339  	s.at(r)
   340  	defer s.pop(s.mark())
   341  	val, _ := indirect(s.evalPipeline(dot, r.Pipe))
   342  	// mark top of stack before any variables in the body are pushed.
   343  	mark := s.mark()
   344  	oneIteration := func(index, elem reflect.Value) {
   345  		// Set top var (lexically the second if there are two) to the element.
   346  		if len(r.Pipe.Decl) > 0 {
   347  			s.setTopVar(1, elem)
   348  		}
   349  		// Set next var (lexically the first if there are two) to the index.
   350  		if len(r.Pipe.Decl) > 1 {
   351  			s.setTopVar(2, index)
   352  		}
   353  		s.walk(elem, r.List)
   354  		s.pop(mark)
   355  	}
   356  	switch val.Kind() {
   357  	case reflect.Array, reflect.Slice:
   358  		if val.Len() == 0 {
   359  			break
   360  		}
   361  		for i := 0; i < val.Len(); i++ {
   362  			oneIteration(reflect.ValueOf(i), val.Index(i))
   363  		}
   364  		return
   365  	case reflect.Map:
   366  		if val.Len() == 0 {
   367  			break
   368  		}
   369  		om := fmtsort.Sort(val)
   370  		for i, key := range om.Key {
   371  			oneIteration(key, om.Value[i])
   372  		}
   373  		return
   374  	case reflect.Chan:
   375  		if val.IsNil() {
   376  			break
   377  		}
   378  		i := 0
   379  		for ; ; i++ {
   380  			elem, ok := val.Recv()
   381  			if !ok {
   382  				break
   383  			}
   384  			oneIteration(reflect.ValueOf(i), elem)
   385  		}
   386  		if i == 0 {
   387  			break
   388  		}
   389  		return
   390  	case reflect.Invalid:
   391  		break // An invalid value is likely a nil map, etc. and acts like an empty map.
   392  	default:
   393  		s.errorf("range can't iterate over %v", val)
   394  	}
   395  	if r.ElseList != nil {
   396  		s.walk(dot, r.ElseList)
   397  	}
   398  }
   399  
   400  func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
   401  	s.at(t)
   402  	tmpl := s.tmpl.tmpl[t.Name]
   403  	if tmpl == nil {
   404  		s.errorf("template %q not defined", t.Name)
   405  	}
   406  	if s.depth == maxExecDepth {
   407  		s.errorf("exceeded maximum template depth (%v)", maxExecDepth)
   408  	}
   409  	// Variables declared by the pipeline persist.
   410  	dot = s.evalPipeline(dot, t.Pipe)
   411  	newState := *s
   412  	newState.depth++
   413  	newState.tmpl = tmpl
   414  	// No dynamic scoping: template invocations inherit no variables.
   415  	newState.vars = []variable{{"$", dot}}
   416  	newState.walk(dot, tmpl.Root)
   417  }
   418  
   419  // Eval functions evaluate pipelines, commands, and their elements and extract
   420  // values from the data structure by examining fields, calling methods, and so on.
   421  // The printing of those values happens only through walk functions.
   422  
   423  // evalPipeline returns the value acquired by evaluating a pipeline. If the
   424  // pipeline has a variable declaration, the variable will be pushed on the
   425  // stack. Callers should therefore pop the stack after they are finished
   426  // executing commands depending on the pipeline value.
   427  func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
   428  	if pipe == nil {
   429  		return
   430  	}
   431  	s.at(pipe)
   432  	value = missingVal
   433  	for _, cmd := range pipe.Cmds {
   434  		value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
   435  		// If the object has type interface{}, dig down one level to the thing inside.
   436  		if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
   437  			value = reflect.ValueOf(value.Interface()) // lovely!
   438  		}
   439  	}
   440  	for _, variable := range pipe.Decl {
   441  		if pipe.IsAssign {
   442  			s.setVar(variable.Ident[0], value)
   443  		} else {
   444  			s.push(variable.Ident[0], value)
   445  		}
   446  	}
   447  	return value
   448  }
   449  
   450  func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
   451  	if len(args) > 1 || final != missingVal {
   452  		s.errorf("can't give argument to non-function %s", args[0])
   453  	}
   454  }
   455  
   456  func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
   457  	firstWord := cmd.Args[0]
   458  	switch n := firstWord.(type) {
   459  	case *parse.FieldNode:
   460  		return s.evalFieldNode(dot, n, cmd.Args, final)
   461  	case *parse.ChainNode:
   462  		return s.evalChainNode(dot, n, cmd.Args, final)
   463  	case *parse.IdentifierNode:
   464  		// Must be a function.
   465  		return s.evalFunction(dot, n, cmd, cmd.Args, final)
   466  	case *parse.PipeNode:
   467  		// Parenthesized pipeline. The arguments are all inside the pipeline; final is ignored.
   468  		return s.evalPipeline(dot, n)
   469  	case *parse.VariableNode:
   470  		return s.evalVariableNode(dot, n, cmd.Args, final)
   471  	}
   472  	s.at(firstWord)
   473  	s.notAFunction(cmd.Args, final)
   474  	switch word := firstWord.(type) {
   475  	case *parse.BoolNode:
   476  		return reflect.ValueOf(word.True)
   477  	case *parse.DotNode:
   478  		return dot
   479  	case *parse.NilNode:
   480  		s.errorf("nil is not a command")
   481  	case *parse.NumberNode:
   482  		return s.idealConstant(word)
   483  	case *parse.StringNode:
   484  		return reflect.ValueOf(word.Text)
   485  	}
   486  	s.errorf("can't evaluate command %q", firstWord)
   487  	panic("not reached")
   488  }
   489  
   490  // idealConstant is called to return the value of a number in a context where
   491  // we don't know the type. In that case, the syntax of the number tells us
   492  // its type, and we use Go rules to resolve. Note there is no such thing as
   493  // a uint ideal constant in this situation - the value must be of int type.
   494  func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
   495  	// These are ideal constants but we don't know the type
   496  	// and we have no context.  (If it was a method argument,
   497  	// we'd know what we need.) The syntax guides us to some extent.
   498  	s.at(constant)
   499  	switch {
   500  	case constant.IsComplex:
   501  		return reflect.ValueOf(constant.Complex128) // incontrovertible.
   502  	case constant.IsFloat && !isHexInt(constant.Text) && strings.ContainsAny(constant.Text, ".eEpP"):
   503  		return reflect.ValueOf(constant.Float64)
   504  	case constant.IsInt:
   505  		n := int(constant.Int64)
   506  		if int64(n) != constant.Int64 {
   507  			s.errorf("%s overflows int", constant.Text)
   508  		}
   509  		return reflect.ValueOf(n)
   510  	case constant.IsUint:
   511  		s.errorf("%s overflows int", constant.Text)
   512  	}
   513  	return zero
   514  }
   515  
   516  func isHexInt(s string) bool {
   517  	return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X') && !strings.ContainsAny(s, "pP")
   518  }
   519  
   520  func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
   521  	s.at(field)
   522  	return s.evalFieldChain(dot, dot, field, field.Ident, args, final)
   523  }
   524  
   525  func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
   526  	s.at(chain)
   527  	if len(chain.Field) == 0 {
   528  		s.errorf("internal error: no fields in evalChainNode")
   529  	}
   530  	if chain.Node.Type() == parse.NodeNil {
   531  		s.errorf("indirection through explicit nil in %s", chain)
   532  	}
   533  	// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
   534  	pipe := s.evalArg(dot, nil, chain.Node)
   535  	return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
   536  }
   537  
   538  func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
   539  	// $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
   540  	s.at(variable)
   541  	value := s.varValue(variable.Ident[0])
   542  	if len(variable.Ident) == 1 {
   543  		s.notAFunction(args, final)
   544  		return value
   545  	}
   546  	return s.evalFieldChain(dot, value, variable, variable.Ident[1:], args, final)
   547  }
   548  
   549  // evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
   550  // dot is the environment in which to evaluate arguments, while
   551  // receiver is the value being walked along the chain.
   552  func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
   553  	n := len(ident)
   554  	for i := 0; i < n-1; i++ {
   555  		receiver = s.evalField(dot, ident[i], node, nil, missingVal, receiver)
   556  	}
   557  	// Now if it's a method, it gets the arguments.
   558  	return s.evalField(dot, ident[n-1], node, args, final, receiver)
   559  }
   560  
   561  func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
   562  	s.at(node)
   563  	name := node.Ident
   564  	function, ok := findFunction(name, s.tmpl)
   565  	if !ok {
   566  		s.errorf("%q is not a defined function", name)
   567  	}
   568  	return s.evalCall(dot, function, cmd, name, args, final)
   569  }
   570  
   571  // evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
   572  // The 'final' argument represents the return value from the preceding
   573  // value of the pipeline, if any.
   574  func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
   575  	if !receiver.IsValid() {
   576  		if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
   577  			s.errorf("nil data; no entry for key %q", fieldName)
   578  		}
   579  		return zero
   580  	}
   581  	typ := receiver.Type()
   582  	receiver, isNil := indirect(receiver)
   583  	if receiver.Kind() == reflect.Interface && isNil {
   584  		// Calling a method on a nil interface can't work. The
   585  		// MethodByName method call below would panic.
   586  		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
   587  		return zero
   588  	}
   589  
   590  	// Unless it's an interface, need to get to a value of type *T to guarantee
   591  	// we see all methods of T and *T.
   592  	ptr := receiver
   593  	if ptr.Kind() != reflect.Interface && ptr.Kind() != reflect.Ptr && ptr.CanAddr() {
   594  		ptr = ptr.Addr()
   595  	}
   596  	if method := ptr.MethodByName(fieldName); method.IsValid() {
   597  		return s.evalCall(dot, method, node, fieldName, args, final)
   598  	}
   599  	hasArgs := len(args) > 1 || final != missingVal
   600  	// It's not a method; must be a field of a struct or an element of a map.
   601  	switch receiver.Kind() {
   602  	case reflect.Struct:
   603  		tField, ok := receiver.Type().FieldByName(fieldName)
   604  		if ok {
   605  			field := receiver.FieldByIndex(tField.Index)
   606  			if tField.PkgPath != "" { // field is unexported
   607  				s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
   608  			}
   609  			// If it's a function, we must call it.
   610  			if hasArgs {
   611  				s.errorf("%s has arguments but cannot be invoked as function", fieldName)
   612  			}
   613  			return field
   614  		}
   615  	case reflect.Map:
   616  		// If it's a map, attempt to use the field name as a key.
   617  		nameVal := reflect.ValueOf(fieldName)
   618  		if nameVal.Type().AssignableTo(receiver.Type().Key()) {
   619  			if hasArgs {
   620  				s.errorf("%s is not a method but has arguments", fieldName)
   621  			}
   622  			result := receiver.MapIndex(nameVal)
   623  			if !result.IsValid() {
   624  				switch s.tmpl.option.missingKey {
   625  				case mapInvalid:
   626  					// Just use the invalid value.
   627  				case mapZeroValue:
   628  					result = reflect.Zero(receiver.Type().Elem())
   629  				case mapError:
   630  					s.errorf("map has no entry for key %q", fieldName)
   631  				}
   632  			}
   633  			return result
   634  		}
   635  	case reflect.Ptr:
   636  		etyp := receiver.Type().Elem()
   637  		if etyp.Kind() == reflect.Struct {
   638  			if _, ok := etyp.FieldByName(fieldName); !ok {
   639  				// If there's no such field, say "can't evaluate"
   640  				// instead of "nil pointer evaluating".
   641  				break
   642  			}
   643  		}
   644  		if isNil {
   645  			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
   646  		}
   647  	}
   648  	s.errorf("can't evaluate field %s in type %s", fieldName, typ)
   649  	panic("not reached")
   650  }
   651  
   652  var (
   653  	errorType        = reflect.TypeOf((*error)(nil)).Elem()
   654  	fmtStringerType  = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
   655  	reflectValueType = reflect.TypeOf((*reflect.Value)(nil)).Elem()
   656  )
   657  
   658  // evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
   659  // it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
   660  // as the function itself.
   661  func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
   662  	if args != nil {
   663  		args = args[1:] // Zeroth arg is function name/node; not passed to function.
   664  	}
   665  	typ := fun.Type()
   666  	numIn := len(args)
   667  	if final != missingVal {
   668  		numIn++
   669  	}
   670  	numFixed := len(args)
   671  	if typ.IsVariadic() {
   672  		numFixed = typ.NumIn() - 1 // last arg is the variadic one.
   673  		if numIn < numFixed {
   674  			s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
   675  		}
   676  	} else if numIn != typ.NumIn() {
   677  		s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), numIn)
   678  	}
   679  	if !goodFunc(typ) {
   680  		// TODO: This could still be a confusing error; maybe goodFunc should provide info.
   681  		s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
   682  	}
   683  	// Build the arg list.
   684  	argv := make([]reflect.Value, numIn)
   685  	// Args must be evaluated. Fixed args first.
   686  	i := 0
   687  	for ; i < numFixed && i < len(args); i++ {
   688  		argv[i] = s.evalArg(dot, typ.In(i), args[i])
   689  	}
   690  	// Now the ... args.
   691  	if typ.IsVariadic() {
   692  		argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
   693  		for ; i < len(args); i++ {
   694  			argv[i] = s.evalArg(dot, argType, args[i])
   695  		}
   696  	}
   697  	// Add final value if necessary.
   698  	if final != missingVal {
   699  		t := typ.In(typ.NumIn() - 1)
   700  		if typ.IsVariadic() {
   701  			if numIn-1 < numFixed {
   702  				// The added final argument corresponds to a fixed parameter of the function.
   703  				// Validate against the type of the actual parameter.
   704  				t = typ.In(numIn - 1)
   705  			} else {
   706  				// The added final argument corresponds to the variadic part.
   707  				// Validate against the type of the elements of the variadic slice.
   708  				t = t.Elem()
   709  			}
   710  		}
   711  		argv[i] = s.validateType(final, t)
   712  	}
   713  	v, err := safeCall(fun, argv)
   714  	// If we have an error that is not nil, stop execution and return that
   715  	// error to the caller.
   716  	if err != nil {
   717  		s.at(node)
   718  		s.errorf("error calling %s: %v", name, err)
   719  	}
   720  	if v.Type() == reflectValueType {
   721  		v = v.Interface().(reflect.Value)
   722  	}
   723  	return v
   724  }
   725  
   726  // canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
   727  func canBeNil(typ reflect.Type) bool {
   728  	switch typ.Kind() {
   729  	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
   730  		return true
   731  	case reflect.Struct:
   732  		return typ == reflectValueType
   733  	}
   734  	return false
   735  }
   736  
   737  // validateType guarantees that the value is valid and assignable to the type.
   738  func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
   739  	if !value.IsValid() {
   740  		if typ == nil {
   741  			// An untyped nil interface{}. Accept as a proper nil value.
   742  			return reflect.ValueOf(nil)
   743  		}
   744  		if canBeNil(typ) {
   745  			// Like above, but use the zero value of the non-nil type.
   746  			return reflect.Zero(typ)
   747  		}
   748  		s.errorf("invalid value; expected %s", typ)
   749  	}
   750  	if typ == reflectValueType && value.Type() != typ {
   751  		return reflect.ValueOf(value)
   752  	}
   753  	if typ != nil && !value.Type().AssignableTo(typ) {
   754  		if value.Kind() == reflect.Interface && !value.IsNil() {
   755  			value = value.Elem()
   756  			if value.Type().AssignableTo(typ) {
   757  				return value
   758  			}
   759  			// fallthrough
   760  		}
   761  		// Does one dereference or indirection work? We could do more, as we
   762  		// do with method receivers, but that gets messy and method receivers
   763  		// are much more constrained, so it makes more sense there than here.
   764  		// Besides, one is almost always all you need.
   765  		switch {
   766  		case value.Kind() == reflect.Ptr && value.Type().Elem().AssignableTo(typ):
   767  			value = value.Elem()
   768  			if !value.IsValid() {
   769  				s.errorf("dereference of nil pointer of type %s", typ)
   770  			}
   771  		case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
   772  			value = value.Addr()
   773  		default:
   774  			s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
   775  		}
   776  	}
   777  	return value
   778  }
   779  
   780  func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
   781  	s.at(n)
   782  	switch arg := n.(type) {
   783  	case *parse.DotNode:
   784  		return s.validateType(dot, typ)
   785  	case *parse.NilNode:
   786  		if canBeNil(typ) {
   787  			return reflect.Zero(typ)
   788  		}
   789  		s.errorf("cannot assign nil to %s", typ)
   790  	case *parse.FieldNode:
   791  		return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, missingVal), typ)
   792  	case *parse.VariableNode:
   793  		return s.validateType(s.evalVariableNode(dot, arg, nil, missingVal), typ)
   794  	case *parse.PipeNode:
   795  		return s.validateType(s.evalPipeline(dot, arg), typ)
   796  	case *parse.IdentifierNode:
   797  		return s.validateType(s.evalFunction(dot, arg, arg, nil, missingVal), typ)
   798  	case *parse.ChainNode:
   799  		return s.validateType(s.evalChainNode(dot, arg, nil, missingVal), typ)
   800  	}
   801  	switch typ.Kind() {
   802  	case reflect.Bool:
   803  		return s.evalBool(typ, n)
   804  	case reflect.Complex64, reflect.Complex128:
   805  		return s.evalComplex(typ, n)
   806  	case reflect.Float32, reflect.Float64:
   807  		return s.evalFloat(typ, n)
   808  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   809  		return s.evalInteger(typ, n)
   810  	case reflect.Interface:
   811  		if typ.NumMethod() == 0 {
   812  			return s.evalEmptyInterface(dot, n)
   813  		}
   814  	case reflect.Struct:
   815  		if typ == reflectValueType {
   816  			return reflect.ValueOf(s.evalEmptyInterface(dot, n))
   817  		}
   818  	case reflect.String:
   819  		return s.evalString(typ, n)
   820  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   821  		return s.evalUnsignedInteger(typ, n)
   822  	}
   823  	s.errorf("can't handle %s for arg of type %s", n, typ)
   824  	panic("not reached")
   825  }
   826  
   827  func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
   828  	s.at(n)
   829  	if n, ok := n.(*parse.BoolNode); ok {
   830  		value := reflect.New(typ).Elem()
   831  		value.SetBool(n.True)
   832  		return value
   833  	}
   834  	s.errorf("expected bool; found %s", n)
   835  	panic("not reached")
   836  }
   837  
   838  func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
   839  	s.at(n)
   840  	if n, ok := n.(*parse.StringNode); ok {
   841  		value := reflect.New(typ).Elem()
   842  		value.SetString(n.Text)
   843  		return value
   844  	}
   845  	s.errorf("expected string; found %s", n)
   846  	panic("not reached")
   847  }
   848  
   849  func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
   850  	s.at(n)
   851  	if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
   852  		value := reflect.New(typ).Elem()
   853  		value.SetInt(n.Int64)
   854  		return value
   855  	}
   856  	s.errorf("expected integer; found %s", n)
   857  	panic("not reached")
   858  }
   859  
   860  func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
   861  	s.at(n)
   862  	if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
   863  		value := reflect.New(typ).Elem()
   864  		value.SetUint(n.Uint64)
   865  		return value
   866  	}
   867  	s.errorf("expected unsigned integer; found %s", n)
   868  	panic("not reached")
   869  }
   870  
   871  func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
   872  	s.at(n)
   873  	if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
   874  		value := reflect.New(typ).Elem()
   875  		value.SetFloat(n.Float64)
   876  		return value
   877  	}
   878  	s.errorf("expected float; found %s", n)
   879  	panic("not reached")
   880  }
   881  
   882  func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
   883  	if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
   884  		value := reflect.New(typ).Elem()
   885  		value.SetComplex(n.Complex128)
   886  		return value
   887  	}
   888  	s.errorf("expected complex; found %s", n)
   889  	panic("not reached")
   890  }
   891  
   892  func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
   893  	s.at(n)
   894  	switch n := n.(type) {
   895  	case *parse.BoolNode:
   896  		return reflect.ValueOf(n.True)
   897  	case *parse.DotNode:
   898  		return dot
   899  	case *parse.FieldNode:
   900  		return s.evalFieldNode(dot, n, nil, missingVal)
   901  	case *parse.IdentifierNode:
   902  		return s.evalFunction(dot, n, n, nil, missingVal)
   903  	case *parse.NilNode:
   904  		// NilNode is handled in evalArg, the only place that calls here.
   905  		s.errorf("evalEmptyInterface: nil (can't happen)")
   906  	case *parse.NumberNode:
   907  		return s.idealConstant(n)
   908  	case *parse.StringNode:
   909  		return reflect.ValueOf(n.Text)
   910  	case *parse.VariableNode:
   911  		return s.evalVariableNode(dot, n, nil, missingVal)
   912  	case *parse.PipeNode:
   913  		return s.evalPipeline(dot, n)
   914  	}
   915  	s.errorf("can't handle assignment of %s to empty interface argument", n)
   916  	panic("not reached")
   917  }
   918  
   919  // indirect returns the item at the end of indirection, and a bool to indicate
   920  // if it's nil. If the returned bool is true, the returned value's kind will be
   921  // either a pointer or interface.
   922  func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
   923  	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
   924  		if v.IsNil() {
   925  			return v, true
   926  		}
   927  	}
   928  	return v, false
   929  }
   930  
   931  // indirectInterface returns the concrete value in an interface value,
   932  // or else the zero reflect.Value.
   933  // That is, if v represents the interface value x, the result is the same as reflect.ValueOf(x):
   934  // the fact that x was an interface value is forgotten.
   935  func indirectInterface(v reflect.Value) reflect.Value {
   936  	if v.Kind() != reflect.Interface {
   937  		return v
   938  	}
   939  	if v.IsNil() {
   940  		return reflect.Value{}
   941  	}
   942  	return v.Elem()
   943  }
   944  
   945  // printValue writes the textual representation of the value to the output of
   946  // the template.
   947  func (s *state) printValue(n parse.Node, v reflect.Value) {
   948  	s.at(n)
   949  	iface, ok := printableValue(v)
   950  	if !ok {
   951  		s.errorf("can't print %s of type %s", n, v.Type())
   952  	}
   953  	_, err := fmt.Fprint(s.wr, iface)
   954  	if err != nil {
   955  		s.writeError(err)
   956  	}
   957  }
   958  
   959  // printableValue returns the, possibly indirected, interface value inside v that
   960  // is best for a call to formatted printer.
   961  func printableValue(v reflect.Value) (interface{}, bool) {
   962  	if v.Kind() == reflect.Ptr {
   963  		v, _ = indirect(v) // fmt.Fprint handles nil.
   964  	}
   965  	if !v.IsValid() {
   966  		return "<no value>", true
   967  	}
   968  
   969  	if !v.Type().Implements(errorType) && !v.Type().Implements(fmtStringerType) {
   970  		if v.CanAddr() && (reflect.PtrTo(v.Type()).Implements(errorType) || reflect.PtrTo(v.Type()).Implements(fmtStringerType)) {
   971  			v = v.Addr()
   972  		} else {
   973  			switch v.Kind() {
   974  			case reflect.Chan, reflect.Func:
   975  				return nil, false
   976  			}
   977  		}
   978  	}
   979  	return v.Interface(), true
   980  }
   981  

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