Source: exec.go in package text/template


package template

import (
	"fmt"
	"internal/fmtsort"
	"io"
	"reflect"
	"runtime"
	"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.

var maxExecDepth = initMaxExecDepth()

func initMaxExecDepth() int {
	if runtime.GOARCH == "wasm" {
		return 1000
	}
	return 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 last declared variable with the given name. Used by variable assignments.

func (s *state) setVar(name string, value reflect.Value) {
	for i := s.mark() - 1; i >= 0; i-- {
		if s.vars[i].name == name {
			s.vars[i].value = value
			return
		}
	}
	s.errorf("undefined variable: %s", name)
}

setTopVar overwrites the top-nth variable on the stack. Used by range iterations.

func (s *state) setTopVar(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

type missingValType struct{}

var missingVal = reflect.ValueOf(missingValType{})

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 {
	return strings.ReplaceAll(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()
}

func (e ExecError) Unwrap() error {
	return e.Err
}

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, although if parallel executions share a Writer the output may be interleaved.

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, although if parallel executions share a Writer the output may be interleaved. 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 strings.Builder
	for name, tmpl := range t.tmpl {
		if tmpl.Tree == nil || tmpl.Root == nil {
			continue
		}
		if b.Len() == 0 {
			b.WriteString("; defined templates are: ")
		} else {
			b.WriteString(", ")
		}
		fmt.Fprintf(&b, "%q", name)
	}
	return b.String()
}

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) {

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.CommentNode:
	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(indirectInterface(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) {

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())

mark top of stack before any variables in the body are pushed.

	mark := s.mark()

Set top var (lexically the second if there are two) to the element.

		if len(r.Pipe.Decl) > 0 {
			s.setTopVar(1, elem)

Set next var (lexically the first if there are two) to the index.

		if len(r.Pipe.Decl) > 1 {
			s.setTopVar(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
		}
		om := fmtsort.Sort(val)
		for i, key := range om.Key {
			oneIteration(key, om.Value[i])
		}
		return
	case reflect.Chan:
		if val.IsNil() {
			break
		}
		if val.Type().ChanDir() == reflect.SendDir {
			s.errorf("range over send-only channel %v", val)
			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++

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)
	value = missingVal
	for _, cmd := range pipe.Cmds {

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 {
		if pipe.IsAssign {
			s.setVar(variable.Ident[0], value)
		} else {
			s.push(variable.Ident[0], value)
		}
	}
	return value
}

func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
	if len(args) > 1 || final != missingVal {
		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)

Must be a function.

		return s.evalFunction(dot, n, cmd, cmd.Args, final)

Parenthesized pipeline. The arguments are all inside the pipeline; final must be absent.

		s.notAFunction(cmd.Args, final)
		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.

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 &&
		!isHexInt(constant.Text) && !isRuneInt(constant.Text) &&
		strings.ContainsAny(constant.Text, ".eEpP"):
		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 isRuneInt(s string) bool {
	return len(s) > 0 && s[0] == '\''
}

func isHexInt(s string) bool {
	return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X') && !strings.ContainsAny(s, "pP")
}

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)
}

$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, missingVal, 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)

Calling a method on a nil interface can't work. The MethodByName method call below would panic.

		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
		return zero
	}

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.Kind() != reflect.Ptr && ptr.CanAddr() {
		ptr = ptr.Addr()
	}
	if method := ptr.MethodByName(fieldName); method.IsValid() {
		return s.evalCall(dot, method, node, fieldName, args, final)
	}

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 {
			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
		}

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 {

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
		}
	case reflect.Ptr:
		etyp := receiver.Type().Elem()
		if etyp.Kind() == reflect.Struct {

If there's no such field, say "can't evaluate" instead of "nil pointer evaluating".

				break
			}
		}
		if isNil {
			s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
		}
	}
	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 != missingVal {
		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() {
		s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), numIn)
	}

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.

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 != missingVal {
		t := typ.In(typ.NumIn() - 1)
		if typ.IsVariadic() {

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)

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)
	}

If we have an error that is not nil, stop execution and return that error to the caller.

	if err != nil {
		s.at(node)
		s.errorf("error calling %s: %v", name, err)
	}
	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() {

An untyped nil interface{}. Accept as a proper nil value.

			return reflect.ValueOf(nil)
		}

Like above, but use the zero value of the non-nil type.

			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}, missingVal), typ)
	case *parse.VariableNode:
		return s.validateType(s.evalVariableNode(dot, arg, nil, missingVal), typ)
	case *parse.PipeNode:
		return s.validateType(s.evalPipeline(dot, arg), typ)
	case *parse.IdentifierNode:
		return s.validateType(s.evalFunction(dot, arg, arg, nil, missingVal), typ)
	case *parse.ChainNode:
		return s.validateType(s.evalChainNode(dot, arg, nil, missingVal), 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, missingVal)
	case *parse.IdentifierNode:
		return s.evalFunction(dot, n, n, nil, missingVal)

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, missingVal)
	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. If the returned bool is true, the returned value's kind will be either a pointer or interface.

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