src/text/template/funcs.go - go - Git at Google

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package template

 import (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"net/url"
 	"reflect"
 	"strings"
 	"unicode"
 	"unicode/utf8"
 )

 // FuncMap is the type of the map defining the mapping from names to functions.
 // Each function must have either a single return value, or two return values of
 // which the second has type error. In that case, if the second (error)
 // return value evaluates to non-nil during execution, execution terminates and
 // Execute returns that error.
 //
 // When template execution invokes a function with an argument list, that list
 // must be assignable to the function's parameter types. Functions meant to
 // apply to arguments of arbitrary type can use parameters of type interface{} or
 // of type reflect.Value. Similarly, functions meant to return a result of arbitrary
 // type can return interface{} or reflect.Value.
 type FuncMap map[string]interface{}

 var builtins = FuncMap{
 	"and":      and,
 	"call":     call,
 	"html":     HTMLEscaper,
 	"index":    index,
 	"js":       JSEscaper,
 	"len":      length,
 	"not":      not,
 	"or":       or,
 	"print":    fmt.Sprint,
 	"printf":   fmt.Sprintf,
 	"println":  fmt.Sprintln,
 	"urlquery": URLQueryEscaper,

 	// Comparisons
 	"eq": eq, // ==
 	"ge": ge, // >=
 	"gt": gt, // >
 	"le": le, // <=
 	"lt": lt, // <
 	"ne": ne, // !=
 }

 var builtinFuncs = createValueFuncs(builtins)

 // createValueFuncs turns a FuncMap into a map[string]reflect.Value
 func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
 	m := make(map[string]reflect.Value)
 	addValueFuncs(m, funcMap)
 	return m
 }

 // addValueFuncs adds to values the functions in funcs, converting them to reflect.Values.
 func addValueFuncs(out map[string]reflect.Value, in FuncMap) {
 	for name, fn := range in {
 		if !goodName(name) {
 			panic(fmt.Errorf("function name %s is not a valid identifier", name))
 		}
 		v := reflect.ValueOf(fn)
 		if v.Kind() != reflect.Func {
 			panic("value for " + name + " not a function")
 		}
 		if !goodFunc(v.Type()) {
 			panic(fmt.Errorf("can't install method/function %q with %d results", name, v.Type().NumOut()))
 		}
 		out[name] = v
 	}
 }

 // addFuncs adds to values the functions in funcs. It does no checking of the input -
 // call addValueFuncs first.
 func addFuncs(out, in FuncMap) {
 	for name, fn := range in {
 		out[name] = fn
 	}
 }

 // goodFunc reports whether the function or method has the right result signature.
 func goodFunc(typ reflect.Type) bool {
 	// We allow functions with 1 result or 2 results where the second is an error.
 	switch {
 	case typ.NumOut() == 1:
 		return true
 	case typ.NumOut() == 2 && typ.Out(1) == errorType:
 		return true
 	}
 	return false
 }

 // goodName reports whether the function name is a valid identifier.
 func goodName(name string) bool {
 	if name == "" {
 		return false
 	}
 	for i, r := range name {
 		switch {
 		case r == '_':
 		case i == 0 && !unicode.IsLetter(r):
 			return false
 		case !unicode.IsLetter(r) && !unicode.IsDigit(r):
 			return false
 		}
 	}
 	return true
 }

 // findFunction looks for a function in the template, and global map.
 func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
 	if tmpl != nil && tmpl.common != nil {
 		tmpl.muFuncs.RLock()
 		defer tmpl.muFuncs.RUnlock()
 		if fn := tmpl.execFuncs[name]; fn.IsValid() {
 			return fn, true
 		}
 	}
 	if fn := builtinFuncs[name]; fn.IsValid() {
 		return fn, true
 	}
 	return reflect.Value{}, false
 }

 // prepareArg checks if value can be used as an argument of type argType, and
 // converts an invalid value to appropriate zero if possible.
 func prepareArg(value reflect.Value, argType reflect.Type) (reflect.Value, error) {
 	if !value.IsValid() {
 		if !canBeNil(argType) {
 			return reflect.Value{}, fmt.Errorf("value is nil; should be of type %s", argType)
 		}
 		value = reflect.Zero(argType)
 	}
 	if value.Type().AssignableTo(argType) {
 		return value, nil
 	}
 	if intLike(value.Kind()) && intLike(argType.Kind()) && value.Type().ConvertibleTo(argType) {
 		value = value.Convert(argType)
 		return value, nil
 	}
 	return reflect.Value{}, fmt.Errorf("value has type %s; should be %s", value.Type(), argType)
 }

 func intLike(typ reflect.Kind) bool {
 	switch typ {
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		return true
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 		return true
 	}
 	return false
 }

 // Indexing.

 // index returns the result of indexing its first argument by the following
 // arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each
 // indexed item must be a map, slice, or array.
 func index(item reflect.Value, indices ...reflect.Value) (reflect.Value, error) {
 	v := indirectInterface(item)
 	if !v.IsValid() {
 		return reflect.Value{}, fmt.Errorf("index of untyped nil")
 	}
 	for _, i := range indices {
 		index := indirectInterface(i)
 		var isNil bool
 		if v, isNil = indirect(v); isNil {
 			return reflect.Value{}, fmt.Errorf("index of nil pointer")
 		}
 		switch v.Kind() {
 		case reflect.Array, reflect.Slice, reflect.String:
 			var x int64
 			switch index.Kind() {
 			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 				x = index.Int()
 			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 				x = int64(index.Uint())
 			case reflect.Invalid:
 				return reflect.Value{}, fmt.Errorf("cannot index slice/array with nil")
 			default:
 				return reflect.Value{}, fmt.Errorf("cannot index slice/array with type %s", index.Type())
 			}
 			if x < 0 || x >= int64(v.Len()) {
 				return reflect.Value{}, fmt.Errorf("index out of range: %d", x)
 			}
 			v = v.Index(int(x))
 		case reflect.Map:
 			index, err := prepareArg(index, v.Type().Key())
 			if err != nil {
 				return reflect.Value{}, err
 			}
 			if x := v.MapIndex(index); x.IsValid() {
 				v = x
 			} else {
 				v = reflect.Zero(v.Type().Elem())
 			}
 		case reflect.Invalid:
 			// the loop holds invariant: v.IsValid()
 			panic("unreachable")
 		default:
 			return reflect.Value{}, fmt.Errorf("can't index item of type %s", v.Type())
 		}
 	}
 	return v, nil
 }

 // Length

 // length returns the length of the item, with an error if it has no defined length.
 func length(item interface{}) (int, error) {
 	v := reflect.ValueOf(item)
 	if !v.IsValid() {
 		return 0, fmt.Errorf("len of untyped nil")
 	}
 	v, isNil := indirect(v)
 	if isNil {
 		return 0, fmt.Errorf("len of nil pointer")
 	}
 	switch v.Kind() {
 	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
 		return v.Len(), nil
 	}
 	return 0, fmt.Errorf("len of type %s", v.Type())
 }

 // Function invocation

 // call returns the result of evaluating the first argument as a function.
 // The function must return 1 result, or 2 results, the second of which is an error.
 func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
 	v := indirectInterface(fn)
 	if !v.IsValid() {
 		return reflect.Value{}, fmt.Errorf("call of nil")
 	}
 	typ := v.Type()
 	if typ.Kind() != reflect.Func {
 		return reflect.Value{}, fmt.Errorf("non-function of type %s", typ)
 	}
 	if !goodFunc(typ) {
 		return reflect.Value{}, fmt.Errorf("function called with %d args; should be 1 or 2", typ.NumOut())
 	}
 	numIn := typ.NumIn()
 	var dddType reflect.Type
 	if typ.IsVariadic() {
 		if len(args) < numIn-1 {
 			return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want at least %d", len(args), numIn-1)
 		}
 		dddType = typ.In(numIn - 1).Elem()
 	} else {
 		if len(args) != numIn {
 			return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want %d", len(args), numIn)
 		}
 	}
 	argv := make([]reflect.Value, len(args))
 	for i, arg := range args {
 		value := indirectInterface(arg)
 		// Compute the expected type. Clumsy because of variadics.
 		var argType reflect.Type
 		if !typ.IsVariadic() || i < numIn-1 {
 			argType = typ.In(i)
 		} else {
 			argType = dddType
 		}

 		var err error
 		if argv[i], err = prepareArg(value, argType); err != nil {
 			return reflect.Value{}, fmt.Errorf("arg %d: %s", i, err)
 		}
 	}
 	result := v.Call(argv)
 	if len(result) == 2 && !result[1].IsNil() {
 		return result[0], result[1].Interface().(error)
 	}
 	return result[0], nil
 }

 // Boolean logic.

 func truth(arg reflect.Value) bool {
 	t, _ := isTrue(indirectInterface(arg))
 	return t
 }

 // and computes the Boolean AND of its arguments, returning
 // the first false argument it encounters, or the last argument.
 func and(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
 	if !truth(arg0) {
 		return arg0
 	}
 	for i := range args {
 		arg0 = args[i]
 		if !truth(arg0) {
 			break
 		}
 	}
 	return arg0
 }

 // or computes the Boolean OR of its arguments, returning
 // the first true argument it encounters, or the last argument.
 func or(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
 	if truth(arg0) {
 		return arg0
 	}
 	for i := range args {
 		arg0 = args[i]
 		if truth(arg0) {
 			break
 		}
 	}
 	return arg0
 }

 // not returns the Boolean negation of its argument.
 func not(arg reflect.Value) bool {
 	return !truth(arg)
 }

 // Comparison.

 // TODO: Perhaps allow comparison between signed and unsigned integers.

 var (
 	errBadComparisonType = errors.New("invalid type for comparison")
 	errBadComparison     = errors.New("incompatible types for comparison")
 	errNoComparison      = errors.New("missing argument for comparison")
 )

 type kind int

 const (
 	invalidKind kind = iota
 	boolKind
 	complexKind
 	intKind
 	floatKind
 	stringKind
 	uintKind
 )

 func basicKind(v reflect.Value) (kind, error) {
 	switch v.Kind() {
 	case reflect.Bool:
 		return boolKind, nil
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		return intKind, nil
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 		return uintKind, nil
 	case reflect.Float32, reflect.Float64:
 		return floatKind, nil
 	case reflect.Complex64, reflect.Complex128:
 		return complexKind, nil
 	case reflect.String:
 		return stringKind, nil
 	}
 	return invalidKind, errBadComparisonType
 }

 // eq evaluates the comparison a == b || a == c || ...
 func eq(arg1 reflect.Value, arg2 ...reflect.Value) (bool, error) {
 	v1 := indirectInterface(arg1)
 	k1, err := basicKind(v1)
 	if err != nil {
 		return false, err
 	}
 	if len(arg2) == 0 {
 		return false, errNoComparison
 	}
 	for _, arg := range arg2 {
 		v2 := indirectInterface(arg)
 		k2, err := basicKind(v2)
 		if err != nil {
 			return false, err
 		}
 		truth := false
 		if k1 != k2 {
 			// Special case: Can compare integer values regardless of type's sign.
 			switch {
 			case k1 == intKind && k2 == uintKind:
 				truth = v1.Int() >= 0 && uint64(v1.Int()) == v2.Uint()
 			case k1 == uintKind && k2 == intKind:
 				truth = v2.Int() >= 0 && v1.Uint() == uint64(v2.Int())
 			default:
 				return false, errBadComparison
 			}
 		} else {
 			switch k1 {
 			case boolKind:
 				truth = v1.Bool() == v2.Bool()
 			case complexKind:
 				truth = v1.Complex() == v2.Complex()
 			case floatKind:
 				truth = v1.Float() == v2.Float()
 			case intKind:
 				truth = v1.Int() == v2.Int()
 			case stringKind:
 				truth = v1.String() == v2.String()
 			case uintKind:
 				truth = v1.Uint() == v2.Uint()
 			default:
 				panic("invalid kind")
 			}
 		}
 		if truth {
 			return true, nil
 		}
 	}
 	return false, nil
 }

 // ne evaluates the comparison a != b.
 func ne(arg1, arg2 reflect.Value) (bool, error) {
 	// != is the inverse of ==.
 	equal, err := eq(arg1, arg2)
 	return !equal, err
 }

 // lt evaluates the comparison a < b.
 func lt(arg1, arg2 reflect.Value) (bool, error) {
 	v1 := indirectInterface(arg1)
 	k1, err := basicKind(v1)
 	if err != nil {
 		return false, err
 	}
 	v2 := indirectInterface(arg2)
 	k2, err := basicKind(v2)
 	if err != nil {
 		return false, err
 	}
 	truth := false
 	if k1 != k2 {
 		// Special case: Can compare integer values regardless of type's sign.
 		switch {
 		case k1 == intKind && k2 == uintKind:
 			truth = v1.Int() < 0 || uint64(v1.Int()) < v2.Uint()
 		case k1 == uintKind && k2 == intKind:
 			truth = v2.Int() >= 0 && v1.Uint() < uint64(v2.Int())
 		default:
 			return false, errBadComparison
 		}
 	} else {
 		switch k1 {
 		case boolKind, complexKind:
 			return false, errBadComparisonType
 		case floatKind:
 			truth = v1.Float() < v2.Float()
 		case intKind:
 			truth = v1.Int() < v2.Int()
 		case stringKind:
 			truth = v1.String() < v2.String()
 		case uintKind:
 			truth = v1.Uint() < v2.Uint()
 		default:
 			panic("invalid kind")
 		}
 	}
 	return truth, nil
 }

 // le evaluates the comparison <= b.
 func le(arg1, arg2 reflect.Value) (bool, error) {
 	// <= is < or ==.
 	lessThan, err := lt(arg1, arg2)
 	if lessThan || err != nil {
 		return lessThan, err
 	}
 	return eq(arg1, arg2)
 }

 // gt evaluates the comparison a > b.
 func gt(arg1, arg2 reflect.Value) (bool, error) {
 	// > is the inverse of <=.
 	lessOrEqual, err := le(arg1, arg2)
 	if err != nil {
 		return false, err
 	}
 	return !lessOrEqual, nil
 }

 // ge evaluates the comparison a >= b.
 func ge(arg1, arg2 reflect.Value) (bool, error) {
 	// >= is the inverse of <.
 	lessThan, err := lt(arg1, arg2)
 	if err != nil {
 		return false, err
 	}
 	return !lessThan, nil
 }

 // HTML escaping.

 var (
 	htmlQuot = []byte("&#34;") // shorter than "&quot;"
 	htmlApos = []byte("&#39;") // shorter than "&apos;" and apos was not in HTML until HTML5
 	htmlAmp  = []byte("&amp;")
 	htmlLt   = []byte("&lt;")
 	htmlGt   = []byte("&gt;")
 	htmlNull = []byte("\uFFFD")
 )

 // HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
 func HTMLEscape(w io.Writer, b []byte) {
 	last := 0
 	for i, c := range b {
 		var html []byte
 		switch c {
 		case '\000':
 			html = htmlNull
 		case '"':
 			html = htmlQuot
 		case '\'':
 			html = htmlApos
 		case '&':
 			html = htmlAmp
 		case '<':
 			html = htmlLt
 		case '>':
 			html = htmlGt
 		default:
 			continue
 		}
 		w.Write(b[last:i])
 		w.Write(html)
 		last = i + 1
 	}
 	w.Write(b[last:])
 }

 // HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
 func HTMLEscapeString(s string) string {
 	// Avoid allocation if we can.
 	if !strings.ContainsAny(s, "'\"&<>\000") {
 		return s
 	}
 	var b bytes.Buffer
 	HTMLEscape(&b, []byte(s))
 	return b.String()
 }

 // HTMLEscaper returns the escaped HTML equivalent of the textual
 // representation of its arguments.
 func HTMLEscaper(args ...interface{}) string {
 	return HTMLEscapeString(evalArgs(args))
 }

 // JavaScript escaping.

 var (
 	jsLowUni = []byte(`\u00`)
 	hex      = []byte("0123456789ABCDEF")

 	jsBackslash = []byte(`\\`)
 	jsApos      = []byte(`\'`)
 	jsQuot      = []byte(`\"`)
 	jsLt        = []byte(`\x3C`)
 	jsGt        = []byte(`\x3E`)
 )

 // JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
 func JSEscape(w io.Writer, b []byte) {
 	last := 0
 	for i := 0; i < len(b); i++ {
 		c := b[i]

 		if !jsIsSpecial(rune(c)) {
 			// fast path: nothing to do
 			continue
 		}
 		w.Write(b[last:i])

 		if c < utf8.RuneSelf {
 			// Quotes, slashes and angle brackets get quoted.
 			// Control characters get written as \u00XX.
 			switch c {
 			case '\\':
 				w.Write(jsBackslash)
 			case '\'':
 				w.Write(jsApos)
 			case '"':
 				w.Write(jsQuot)
 			case '<':
 				w.Write(jsLt)
 			case '>':
 				w.Write(jsGt)
 			default:
 				w.Write(jsLowUni)
 				t, b := c>>4, c&0x0f
 				w.Write(hex[t : t+1])
 				w.Write(hex[b : b+1])
 			}
 		} else {
 			// Unicode rune.
 			r, size := utf8.DecodeRune(b[i:])
 			if unicode.IsPrint(r) {
 				w.Write(b[i : i+size])
 			} else {
 				fmt.Fprintf(w, "\\u%04X", r)
 			}
 			i += size - 1
 		}
 		last = i + 1
 	}
 	w.Write(b[last:])
 }

 // JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
 func JSEscapeString(s string) string {
 	// Avoid allocation if we can.
 	if strings.IndexFunc(s, jsIsSpecial) < 0 {
 		return s
 	}
 	var b bytes.Buffer
 	JSEscape(&b, []byte(s))
 	return b.String()
 }

 func jsIsSpecial(r rune) bool {
 	switch r {
 	case '\\', '\'', '"', '<', '>':
 		return true
 	}
 	return r < ' ' || utf8.RuneSelf <= r
 }

 // JSEscaper returns the escaped JavaScript equivalent of the textual
 // representation of its arguments.
 func JSEscaper(args ...interface{}) string {
 	return JSEscapeString(evalArgs(args))
 }

 // URLQueryEscaper returns the escaped value of the textual representation of
 // its arguments in a form suitable for embedding in a URL query.
 func URLQueryEscaper(args ...interface{}) string {
 	return url.QueryEscape(evalArgs(args))
 }

 // evalArgs formats the list of arguments into a string. It is therefore equivalent to
 //	fmt.Sprint(args...)
 // except that each argument is indirected (if a pointer), as required,
 // using the same rules as the default string evaluation during template
 // execution.
 func evalArgs(args []interface{}) string {
 	ok := false
 	var s string
 	// Fast path for simple common case.
 	if len(args) == 1 {
 		s, ok = args[0].(string)
 	}
 	if !ok {
 		for i, arg := range args {
 			a, ok := printableValue(reflect.ValueOf(arg))
 			if ok {
 				args[i] = a
 			} // else let fmt do its thing
 		}
 		s = fmt.Sprint(args...)
 	}
 	return s
 }
	// Copyright 2011 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package template

	import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"net/url"
	"reflect"
	"strings"
	"unicode"
	"unicode/utf8"
	)

	// FuncMap is the type of the map defining the mapping from names to functions.
	// Each function must have either a single return value, or two return values of
	// which the second has type error. In that case, if the second (error)
	// return value evaluates to non-nil during execution, execution terminates and
	// Execute returns that error.
	//
	// When template execution invokes a function with an argument list, that list
	// must be assignable to the function's parameter types. Functions meant to
	// apply to arguments of arbitrary type can use parameters of type interface{} or
	// of type reflect.Value. Similarly, functions meant to return a result of arbitrary
	// type can return interface{} or reflect.Value.
	type FuncMap map[string]interface{}

	var builtins = FuncMap{
	"and": and,
	"call": call,
	"html": HTMLEscaper,
	"index": index,
	"js": JSEscaper,
	"len": length,
	"not": not,
	"or": or,
	"print": fmt.Sprint,
	"printf": fmt.Sprintf,
	"println": fmt.Sprintln,
	"urlquery": URLQueryEscaper,

	// Comparisons
	"eq": eq, // ==
	"ge": ge, // >=
	"gt": gt, // >
	"le": le, // <=
	"lt": lt, // <
	"ne": ne, // !=
	}

	var builtinFuncs = createValueFuncs(builtins)

	// createValueFuncs turns a FuncMap into a map[string]reflect.Value
	func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
	m := make(map[string]reflect.Value)
	addValueFuncs(m, funcMap)
	return m
	}

	// addValueFuncs adds to values the functions in funcs, converting them to reflect.Values.
	func addValueFuncs(out map[string]reflect.Value, in FuncMap) {
	for name, fn := range in {
	if !goodName(name) {
	panic(fmt.Errorf("function name %s is not a valid identifier", name))
	}
	v := reflect.ValueOf(fn)
	if v.Kind() != reflect.Func {
	panic("value for " + name + " not a function")
	}
	if !goodFunc(v.Type()) {
	panic(fmt.Errorf("can't install method/function %q with %d results", name, v.Type().NumOut()))
	}
	out[name] = v
	}
	}

	// addFuncs adds to values the functions in funcs. It does no checking of the input -
	// call addValueFuncs first.
	func addFuncs(out, in FuncMap) {
	for name, fn := range in {
	out[name] = fn
	}
	}

	// goodFunc reports whether the function or method has the right result signature.
	func goodFunc(typ reflect.Type) bool {
	// We allow functions with 1 result or 2 results where the second is an error.
	switch {
	case typ.NumOut() == 1:
	return true
	case typ.NumOut() == 2 && typ.Out(1) == errorType:
	return true
	}
	return false
	}

	// goodName reports whether the function name is a valid identifier.
	func goodName(name string) bool {
	if name == "" {
	return false
	}
	for i, r := range name {
	switch {
	case r == '_':
	case i == 0 && !unicode.IsLetter(r):
	return false
	case !unicode.IsLetter(r) && !unicode.IsDigit(r):
	return false
	}
	}
	return true
	}

	// findFunction looks for a function in the template, and global map.
	func findFunction(name string, tmpl *Template) (reflect.Value, bool) {
	if tmpl != nil && tmpl.common != nil {
	tmpl.muFuncs.RLock()
	defer tmpl.muFuncs.RUnlock()
	if fn := tmpl.execFuncs[name]; fn.IsValid() {
	return fn, true
	}
	}
	if fn := builtinFuncs[name]; fn.IsValid() {
	return fn, true
	}
	return reflect.Value{}, false
	}

	// prepareArg checks if value can be used as an argument of type argType, and
	// converts an invalid value to appropriate zero if possible.
	func prepareArg(value reflect.Value, argType reflect.Type) (reflect.Value, error) {
	if !value.IsValid() {
	if !canBeNil(argType) {
	return reflect.Value{}, fmt.Errorf("value is nil; should be of type %s", argType)
	}
	value = reflect.Zero(argType)
	}
	if value.Type().AssignableTo(argType) {
	return value, nil
	}
	if intLike(value.Kind()) && intLike(argType.Kind()) && value.Type().ConvertibleTo(argType) {
	value = value.Convert(argType)
	return value, nil
	}
	return reflect.Value{}, fmt.Errorf("value has type %s; should be %s", value.Type(), argType)
	}

	func intLike(typ reflect.Kind) bool {
	switch typ {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
	return true
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
	return true
	}
	return false
	}

	// Indexing.

	// index returns the result of indexing its first argument by the following
	// arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each
	// indexed item must be a map, slice, or array.
	func index(item reflect.Value, indices ...reflect.Value) (reflect.Value, error) {
	v := indirectInterface(item)
	if !v.IsValid() {
	return reflect.Value{}, fmt.Errorf("index of untyped nil")
	}
	for _, i := range indices {
	index := indirectInterface(i)
	var isNil bool
	if v, isNil = indirect(v); isNil {
	return reflect.Value{}, fmt.Errorf("index of nil pointer")
	}
	switch v.Kind() {
	case reflect.Array, reflect.Slice, reflect.String:
	var x int64
	switch index.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
	x = index.Int()
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
	x = int64(index.Uint())
	case reflect.Invalid:
	return reflect.Value{}, fmt.Errorf("cannot index slice/array with nil")
	default:
	return reflect.Value{}, fmt.Errorf("cannot index slice/array with type %s", index.Type())
	}
	if x < 0 \|\| x >= int64(v.Len()) {
	return reflect.Value{}, fmt.Errorf("index out of range: %d", x)
	}
	v = v.Index(int(x))
	case reflect.Map:
	index, err := prepareArg(index, v.Type().Key())
	if err != nil {
	return reflect.Value{}, err
	}
	if x := v.MapIndex(index); x.IsValid() {
	v = x
	} else {
	v = reflect.Zero(v.Type().Elem())
	}
	case reflect.Invalid:
	// the loop holds invariant: v.IsValid()
	panic("unreachable")
	default:
	return reflect.Value{}, fmt.Errorf("can't index item of type %s", v.Type())
	}
	}
	return v, nil
	}

	// Length

	// length returns the length of the item, with an error if it has no defined length.
	func length(item interface{}) (int, error) {
	v := reflect.ValueOf(item)
	if !v.IsValid() {
	return 0, fmt.Errorf("len of untyped nil")
	}
	v, isNil := indirect(v)
	if isNil {
	return 0, fmt.Errorf("len of nil pointer")
	}
	switch v.Kind() {
	case reflect.Array, reflect.Chan, reflect.Map, reflect.Slice, reflect.String:
	return v.Len(), nil
	}
	return 0, fmt.Errorf("len of type %s", v.Type())
	}

	// Function invocation

	// call returns the result of evaluating the first argument as a function.
	// The function must return 1 result, or 2 results, the second of which is an error.
	func call(fn reflect.Value, args ...reflect.Value) (reflect.Value, error) {
	v := indirectInterface(fn)
	if !v.IsValid() {
	return reflect.Value{}, fmt.Errorf("call of nil")
	}
	typ := v.Type()
	if typ.Kind() != reflect.Func {
	return reflect.Value{}, fmt.Errorf("non-function of type %s", typ)
	}
	if !goodFunc(typ) {
	return reflect.Value{}, fmt.Errorf("function called with %d args; should be 1 or 2", typ.NumOut())
	}
	numIn := typ.NumIn()
	var dddType reflect.Type
	if typ.IsVariadic() {
	if len(args) < numIn-1 {
	return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want at least %d", len(args), numIn-1)
	}
	dddType = typ.In(numIn - 1).Elem()
	} else {
	if len(args) != numIn {
	return reflect.Value{}, fmt.Errorf("wrong number of args: got %d want %d", len(args), numIn)
	}
	}
	argv := make([]reflect.Value, len(args))
	for i, arg := range args {
	value := indirectInterface(arg)
	// Compute the expected type. Clumsy because of variadics.
	var argType reflect.Type
	if !typ.IsVariadic() \|\| i < numIn-1 {
	argType = typ.In(i)
	} else {
	argType = dddType
	}

	var err error
	if argv[i], err = prepareArg(value, argType); err != nil {
	return reflect.Value{}, fmt.Errorf("arg %d: %s", i, err)
	}
	}
	result := v.Call(argv)
	if len(result) == 2 && !result[1].IsNil() {
	return result[0], result[1].Interface().(error)
	}
	return result[0], nil
	}

	// Boolean logic.

	func truth(arg reflect.Value) bool {
	t, _ := isTrue(indirectInterface(arg))
	return t
	}

	// and computes the Boolean AND of its arguments, returning
	// the first false argument it encounters, or the last argument.
	func and(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
	if !truth(arg0) {
	return arg0
	}
	for i := range args {
	arg0 = args[i]
	if !truth(arg0) {
	break
	}
	}
	return arg0
	}

	// or computes the Boolean OR of its arguments, returning
	// the first true argument it encounters, or the last argument.
	func or(arg0 reflect.Value, args ...reflect.Value) reflect.Value {
	if truth(arg0) {
	return arg0
	}
	for i := range args {
	arg0 = args[i]
	if truth(arg0) {
	break
	}
	}
	return arg0
	}

	// not returns the Boolean negation of its argument.
	func not(arg reflect.Value) bool {
	return !truth(arg)
	}

	// Comparison.

	// TODO: Perhaps allow comparison between signed and unsigned integers.

	var (
	errBadComparisonType = errors.New("invalid type for comparison")
	errBadComparison = errors.New("incompatible types for comparison")
	errNoComparison = errors.New("missing argument for comparison")
	)

	type kind int

	const (
	invalidKind kind = iota
	boolKind
	complexKind
	intKind
	floatKind
	stringKind
	uintKind
	)

	func basicKind(v reflect.Value) (kind, error) {
	switch v.Kind() {
	case reflect.Bool:
	return boolKind, nil
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
	return intKind, nil
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
	return uintKind, nil
	case reflect.Float32, reflect.Float64:
	return floatKind, nil
	case reflect.Complex64, reflect.Complex128:
	return complexKind, nil
	case reflect.String:
	return stringKind, nil
	}
	return invalidKind, errBadComparisonType
	}

	// eq evaluates the comparison a == b \|\| a == c \|\| ...
	func eq(arg1 reflect.Value, arg2 ...reflect.Value) (bool, error) {
	v1 := indirectInterface(arg1)
	k1, err := basicKind(v1)
	if err != nil {
	return false, err
	}
	if len(arg2) == 0 {
	return false, errNoComparison
	}
	for _, arg := range arg2 {
	v2 := indirectInterface(arg)
	k2, err := basicKind(v2)
	if err != nil {
	return false, err
	}
	truth := false
	if k1 != k2 {
	// Special case: Can compare integer values regardless of type's sign.
	switch {
	case k1 == intKind && k2 == uintKind:
	truth = v1.Int() >= 0 && uint64(v1.Int()) == v2.Uint()
	case k1 == uintKind && k2 == intKind:
	truth = v2.Int() >= 0 && v1.Uint() == uint64(v2.Int())
	default:
	return false, errBadComparison
	}
	} else {
	switch k1 {
	case boolKind:
	truth = v1.Bool() == v2.Bool()
	case complexKind:
	truth = v1.Complex() == v2.Complex()
	case floatKind:
	truth = v1.Float() == v2.Float()
	case intKind:
	truth = v1.Int() == v2.Int()
	case stringKind:
	truth = v1.String() == v2.String()
	case uintKind:
	truth = v1.Uint() == v2.Uint()
	default:
	panic("invalid kind")
	}
	}
	if truth {
	return true, nil
	}
	}
	return false, nil
	}

	// ne evaluates the comparison a != b.
	func ne(arg1, arg2 reflect.Value) (bool, error) {
	// != is the inverse of ==.
	equal, err := eq(arg1, arg2)
	return !equal, err
	}

	// lt evaluates the comparison a < b.
	func lt(arg1, arg2 reflect.Value) (bool, error) {
	v1 := indirectInterface(arg1)
	k1, err := basicKind(v1)
	if err != nil {
	return false, err
	}
	v2 := indirectInterface(arg2)
	k2, err := basicKind(v2)
	if err != nil {
	return false, err
	}
	truth := false
	if k1 != k2 {
	// Special case: Can compare integer values regardless of type's sign.
	switch {
	case k1 == intKind && k2 == uintKind:
	truth = v1.Int() < 0 \|\| uint64(v1.Int()) < v2.Uint()
	case k1 == uintKind && k2 == intKind:
	truth = v2.Int() >= 0 && v1.Uint() < uint64(v2.Int())
	default:
	return false, errBadComparison
	}
	} else {
	switch k1 {
	case boolKind, complexKind:
	return false, errBadComparisonType
	case floatKind:
	truth = v1.Float() < v2.Float()
	case intKind:
	truth = v1.Int() < v2.Int()
	case stringKind:
	truth = v1.String() < v2.String()
	case uintKind:
	truth = v1.Uint() < v2.Uint()
	default:
	panic("invalid kind")
	}
	}
	return truth, nil
	}

	// le evaluates the comparison <= b.
	func le(arg1, arg2 reflect.Value) (bool, error) {
	// <= is < or ==.
	lessThan, err := lt(arg1, arg2)
	if lessThan \|\| err != nil {
	return lessThan, err
	}
	return eq(arg1, arg2)
	}

	// gt evaluates the comparison a > b.
	func gt(arg1, arg2 reflect.Value) (bool, error) {
	// > is the inverse of <=.
	lessOrEqual, err := le(arg1, arg2)
	if err != nil {
	return false, err
	}
	return !lessOrEqual, nil
	}

	// ge evaluates the comparison a >= b.
	func ge(arg1, arg2 reflect.Value) (bool, error) {
	// >= is the inverse of <.
	lessThan, err := lt(arg1, arg2)
	if err != nil {
	return false, err
	}
	return !lessThan, nil
	}

	// HTML escaping.

	var (
	htmlQuot = []byte(""") // shorter than """
	htmlApos = []byte("'") // shorter than "'" and apos was not in HTML until HTML5
	htmlAmp = []byte("&")
	htmlLt = []byte("<")
	htmlGt = []byte(">")
	htmlNull = []byte("\uFFFD")
	)

	// HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
	func HTMLEscape(w io.Writer, b []byte) {
	last := 0
	for i, c := range b {
	var html []byte
	switch c {
	case '\000':
	html = htmlNull
	case '"':
	html = htmlQuot
	case '\'':
	html = htmlApos
	case '&':
	html = htmlAmp
	case '<':
	html = htmlLt
	case '>':
	html = htmlGt
	default:
	continue
	}
	w.Write(b[last:i])
	w.Write(html)
	last = i + 1
	}
	w.Write(b[last:])
	}

	// HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
	func HTMLEscapeString(s string) string {
	// Avoid allocation if we can.
	if !strings.ContainsAny(s, "'\"&<>\000") {
	return s
	}
	var b bytes.Buffer
	HTMLEscape(&b, []byte(s))
	return b.String()
	}

	// HTMLEscaper returns the escaped HTML equivalent of the textual
	// representation of its arguments.
	func HTMLEscaper(args ...interface{}) string {
	return HTMLEscapeString(evalArgs(args))
	}

	// JavaScript escaping.

	var (
	jsLowUni = []byte(`\u00`)
	hex = []byte("0123456789ABCDEF")

	jsBackslash = []byte(`\\`)
	jsApos = []byte(`\'`)
	jsQuot = []byte(`\"`)
	jsLt = []byte(`\x3C`)
	jsGt = []byte(`\x3E`)
	)

	// JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.
	func JSEscape(w io.Writer, b []byte) {
	last := 0
	for i := 0; i < len(b); i++ {
	c := b[i]

	if !jsIsSpecial(rune(c)) {
	// fast path: nothing to do
	continue
	}
	w.Write(b[last:i])

	if c < utf8.RuneSelf {
	// Quotes, slashes and angle brackets get quoted.
	// Control characters get written as \u00XX.
	switch c {
	case '\\':
	w.Write(jsBackslash)
	case '\'':
	w.Write(jsApos)
	case '"':
	w.Write(jsQuot)
	case '<':
	w.Write(jsLt)
	case '>':
	w.Write(jsGt)
	default:
	w.Write(jsLowUni)
	t, b := c>>4, c&0x0f
	w.Write(hex[t : t+1])
	w.Write(hex[b : b+1])
	}
	} else {
	// Unicode rune.
	r, size := utf8.DecodeRune(b[i:])
	if unicode.IsPrint(r) {
	w.Write(b[i : i+size])
	} else {
	fmt.Fprintf(w, "\\u%04X", r)
	}
	i += size - 1
	}
	last = i + 1
	}
	w.Write(b[last:])
	}

	// JSEscapeString returns the escaped JavaScript equivalent of the plain text data s.
	func JSEscapeString(s string) string {
	// Avoid allocation if we can.
	if strings.IndexFunc(s, jsIsSpecial) < 0 {
	return s
	}
	var b bytes.Buffer
	JSEscape(&b, []byte(s))
	return b.String()
	}

	func jsIsSpecial(r rune) bool {
	switch r {
	case '\\', '\'', '"', '<', '>':
	return true
	}
	return r < ' ' \|\| utf8.RuneSelf <= r
	}

	// JSEscaper returns the escaped JavaScript equivalent of the textual
	// representation of its arguments.
	func JSEscaper(args ...interface{}) string {
	return JSEscapeString(evalArgs(args))
	}

	// URLQueryEscaper returns the escaped value of the textual representation of
	// its arguments in a form suitable for embedding in a URL query.
	func URLQueryEscaper(args ...interface{}) string {
	return url.QueryEscape(evalArgs(args))
	}

	// evalArgs formats the list of arguments into a string. It is therefore equivalent to
	// fmt.Sprint(args...)
	// except that each argument is indirected (if a pointer), as required,
	// using the same rules as the default string evaluation during template
	// execution.
	func evalArgs(args []interface{}) string {
	ok := false
	var s string
	// Fast path for simple common case.
	if len(args) == 1 {
	s, ok = args[0].(string)
	}
	if !ok {
	for i, arg := range args {
	a, ok := printableValue(reflect.ValueOf(arg))
	if ok {
	args[i] = a
	} // else let fmt do its thing
	}
	s = fmt.Sprint(args...)
	}
	return s
	}