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我们在使用go语言的模板的时候对于内置函数我们是可以直接在模板里面使用的， 如 html 他实际上是绑定的函数 func HTMLEscaper(args ...any) string。 另外模板里面的比较运算符，如eq等于 ne不等于 gt大于 lt小于等 他们实际上也是一个个对应的函数。

template 模板内置函数FuncMap 

golang里面的内置函数就是一个FuncMap, 其实就是一个这个类型（ map[string]any ）的map而已，    默认的内置函数如下

FuncMap{
		"and":      and,
		"call":     call,
		"html":     HTMLEscaper,
		"index":    index,
		"slice":    slice,
		"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, // !=
	}


// FuncMap的定义 type FuncMap map[string]any

golang默认内置函数html实现代码

这个函数应该是我们在使用go模板的时候使用最为频繁的一个函数了,他的功能实际上就是对我们传递的字符串进行html实体编码，以消除安全隐患，以下就是他的相关实现代码！

// 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 strings.Builder
	HTMLEscape(&b, []byte(s))
	return b.String()
}

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

golang默认内置函数call实现代码

这个内置函数 call 是一个比较经典的一个reflect 反射使用函数，他可以让你在模板里面调用一个必须具有1个或者2个返回值（第二个必须是error）的函数。

// 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) {
	fn = indirectInterface(fn)
	if !fn.IsValid() {
		return reflect.Value{}, fmt.Errorf("call of nil")
	}
	typ := fn.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 {
		arg = indirectInterface(arg)
		// Compute the expected type. Clumsy because of variadics.
		argType := dddType
		if !typ.IsVariadic() || i < numIn-1 {
			argType = typ.In(i)
		}

		var err error
		if argv[i], err = prepareArg(arg, argType); err != nil {
			return reflect.Value{}, fmt.Errorf("arg %d: %w", i, err)
		}
	}
	return safeCall(fn, argv)
}

// safeCall runs fun.Call(args), and returns the resulting value and error, if
// any. If the call panics, the panic value is returned as an error.
func safeCall(fun reflect.Value, args []reflect.Value) (val reflect.Value, err error) {
	defer func() {
		if r := recover(); r != nil {
			if e, ok := r.(error); ok {
				err = e
			} else {
				err = fmt.Errorf("%v", r)
			}
		}
	}()
	ret := fun.Call(args)
	if len(ret) == 2 && !ret[1].IsNil() {
		return ret[0], ret[1].Interface().(error)
	}
	return ret[0], nil
}

golang模板中的比较运算符实现代码

在go模板中，我们可以使用 以下6种比较运算符，他们分别是

eq  相等 ==
ne  不等于 !=
gt  大于 >
lt  小于 <
ge  大于等于 >=
le  小于等于 <=

他们在go模板里面实际上都是一些内置函数， 对应的实现代码如下：

// 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
}

// isNil returns true if v is the zero reflect.Value, or nil of its type.
func isNil(v reflect.Value) bool {
	if !v.IsValid() {
		return true
	}
	switch v.Kind() {
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
		return v.IsNil()
	}
	return false
}

// canCompare reports whether v1 and v2 are both the same kind, or one is nil.
// Called only when dealing with nillable types, or there's about to be an error.
func canCompare(v1, v2 reflect.Value) bool {
	k1 := v1.Kind()
	k2 := v2.Kind()
	if k1 == k2 {
		return true
	}
	// We know the type can be compared to nil.
	return k1 == reflect.Invalid || k2 == reflect.Invalid
}

// eq evaluates the comparison a == b || a == c || ...
func eq(arg1 reflect.Value, arg2 ...reflect.Value) (bool, error) {
	arg1 = indirectInterface(arg1)
	if len(arg2) == 0 {
		return false, errNoComparison
	}
	k1, _ := basicKind(arg1)
	for _, arg := range arg2 {
		arg = indirectInterface(arg)
		k2, _ := basicKind(arg)
		truth := false
		if k1 != k2 {
			// Special case: Can compare integer values regardless of type's sign.
			switch {
			case k1 == intKind && k2 == uintKind:
				truth = arg1.Int() >= 0 && uint64(arg1.Int()) == arg.Uint()
			case k1 == uintKind && k2 == intKind:
				truth = arg.Int() >= 0 && arg1.Uint() == uint64(arg.Int())
			default:
				if arg1 != zero && arg != zero {
					return false, errBadComparison
				}
			}
		} else {
			switch k1 {
			case boolKind:
				truth = arg1.Bool() == arg.Bool()
			case complexKind:
				truth = arg1.Complex() == arg.Complex()
			case floatKind:
				truth = arg1.Float() == arg.Float()
			case intKind:
				truth = arg1.Int() == arg.Int()
			case stringKind:
				truth = arg1.String() == arg.String()
			case uintKind:
				truth = arg1.Uint() == arg.Uint()
			default:
				if !canCompare(arg1, arg) {
					return false, fmt.Errorf("non-comparable types %s: %v, %s: %v", arg1, arg1.Type(), arg.Type(), arg)
				}
				if isNil(arg1) || isNil(arg) {
					truth = isNil(arg) == isNil(arg1)
				} else {
					if !arg.Type().Comparable() {
						return false, fmt.Errorf("non-comparable type %s: %v", arg, arg.Type())
					}
					truth = arg1.Interface() == arg.Interface()
				}
			}
		}
		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) {
	arg1 = indirectInterface(arg1)
	k1, err := basicKind(arg1)
	if err != nil {
		return false, err
	}
	arg2 = indirectInterface(arg2)
	k2, err := basicKind(arg2)
	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 = arg1.Int() < 0 || uint64(arg1.Int()) < arg2.Uint()
		case k1 == uintKind && k2 == intKind:
			truth = arg2.Int() >= 0 && arg1.Uint() < uint64(arg2.Int())
		default:
			return false, errBadComparison
		}
	} else {
		switch k1 {
		case boolKind, complexKind:
			return false, errBadComparisonType
		case floatKind:
			truth = arg1.Float() < arg2.Float()
		case intKind:
			truth = arg1.Int() < arg2.Int()
		case stringKind:
			truth = arg1.String() < arg2.String()
		case uintKind:
			truth = arg1.Uint() < arg2.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
}

更多的go语言模板内置函数实现可参考go源码中的 src/text/template/funcs.go 文件。