英文:
How to define a function type which accepts any number of arguments in Go?
问题
我试图编写一个函数,它接受任何其他函数并在其周围包装一个新函数。这是我目前尝试的代码:
package main
import (
"fmt"
)
func protect(unprotected func(...interface{})) func(...interface{}) {
return func(args ...interface{}) {
fmt.Println("protected")
unprotected(args...)
}
}
func main() {
a := func() {
fmt.Println("unprotected")
}
b := protect(a)
b()
}
当我编译时,我得到以下错误:
cannot use a (type func()) as type func(...interface { }) in function argument
为什么没有参数的函数与具有可变数量参数的函数不兼容?我该如何使它们兼容?
更新:
受保护的函数应与原始函数兼容:
func take_func_int_int(f func(x int) int) int {
return f(1)
}
func main() {
a := func(x int) int {
return 2 * x
}
b := protect(a)
take_func_int_int(a)
take_func_int_int(b)
}
英文:
I try to write a function which takes any other function and wraps a new function around it. This is what I have tried so far:
package main
import (
"fmt"
)
func protect (unprotected func (...interface{})) (func (...interface{})) {
return func (args ...interface{}) {
fmt.Println ("protected");
unprotected (args...);
};
}
func main () {
a := func () {
fmt.Println ("unprotected");
};
b := protect (a);
b ();
}
When I compile this I get the error:
cannot use a (type func()) as type func(...interface { }) in function argument
Why is a function without arguments not compatible to a function with a variable number of arguments? What can I do to make them compatible?
Update:
The protected function should be compatible with the original:
func take_func_int_int (f func (x int) (y int)) (int) {
return f (1)
}
func main () {
a := func (x int) (y int) {
return 2 * x
}
b := protect (a)
take_func_int_int (a)
take_func_int_int (b)
}
答案1
得分: 17
在Go语言中,类型是非常具体的。你可以尝试使用以下代码:
a := func(_ ...interface{}) {
fmt.Println("unprotected")
}
func (...interface{})
并不意味着“任何接受任意数量任何类型参数的函数”,而是意味着“只有接受可变数量interface{}类型参数的函数”。
或者,你可以使用interface{}
和reflect
包,而不是func(...interface{})
。可以参考http://github.com/hoisie/web.go中的示例。
编辑:具体来说,是这样的:
package main
import (
"fmt"
"reflect"
)
func protect(oldfunc interface{}) (func (...interface{})) {
if reflect.TypeOf(oldfunc).Kind() != reflect.Func {
panic("protected item is not a function")
}
return func (args ...interface{}) {
fmt.Println("Protected")
vargs := make([]reflect.Value, len(args))
for n, v := range args {
vargs[n] = reflect.ValueOf(v)
}
reflect.ValueOf(oldfunc).Call(vargs)
}
}
func main() {
a := func() {
fmt.Println("unprotected")
}
b := func(s string) {
fmt.Println(s)
}
c := protect(a)
d := protect(b)
c()
d("hello")
}
输出结果为:
Protected
unprotected
Protected
hello
编辑:回答更新的问题
就像我之前说的,Go语言中的类型是非常具体的。protect函数返回的类型是func(...interface{})
,它永远不会被赋值给func(int)int
。我认为你可能要么过度设计了你的问题,要么对问题有误解。然而,下面是一个强烈不推荐的代码片段,可以使其工作。
首先,修改protect函数以返回值:
func protect(oldfunc interface{}) (func (...interface{}) []interface{}) {
if reflect.TypeOf(oldfunc).Kind() != reflect.Func {
panic("protected item is not a function")
}
return func (args ...interface{}) []interface{} {
fmt.Println("Protected")
vargs := make([]reflect.Value, len(args))
for n, v := range args {
vargs[n] = reflect.ValueOf(v)
}
ret_vals := reflect.ValueOf(oldfunc).Call(vargs)
to_return := make([]interface{}, len(ret_vals))
for n, v := range ret_vals {
to_return[n] = v.Interface()
}
return to_return
}
}
然后创建一个转换函数:
func convert(f func(...interface{}) (func(int) int) {
return func(x int) int {
r := f(x)
return r[0].(int)
}
}
然后你的调用将如下所示:
take_func_int_int(convert(b))
但我保证这不是你真正想做的。
退一步,尝试重新思考问题。我在这些示例中完全破坏了类型安全性。你想要实现什么?
英文:
Types are pretty concrete in Go. You could try
a := func(_ ...interface{}) {
fmt.Println("unprotected")
}
func (...interface{})
does not mean "any function that takes any number of any kind of arguments", it means "only a function which takes a variable number of interface{} arguments"
Alternatively rather than func(...interface{})
you can just use interface{}
and the reflect
package. See http://github.com/hoisie/web.go for an example.
EDIT: Specifically, this:
package main
import (
"fmt"
"reflect"
)
func protect(oldfunc interface{}) (func (...interface{})) {
if reflect.TypeOf(oldfunc).Kind() != reflect.Func {
panic("protected item is not a function")
}
return func (args ...interface{}) {
fmt.Println("Protected")
vargs := make([]reflect.Value, len(args))
for n, v := range args {
vargs[n] = reflect.ValueOf(v)
}
reflect.ValueOf(oldfunc).Call(vargs)
}
}
func main() {
a := func() {
fmt.Println("unprotected")
}
b := func(s string) {
fmt.Println(s)
}
c := protect(a)
d := protect(b)
c()
d("hello")
}
Ouput is
Protected
unprotected
Protected
hello
EDIT: To answer the update
Like I said above, types are pretty concrete in Go. The protect function returns a type func(...interface{})
which will never be assignable to func(int)int
. I think you're probably either over-engineering your problem or misunderstanding it. However, here's a highly discouraged code snippet that would make it work.
First change protect to also return values:
func protect(oldfunc interface{}) (func (...interface{}) []interface{}) {
if reflect.TypeOf(oldfunc).Kind() != reflect.Func {
panic("protected item is not a function")
}
return func (args ...interface{}) []interface{} {
fmt.Println("Protected")
vargs := make([]reflect.Value, len(args))
for n, v := range args {
vargs[n] = reflect.ValueOf(v)
}
ret_vals := reflect.ValueOf(oldfunc).Call(vargs)
to_return := make([]interface{}, len(ret_vals))
for n, v := range ret_vals {
to_return[n] = v.Interface()
}
return to_return
}
}
Then make a convert function:
func convert(f func(...interface{}) (func(int) int) {
return func(x int) int {
r := f(x)
return r[0].(int)
}
}
Then your call would look like
take_func_int_int(convert(b))
##But I promise this isn't what you actually want to do.
Step back and try to rework the problem. I've completely killed type-safety in these examples. What are you trying to accomplish?
答案2
得分: 2
包 main
导入 "fmt"
// 这是一个接受任意数量的 int 参数的函数。
func sum(nums ...int) {
fmt.Print(nums, " ")
total := 0
for _, num := range nums {
total += num
}
fmt.Println(total)
}
func main() {
// 可变参数函数可以像通常一样使用单独的参数进行调用。
sum(1, 2)
sum(1, 2, 3)
// 如果您已经有一个包含多个参数的切片,
// 可以使用 `func(slice...)` 将它们应用于可变参数函数,就像这样。
nums := []int{1, 2, 3, 4}
sum(nums...)
}
英文:
package main
import "fmt"
// Here's a function that will take an arbitrary number
// of `int`s as arguments.
func sum(nums ...int) {
fmt.Print(nums, " ")
total := 0
for _, num := range nums {
total += num
}
fmt.Println(total)
}
func main() {
// Variadic functions can be called in the usual way
// with individual arguments.
sum(1, 2)
sum(1, 2, 3)
// If you already have multiple args in a slice,
// apply them to a variadic function using
// `func(slice...)` like this.
nums := []int{1, 2, 3, 4}
sum(nums...)
}
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