英文:
Callback in Golang
问题
我正在使用go-couchbase将数据更新到couchbase，但是在如何使用回调函数方面遇到了问题。
函数Update要求我传递一个回调函数，该函数应为UpdateFunc类型：
func (b *Bucket) Update(k string, exp int, callback UpdateFunc) error
这是我所做的：
首先，我声明了一个类型UpdateFunc：
type UpdateFunc func(current []byte) (updated []byte, err error)
然后在代码中，我添加了以下几行：
fn := UpdateFunc{func(0){}}
然后调用Update函数：
bucket.Update("12345", 0, fn())
但是Go返回了以下错误：
语法错误：意外的字面量0，期望)，对于这行代码fn := UpdateFunc{func(0){}}
那么我做错了什么？如何使回调函数工作？
附加信息
感谢你们的建议。现在我可以按照以下方式运行回调函数：
myfunc := func(current []byte)(updated []byte, err error) {return updated, err }
myb.Update("key123", 1, myfunc)
然而，当我运行bucket的Update函数时，我检查了couch数据库。具有键"key123"的文档消失了。似乎Update函数并没有更新值，而是删除了它。发生了什么？
英文:
I am using go-couchbase to update data to couchbase, however, I have problem in how to use the callback function.
The function Update requires me to pass a callback function in which should be UpdateFunc
func (b *Bucket) Update(k string, exp int, callback UpdateFunc) error
So that's what I have done
First, I declared a type UpdateFunc:
type UpdateFunc func(current []byte) (updated []byte, err error)
Then in the code, I add the following lines:
fn := UpdateFunc{func(0){}}
and then call the Update function:
bucket.Update("12345", 0, fn()}
But the Go returns the following error:
syntax error: unexpected literal 0, expecting ) for this line fn := UpdateFunc{func(0){}}
So what I am doing wrong? So how can I make the callback function work ?
additional information
Thanks all of your suggestion. Now I can run the callback function as follows:
myfunc := func(current []byte)(updated []byte, err error) {return updated, err }
myb.Update("key123", 1, myfunc)
However, when I run the Update function of the bucket. I checked the couch database. The document with the key of "key123" was disappeared. It seems the Update does not update the value but delete it. What happened?
答案1
得分: 10
你需要创建一个与couchbase.UpdateFunc签名匹配的函数，然后将其传递给bucket.Update。
例如：
fn := func(current []byte) (updated []byte, err error) {
updated = make([]byte, len(current))
copy(updated, current)
// 修改 updated
return
}
....
bucket.Update("12345", 0, fn)
请注意，要传递一个函数，只需传递fn而不是fn()，后者实际上会立即调用该函数并传递其返回值。
我强烈建议停下你正在做的一切，阅读Effective Go以及Go的博客上的所有文章，从First Class Functions in Go开始阅读。
英文:
You need to create a function that matches the couchbase.UpdateFunc signature then pass it to bucket.Update.
For example:
fn := func(current []byte) (updated []byte, err error) {
updated = make([]byte, len(current))
copy(updated, current)
//modify updated
return
}
....
bucket.Update("12345",0,fn)
Notice that to pass a function you just pass fn not fn(), that would actually call the function right away and pass the return value of it.
I highly recommend stopping everything you're doing and reading Effective Go and all the posts on Go's blog starting with First Class Functions in Go.
答案2
得分: 4
我在go-nuts列表上发布了相同的答案，以防万一：
你不需要自己定义UpdateFunc类型，它已经在这里定义了：
https://github.com/couchbaselabs/go-couchbase/blob/master/client.go#L608
只需正常定义函数并将其作为参数传递，或者像其他语言中那样传递一个匿名函数。
这里有一个简单的示例：
http://play.golang.org/p/YOKxRtQqmU
英文:
Same answer I posted on the go-nuts list, just in case:
You don't have to define the UpdateFunc type by yourself, it's already defined here:
https://github.com/couchbaselabs/go-couchbase/blob/master/client.go#L608
just define the function normally and pass it as an argument or pass an anonymous function as in other languages.
here's a simple example:
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