Callback in Golang

Question

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?

Answer 1

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.

Answer 2

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:

http://play.golang.org/p/YOKxRtQqmU