variadic function looping through channels

func fanIn(inputs ...<-chan string) <-chan string {
    c := make(chan string)
    for cinput := inputs { // this line gives me error
        go func() {for { c <- <-cinput}}()
    }  
}

It's pretty self explanatory on what I am trying to do but it seems go lang doens't like how i do it. Why?

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评论：

dewey4iv:

func fanIn(inputs ...<-chan string) <-chan string {
    c := make(chan string)
    for _, in := range inputs { // this line gives me error
        go func(ch <-chan string) {
            for {
                c <-ch
            }
        }(in)
    }
}

You were missing the range keyword :).
I'd also like to point out that you don't have any way of signaling when a channel is done sending.

binaryblade:

I like how you attempted to avoid closing over the loop variable but didn't.

dewey4iv:

Yeah, haha - I was in the middle of editing it and lost internet connection. Thanks for paying attention though!

juniorsysadmin1:

Hmm i don't think it's doing what I want it to do. 

I'm trying to write a variadic function of the following function. 

func fanIn(input1,input2 <-chan string) <-chan string{
    c := make(chan string)
    go func() {for { c <- <-input1}}()
    go func() {for { c <- <-input2}}()
    return c
}

So instead of 2 input it can take N inputs.
I then have the following function in my testcode
func boring(msg string, s int) <-chan string { // this return a receive-only channel of strings
    c := make(chan string)
    go func() {
        for i := 0; ; i++ {
            c <- fmt.Sprintf("%s %d", msg, i)
            time.Sleep(time.Second * time.Duration(s))
        }
    }()
    return c // return the channel to th e caller
}

in main I have 
joe:= boring("joe", 1)
ann := boring("ann", 4)
bla := fanIn(joe, ann)
for i := 0; i < 5; i++ {
    fmt.Println(<-bla)
}

Joe should appear 4 times and ann should appear once. The program end up having ann appear 5 times with 4 sec interval. 
My code is in pastebin

dewey4iv:

Take a look at the output here -- is this more what you are looking for?
https://pastebin.com/Ti8fj7uY

juniorsysadmin1:

yea that's what I was trying to achieve. 

dewey4iv:

:thumbsup: 

mr-bosley:

Surely this will memory leak, as the go routine inside...
for _, in := range inputs { // this line gives me error
        go func(ch <-chan string) {
            for {
                c <-ch
            }
        }(in)
}

will never stop?
There's a really good example of fan-in here.
https://blog.golang.org/pipelines

dewey4iv:

I'd also like to point out that you don't have any way of signaling when a channel is done sending.

Oh completely! That's what I was getting at with the above. 

jerf:

Missing "range" on the inputs.
While more complicated to work with, you may prefer either to use reflect.Select to do this all in one goroutine (as they may be cheap but they are not free), or conceivably, to try to make it so the destination channel is shared with the producers in a different way that prevents you from needing this. (Fan-in is a legitimate pattern, but it is not the first thing I'd reach for. You're usually better off trying to get the channels to the right places in the first place rather than building complicated networks in Go.)

LimEJET:

did you just try to link your local copy of the documentation

jerf:

Yes. Yes I did. What, you can't get to it? :)

LimEJET:

I guess that's shame on me for not having a local doc server up constantly.

jammerlt:

Inputs is a slice, so cinput ends up being the index in the slice, and not the channel. You are also missing the range keyword and a return value. So for _, cinput := range inputs

binaryblade:

Just a heads up that channels in an API appear very infrequently in the Stdlib.

uncle_bad_touches:

Look at https://golang.org/pkg/reflect/#Select.  This would allow you to monitor all the channels in a single goroutine.

ChristophBerger:

In addition to all the helpful comments so far, let me mention this blog post from the Go blog that discusses fan-in and fan-out.

disq:

Copying from an example I did for a quickstart session:
func copyData(in, out chan string) {
    for { // infinite loop
        out <- <-in // read from in, write to out
    }
}
func fanIn(channels ...chan string) chan string {
    out := make(chan string)
    for _, ch := range channels { // iterate all params
        go copyData(ch, out)
    }
    return out
}