Daisy chain input,output channels together in golang - go

Question

I have the following interface and struct
type PiplineStep interface {
Do(ctx context.Context, in <-chan Message) (<-chan Message, <-chan error, error)
}
type Pipline struct {
Steps []core.PiplineStep
}
Now I am trying to daisy the interfaces to create a pipeline like the following
for _, step := range p.Steps {
out, errc, err := step.Do(ctx, out)
errcList = append(errcList, errc)
if err != nil {
errc <- err
return
}
select {
case outer <- msg:
case <-ctx.Done():
return
}
}
But the compiler says no is this possible?
I get the following Error 'out declared and not used' i have attempted following but it appears that all steps are receiving the same chan
for _, step := range p.Steps {
var tmpOut <-chan core.Message
tmpOut = out
tmpOut, errcTmp, err := step.Do(ctx, tmpOut)
errcList = append(errcList, errcTmp)
if err != nil {
errc <- err
return
}
select {
case out <- msg:
case <-ctx.Done():
return
}
}

Answer 1

You have to declare your channel variable outside the loop if you want to re-use it in each iteration (errors and context omitted for brevity):
package main
import "fmt"
func main() {
var pipeline Pipeline
pipeline.Steps = append(pipeline.Steps,
AddBang{},
AddBang{},
AddBang{},
)
src := make(chan Message)
pipe := src
for _, s := range pipeline.Steps {
pipe = s.Do(pipe)
}
go func() {
src <- "msg 1"
src <- "msg 2"
src <- "msg 3"
}()
fmt.Println(<-pipe)
fmt.Println(<-pipe)
fmt.Println(<-pipe)
}
type Message string
type Pipeline struct {
Steps []PipelineStep
}
type PipelineStep interface {
Do(in chan Message) chan Message
}
type AddBang struct{}
func (AddBang) Do(in chan Message) chan Message {
out := make(chan Message)
go func() {
defer close(out)
for m := range in {
out <- m + "!"
}
}()
return out
}
Try it on the playground: https://play.golang.org/p/ItVLUBRpNA1