What are directional channels in Go and why use them in function signatures?

Go channels can be typed with a direction: chan<- T (send-only) or <-chan T (receive-only). A bidirectional chan T can be assigned to either. Directional channels enforce access discipline at compile time, making the data-flow contract of each function explicit.

// Producer: only sends to out — compile error if it tries to receive
func produce(out chan<- int) {
    for i := 0; i < 5; i++ { out <- i }
    close(out)   // close is allowed on a send-only channel
    // v := <-out  // COMPILE ERROR: receive from send-only channel
}

// Consumer: only receives from in
func consume(in <-chan int) {
    for v := range in { fmt.Println(v) }
    // in <- 99   // COMPILE ERROR: send to receive-only channel
    // close(in)  // COMPILE ERROR: close of receive-only channel
}

// Bidirectional channel can be passed as either direction
ch := make(chan int, 10) // chan int (bidirectional)
go produce(ch)          // narrowed to chan<- int automatically
consume(ch)             // narrowed to <-chan int automatically

// Pipeline: each stage returns receive-only — caller can't accidentally close
func generator(nums ...int) <-chan int {
    out := make(chan int)
    go func() { defer close(out); for _, n := range nums { out <- n } }()
    return out // exposes only read access
}

func square(in <-chan int) <-chan int {
    out := make(chan int)
    go func() { defer close(out); for v := range in { out <- v * v } }()
    return out
}

for v := range square(generator(2, 3, 4)) {
    fmt.Println(v) // 4 9 16
}

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