The code of chan part in the Reflection chapter is a little bit unintuitive

[UponTheSky]

Hi, it is a great pleasure to go through this wonderful content. I am learning a lot from this.

While I was reading and testing the chan part in the chapter Reflection, the loop part was not much intuitive to me.

for v, ok := val.Recv(); ok; v, ok = val.Recv() {
  // ...
}

So I tried this code instead(at first, I thought it might be a typo):

for v, ok := val.Recv(); ok; {
  // ...
}

However, the test didn't seem to be running. So mimicking the code in the page(the docs doesn't say much about Value.Recv), and I've got this result:

• the code(playground link)

package main

import (
	"fmt"
	"reflect"
)

func main() {
	channel := make(chan int)

	go func() {
		channel <- 42
		channel <- 7
		close(channel)
	}()

	val := reflect.ValueOf(channel)
	count := 0

	for v, ok := val.Recv(); ok; {
		fmt.Print(v)
		count++

		if count > 100 {
			fmt.Println("\nInfinite loop")
			break
		}
	}
}

• the result

4242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242424242
Infinite loop

Program exited.

From my understanding, it seems that Value.Recv() doesn't behave the same as the <- operator when it is in the condition statement of a for loop - not popping the front element from the queue.

However, since the original code passes the test, I tweaked the code a little and printed the values:

• the code(playground)

package main

import (
	"fmt"
	"reflect"
)

func main() {
	channel := make(chan int)

	go func() {
		channel <- 42
		channel <- 7
		close(channel)
	}()

	val := reflect.ValueOf(channel)

	for v, ok := val.Recv(); ok; {
		fmt.Println("first", v)
		v, ok = val.Recv()
		fmt.Println("second", v)
	}
}

• the result is:

first 42
second 7
first 7
second 0

So in the conclusion, the behavior of value.Recv is hardly expectable(Moreover, if I ignore the ok variable in the body as v, _ = val.Recv(), I get the infinite loop again). Perhaps this is why the original author wrote the code like this.

Nevertheless, personally I think this line easily confuses newbies like me. It is neither using the familiar <- operator nor val := range channel. Furthermore, it gives the impression that we should poll the channel twice to get the value we want.

If it is okay, I would like to make a simple fix on this part as a PR.

[UponTheSky]

I close this issue due to merged PR #737