Rewrite the Send/Recv part of csp.md

[wangkuiyi]

Fix #8163

[helinwang]

A side note, in Go, one can still receive valid value from a closed channel:

package main

import (
	"fmt"
)

func main() {
	a := make(chan int, 1)
	a <- 5
	close(a)
	for {
		v, ok := <-a
		if !ok {
			break
		}

		fmt.Println(v)
	}
}

I think we will need similar functionality.

[wangkuiyi]

Yes, you are right -- if there are any residual values left in a closed buffered channel, receive should be able to read them.

ch = fluid.make_channel(INT, 10)
fluid.send(ch, 1)
fluid.send(ch, 2)
fluid.send(ch, 3)
fluid.close(ch)

fluid.recv(ch) # true, 1
fluid.recv(ch) # true, 2
fluid.recv(ch) # true, 3
fluid.recv(ch) # false, 0
fluid.recv(ch) # false, 0

[kavyasrinet]

In addition to above,

1. For unbuffered channels, the current behavior remains the same. We close everything and unblock everyone ?
2. For buffered channels, a send operation after fluid.close(ch) should panic/ throw an error.

Correct me if I am wrong.

[wangkuiyi]

I added detailed explanations to this doc.

[helinwang]

Typically sender will close the channel, since send to a closed channel will panic, but receive from a closed channel will get zero value, optionally one can check if the value is valid:

a := make(chan int)
close(a)
fmt.Println(<-a) // 0
v, ok := <-a
fmt.Println(v, ok) // 0, false

[kavyasrinet]

LGTM

[kavyasrinet]

I think we can reuse these methods as fluid.send() and fluid.receive() for reading and writing in select cases as well.

[wangkuiyi]

It would be great if we could. I don't have an idea how could we. Do you?

[kavyasrinet]

Maybe something like:

sel = fluid.select()
with sel.case(fluid.recv(ch), x): # similar to with sel.read_case(ch, x) or sel.case(ch.read(), x)
  do_something_with(x)

As per the discussion in #8165

[kavyasrinet]

In addition to above,

1. For unbuffered channels, the current behavior remains the same. We close everything and unblock everyone ?
2. For buffered channels, a send operation after fluid.close(ch) should panic/ throw an error.

Correct me if I am wrong.

[helinwang]

LGTM!