"null channel should always block"

[summivox]

Rationale is explained excellently in this article: http://dave.cheney.net/2013/04/30/curious-channels

Since interface for channels are functions (instead of member of channel), could this possibly be implemented with a null check?

[ubolonton]

Something like this should work (instead of "abusing" nil like that Go example):

var NULL_CHANNEL = {
  _put: function(value, handler) {
    return null;
  },

  _take: function(handler) {
    return null;
  },

  is_closed: function() {
    return false;
  },

  close: function() {
    throw new Error("Null channel cannot be closed.");
  }
}

However, waitMany could be implemented by removing the done channel from the list of channels to alts from.

function* waitMany() {
  var chans = Array.prototype.slice.call(arguments, 0);
  while (chans.length > 0) {
    var chan = (yield alts(chans)).channel;
    chans.splice(chans.indexOf(chan), 1);
  }
}

Is there any other better example where a null channel is needed?

[summivox]

From my limited knowledge of CSP, one example is guarded select block:

func maybe(b bool, c chan int) chan int {
    if !b {
        return nil
    }
    return c
}

select {
case <-maybe(val>0, p):
    val--
case <-v:
    val++
}

https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/ChPxr_h8kUM

Of course in javascript we can simply process the channel array before passing it to alts. It's just more convenient and consistent to have some sort of "always blocking" channel (I agree it doesn't have to be literally null).