Ch8, Ex6

declare Channel
local
   proc {MySend C M}
      X in
      {Send C.send send(M#X)}
      {Wait X}
   end   
   fun {Receive C}
      M in
      {Send C.receive receive(M)}
      {Wait M}
      M
   end
   proc {MReceive Cs}
      %see later for MReceive definition
      skip
   end
   fun {New}
      P1 P2 S1 S2
      {NewPort S1 P1} %for receive msgs
      {NewPort S2 P2} %for send msgs
      proc {Iter S1 S2}
         case S1#S2 of (receive(R)|S1s)#(send(X#Y)|S2s)
         then
            R=X
            Y = unit
            {Iter S1s S2s}
         end
      end
   in
      thread {Iter S1 S2} end
      channel(receive:P1 send:P2)
   end
in
   Channel = c(new:New send:MySend
               receive:Receive
               mreceive:MReceive)
end 

From the book its not clear what mreceive is actually supposed to do, following are my various interpretations

mreceive version#1
In this version mreceive starts listeners for all the channels and exits immediately. All the Si will be called as and when messages are received on respective channels.

proc {MReceive Cs}
   for Ci#Si in Cs do
      thread
         {Si {Channel.receive Ci}}
      end
   end
end 

mreceive version#2
In this version mreceive starts listeners for all the channels and waits untill messages are received on all the channels and all corresponding Si are executed.

proc {MReceive Cs}
   Exit Count = {NewCell 0} in
   for Ci#Si in Cs do
      thread
         {Si {Channel.receive Ci}}
         Count := 1+@Count
         if @Count == {List.length Cs} then Exit=unit end
      end
   end
   {Wait Exit}
end 

mreceive version#3
In this version mreceive starts listeners for all the channels and waits. As soon as a message is received any of the channels, corresponding Si is executed and mreceive exits. In this case only one Si is executed on whose channel a message is received first.

proc {MReceive Cs}
   Exit A = {NewCell 0} in
   for Ci#Si in Cs do
      thread
         local M in
            M = {Channel.receive Ci}
            if @A==0 then
               {Exchange A _ 1}
               {Si M}
               Exit = unit
            end
         end
      end
   end
   {Wait Exit}
end 

NOTE: I could not understand what exactly does the extended version of mreceive is supposed to do.

TESTS

%This test proves that receive operation blocks
%until there is a send
local C in
   {Browse started}
   C = {Channel.new}
   thread {Browse rec1#{Channel.receive C}} end
   thread {Browse rec2#{Channel.receive C}} end
   thread {Browse rec3#{Channel.receive C}} end

   {Delay 5000}
   {Channel.send C msg1}
   {Delay 5000}
   {Channel.send C msg2}
   {Delay 5000}
   {Channel.send C msg3}

end

%This test proves that send operation blocks
%until there is a receive
local C in
   {Browse started}
   C = {Channel.new}
   thread {Channel.send C msg1} {Browse send1done} end
   thread {Channel.send C msg2} {Browse send2done} end
   thread {Channel.send C msg3} {Browse send3done} end

   {Delay 5000}
   {Browse {Channel.receive C}}
   {Delay 5000}
   {Browse {Channel.receive C}}
   {Delay 5000}
   {Browse {Channel.receive C}}
end