Ch6, Ex4

An open unbundled stateful version

declare
proc {NewPort S P}
   P = {NewCell S}
end
proc {Send P X}
   S=@P S1 in
   S = X|S1
   P:=S1
end 

A Secure unbundled stateful version

declare
proc {NewWrapper ?Wrap ?Unwrap}
   Key={NewName}
in
   fun {Wrap X}
      fun {$ K} if K==Key then X end end
   end
   fun {Unwrap W}
      {W Key}
   end
end

declare NewPort Send
local Wrap Unwrap in
   {NewWrapper Wrap Unwrap}
   proc {NewPort S P}
      P = {Wrap {NewCell S}}
   end
   proc {Send P X}
      C={Unwrap P} S=@C S1 in
      S = X|S1
      C:=S1
   end
end 

A Secure Bundled stateful version

declare
proc {NewPort S P}
   C = {NewCell S}
   proc {Send X}
      S = @C S1 in
      S = X|S1
      C := S1
   end
in
   P = port(send:Send)
end

%Usage
declare P S
{NewPort S P}
{Browse S}
{P.send a}
{P.send b} 

Note: In chapter 5 it is said that end of port stream is a readonly variable, I don't completely understand the meaning of that and hence above versions don't support that statement, Someday I'll give more thought to it.