Ch8, Ex5

declare
fun {NewMVar}
   P1 P2 Si1 Si2
   {NewPort Si1 P1} %put msgs received on P1
   {NewPort Si2 P2} %get msgs received on P2
   %Put puts the content in C if box is empty or waits
   C = {NewCell _}
   %content of D is either empty or unbound variable
   %signifying empty or full box respectivaly
   D = {NewCell empty}
   proc {PutIter Xs}
      case Xs of (Sync#X)|Xr then
         {Wait @D}
         @C = X
         Sync = ok
         D := _
         {PutIter Xr}
      end
   end
   proc {GetIter Xs}
      case Xs of (Sync#X)|Xr then
         {Wait @C}
         X = @C
         Sync = ok
         C := _
         @D = empty
         {GetIter Xr}
      end
   end
   proc {Put X}
      Sync in
      {Send P1 Sync#X}
      {Wait Sync}
   end
   proc {Get ?X}
      Sync in
      {Send P2 Sync#X}
      {Wait Sync}
   end
in
   thread {PutIter Si1} end
   thread {GetIter Si2} end
   mvar(put:Put get:Get)
end