Event Streams in Lua

DemoChain.lua

function double( x )
  return x * 2;
end

function square( x )
  return x * x;
end

s = EventStream.new();

t = s:map( double );
t:listen( function( ... ) print( "t", ... ) end );

u = t:map( square );
u:listen( function( ... ) print( "u", ... ) end );

s:emit( 3 );
-- prints:
-- t       6
-- u       36

t:emit( 4 );
-- what should this print?

u:emit( 5 );
-- what should this print?

DemoFork.lua

function add( x, y )
  return x + y;
end

function square( x )
  return x * x;
end

function is_odd( x )
  return x % 2 == 1;
end

s = EventStream.new();

t = s:map( add ):filter( is_odd );
t:listen( function( ... ) print( "t", ... ) end );

u = s:map( add ):map( square );
u:listen( function( ... ) print( "u", ... ) end );

s:emit( 3, 4 );
-- prints:
-- t       7
-- u       49

t:emit( 1, 3 );
-- prints:
-- t       1       3
-- should this instead print nothing?

u:emit( 2, 7 );
-- prints:
-- u       2       7
-- should this instead print 81?

EventStream.lua

local EventStream = {};

function EventStream.new()
  local object = { __listeners = {} };
  setmetatable( object, { __index = EventStream } );
  return object;
end

function EventStream:listen( listener )
  table.insert( self.__listeners, listener );
end

function EventStream:emit( ... )
  for _, listener in ipairs( self.__listeners ) do
    listener( ... );
  end
end

function EventStream:map( fn )
  local stream = EventStream:new();

  self:listen( function( ... )
    stream:emit( fn( ... ) );
  end )

  return stream;
end

function EventStream:filter( fn )
  local stream = EventStream:new();

  self:listen( function( ... )
    if fn( ... ) then
      stream:emit( ... );
    end
  end )

  return stream;
end

return EventStream;