'...' is a variable,who can explain

["wj" <jiang_wang@...>]

look at this code:

 

--codes begin

 

function joinFunc(f1,f2)
  return function (...)
           return f1(f2(...)) --...treated as a variable! is it maze!
   end

end

 

f1 = joinFunc(print,math.abs)

 

f1(-2) --out:2

 

--codes end

 

why the