foo = function(a,b) {a+b}

x = 1
n = 4

out = vector("numeric")
steps = seq(1, 4, by = 1)

for( b in steps){
  ## initial value
  if (length(out) == 0){

      out = append(out, values = foo(x, b) )
    }else{

      out = append(out, values = foo( tail( out, 1), b) )
    }

}
tail(out, 1)

foo_n <- reduce(map(1:n, ~partial(foo, b=.x)), compose)