[tylermw]

They are the same thing, minus the corner case of assignment within a function call:

e.g.

  divide = function(x, y) {
    return(x/y)
  }

  divide(y = 2, x = 1)
  divide(y <- 1, x <- 2)

These two calls give the same result, as the second results in assignment and then passing the argument by position. Other than this case, they are exactly interchangeable.