Avoiding If-Then-Else structure in R

The task I'm facing is to return some rows from a list of data frames. Rows to return can be entered as a number or as "first" and "last". I've coded that like this:

showrow <- function(row) {
  if (class(row) == "numeric") {
    getrow <- function(d) {
      d[row,]
    }
    return(getrow)
  } else {
    if (row =="first") {
      getrow <- function(d) {
        head(d, 1)
      }
      return(getrow)
    } else {
      if (row == "last") {
        getrow <- function(d) {
          tail(d, 1)
        }
        return(getrow)
      } else {
        stop("invalid position")
      }
    }
  }
}

And then it's possible to use it like that:

a <- matrix(rnorm(20), 4)
b <- matrix(rnorm(100), 10)
lst <- list(a, b) ; lst
num <- "last" # Or `num <- "first"`, or `num <- 3`, etc
lapply(lst, function(df) { showrow(num)(df) })

The problem is I think that If structure still looks clumsy... Is there any workaround to avoid it in this particular scenario?

(And, on a side note, is it possible to return NAs if the subscript would be out of bounds, like num <- 11, for example?)