triangle <- function(x, y, z) {
  
  checks <- c(x + y < z,
              x + z < y,
              y + z < x,
              x == 0,
              y == 0,
              z == 0)
  
  if (any(checks)) {
    stop("Invalid sides for triangle")
  }
  
  unique_sides <- length(unique(c(x, y, z)))
  
  triangle_class <- switch(unique_sides,
                           c("equilateral", "isosceles"),
                           "isosceles",
                           "scalene")
  
  structure(list(x = x, y = y, z = z), class = triangle_class)

}