In case you want dim to work for vectors

dim-for-vectors.R

dim <- function(x) {
  if (is.vector(x)) {
    return(length(x)) 
  } else { 
    return(.Primitive("dim")(x)) 
  }
}

# Test code
# v <- rnorm(100)
# dim(v)  # returns 100, not NULL
# m <- matrix(1:25, ncol=5)
# dim(m) # returns the dimensions as usual