is_installed.r

is_installed <- function(pkg, version = NULL) {
  system.file(package = pkg) != "" && version_ok(pkg, version)
}

version_ok <- function(pkg, version = NULL) {
  if (is.null(version)) return(TRUE)
  
  pkgVersion(pkg) >= version
}


check_installed <- function(pkg, version = NULL) {
  if (!is_installed(pkg)) {
    stop("Please install ", pkg, call. = FALSE)
  }
  
  if (!version_ok(pkg, version)) {
    stop("Need at least version ", version, " of ", pkg, ". ", 
      "Currently running ", packageVersion(pkg), call. = FALSE)
  }    
}