How to test whether the argument passed to an R function is a raster or a vector

Question

I am writing a function that expects two arguments: X and Y. Argument X should be a raster which can be generated by packages such as raster, terra, or stars.

Argument Y should be a polygon from packages such as sp or sf. All these packages manage spatial data differently and represent this data using objects of different structure.

How can I ensure that the user supplied objects of the right classes to my function?

Currently, I managed only to test whether object X is of class RasterLayer (created by the package raster) and whether the object Y is of class SpatialPolygonsDataFrame (created by the package sp)

e.g.

  if(class(Y) != "SpatialPolygonsDataFrame"){
    stop("Y has to be a spatial object of type SpatialPolygonsDataFrame")
  }
  if(class(X) != "RasterLayer"){
    stop("X has to be an object of class RasterLayer")
  }

Answer 1

I'm doing pretty much the same

if (is(X, "RasterLayer") | is(X, "SpatRaster") | is(X, "stars"))
{
  # handle the different cases with specific code
  # personnally I convert to stars
}

if (is(X, "SpatialPolygonsDataFrame") | is(X, "sf"))
{
  # handle the different cases with specific code
  # personnally I convert to sf
}

In my case my code looks more like

if (inherit(X, "Spatial"))
  X <- sf::st_as_sf(X)

if (inherit(Y, "Raster"))
  Y <- stars::st_as_stars(Y)

if (inherit(Y, "SpatRaster"))
  Y <- stars::st_as_stars(Y) # that one does not exist do it yourself

if(!is(X, "sf"))
   stop("Y has to be a spatial object ")

if(!is(Y, "stars")
   stop("Y has to be a raster object")

geom = sf::st_geometry(X)

if (!is(geom, "sfc_POLYGON") & !is(geom, "sfc_MULTIPOLYGON"))
   stop("Y has to be a spatial polygon object ")

# keep going with sf/stars-based code