# Extending ape::phylo class to NeXML:

## Promoting an S3 class into S4 while maintaining existing methods

While working on the RNeXML package, I have recently I have been puzzling over extending S3 objects to share some of the nice properties of S4 objects and methods while continuing to function with the potentially huge library of functions written to work with them. SO:

To illustrate this issue, consider the S3 class phylo provided by the ape package and used by most of the 30+ packages that reverse depend or reverse import ape. Despite this popularity, the S3 class has quite a few shortcomings (including inconsistent definition in ordering of it’s components, see the phylobase paper). To address these concerns, a hackathon consisting of many of the leading developers in this field created the phylobase package with S4 definitions of phylo objects that extend the class to handle additional types of data and address various other concerns that have arisen. Unfortunately, as far as I can tell, adoption of the new format has been lacking. In my mind, the greatest limitation of the new format is the lack of compatibility with existing methods build on the S3 type.

I never understood why phylobase did not do this. To illustrate the problem, let us load an S3 phylo object and then coerce it into the phylobase S4 type:

library(phylobase)
library(ape)
data(bird.orders)
bird.orders4 <- as(bird.orders, "phylo4") # make ape::phylo tree into phylobase::phylo4 S4 class

The S4 class has explicitly defined it’s own plotting method, so:

plot(bird.orders4) 

However, the original S3 plotting method fails:

plot.phylo(bird.orders4) 
Error: \$ operator not defined for this S4 class

More interesting functions, many of which may not have even existed when phylobase was written, operate only on the S3 class:

 S <- c(10, 47, 69, 214, 161, 17, 355, 51, 56, 10, 39, 152,
6, 143, 358, 103, 319, 23, 291, 313, 196, 1027, 5712)
bd.ext(bird.orders4, S)   # Fails 
Error: object "phy" is not of class "phylo"

While we can work around this with explicit coercion, this requirement is rather crude and breaks existing code users may already rely upon:

bd.ext(as(bird.orders4, "phylo"), S)   # Works only after coercion 

Extended Version of the Birth-Death Models to
Estimate Speciation and Extinction Rates

Data: phylogenetic: as(bird.orders4, "phylo")
taxonomic: S
Number of tips: 23
Deviance: 290.2
Log-likelihood: -145.1
Parameter estimates:
d / b = 2.315e-08   StdErr = 0.1542
b - d = 0.2765   StdErr = 0.009578
(b: speciation rate, d: extinction rate)

It appears this problem can be solved using setOldClass. I’ve defined an the class phyloS4 which inherits all methods for the S3 phylo class without having to explicitly declare those methods. In this way, we have the benefits of an S4 class while maintaining compatibility with all developers who only write functions based on the S3 class. (as long as functions don’t stupidly check the string identity class(obj) == "phylo", instead of using the proper class check is(obj, "phylo") – looking at you ape::skyline and friends….)

setClass("phyloS4",
representation(edge = "matrix",
Nnode = "integer",
tip.label = "character",
edge.length = "numeric"))
setOldClass("phylo", S4Class="phyloS4")
selectMethod("show", "phylo")
removeClass("phyloS4")

1

Now consider using our S4 version, rather than the phylobase S4 version:

a <- new("phylo", bird.orders)
is(a, "phylo")
[1] TRUE
bd.ext(a, S)   

Extended Version of the Birth-Death Models to
Estimate Speciation and Extinction Rates

Data: phylogenetic: a
taxonomic: S
Number of tips: 23
Deviance: 290.2
Log-likelihood: -145.1
Parameter estimates:
d / b = 2.315e-08   StdErr = 0.1542
b - d = 0.2765   StdErr = 0.009578
(b: speciation rate, d: extinction rate)

Class, show, and plot, and additional (bd.ext) methods from the S3 class just work. Now we want to extend the class to contain additional metadata (in my case, the nexml information):

I can then build a new class, nexmlTree by extending this class. Again my new class acts like an S3 phylo in any such functions, but adds a representation containing all the nexml data. This approach doesn’t minimize memory footprint, but usually that is not a concern for R users (otherwise coercion is always an option).

library(RNeXML)
setClass("nexmlTree", representation(nexml = "nexml"), contains="phylo")
setMethod("show", "nexmlTree", function(object) print.phylo(object))
[1] "show"

Again, we check a few methods:

 b <- new("nexmlTree", bird.orders, nexml = as(bird.orders, "nexml"))
is(b, "phylo")
[1] TRUE
bd.ext(b, S)   

Extended Version of the Birth-Death Models to
Estimate Speciation and Extinction Rates

Data: phylogenetic: b
taxonomic: S
Number of tips: 23
Deviance: 290.2
Log-likelihood: -145.1
Parameter estimates:
d / b = 2.315e-08   StdErr = 0.1542
b - d = 0.2765   StdErr = 0.009578
(b: speciation rate, d: extinction rate)

This provides the additional metadata while leaving us with an object that works with all existing functions.

1. I’m not quite sure why this required the show method to be defined such that we wouldn’t get the S4 show method instead. As a default show method seems to be the only method automatically defined for the new class, I gather it is the only one we have to overwrite in preference for the print.phylo() method.