r - Why `data.table::unique` doesn't work?

Question

help(unique) shows that unique function is present in two packages - base and data.table. I would like to use this function from data.table package. I thought that the following syntax - data <- data.table::unique(data) indicates the package to be used. But I get the following error -

'unique' is not an exported object from 'namespace:data.table'

But data <- unique(data) works well.

What is wrong here?

Answer 1

The function in question is really unique.data.table, an S3 method defined in the data.table package. That method is not really intended to be called directly, so it isn't exported. This is typically the case with S3 methods. Instead, the package registers the method as an S3 method, which then allows the S3 generic, base::unique in this case, to dispatch on it. So the right way to call the function is:

library(data.table)
irisDT <- data.table(iris)
unique(irisDT)

We use base::unique, which is exported, and it dispatches data.table:::unique.data.table, which is not exported. The function data.table:::unique does not actually exist (or does it need to).

As eddi points out, base::unique dispatches based on the class of the object called. So base::unique will call data.table:::unique.data.table only if the object is a data.table. You can force a call to that method directly with something like data.table:::unique.data.table(iris), but internally that will mostly likely result in the next method getting called unless your object is actually a data.table.