Difference between 'generic' and 'method'?

Question

I noticed that the following functions in R have two slightly different classifications:

sloop::ftype(t.test)
#> [1] "S3"      "generic"

sloop::ftype(t.data.frame)
#> [1] "S3"     "method"

^{Created on 2021-04-21 by the reprex package (v1.0.0)}

One is a 'generic' and one is a 'method' but I'm struggling to differentiate the two: my understanding of a 'generic' is that it is a method - specifically, a method which acts on an input object according to its class.

Answer 1

A method implements a generic (or, on a more technical level, a method gets called by a generic via UseMethod).

That is, a generic function calls UseMethod; it might look something like this:

foo = function (x, ...) UseMethod('foo')

Whereas a method is a function that implements the generic for a specific S3 class; for instance:

foo.bar = function (x, ...) message('class of x is bar!')