S4 classes and methods with optional arguments

It seems to me like the generic should (always?) just have arguments
used for dispatch -- stream, in this case -- and that methods then
specify default values. 

There are advantages to adding named arguments to a generic to define
the expected interface.  These 'extra' args may not be *needed* for
dispatch in the sense that the first arg may be enough to decide what
method you want.

So IMO, there are two reasons to put an arg in a generic:

1. You really want to dispatch on it.
2. You want to define an interface and can handle the fact that you
   will have to also dispatch on it.

I guess my point is that for downstream developers extending your
generic and for the sake of documentation, relying too much on '...'
can make things difficult.

To also dispatch on the second argument, one
might

setGeneric("rstream.sample",
function( stream, n, ... ) standardGeneric("rstream.sample"))

setMethod("rstream.sample", c( "rstream.sample", "numeric" ),
function( stream, n, ... ) { code } )

setMethod("rstream.sample", c( "rstream.sample", "missing" ),
function( stream, n, ... ) rstream.sample( stream, n = 1 ))

And here I might offer a slight improvement.  Putting the default
value in the signature of the function will give automated tools a
chance to document:

  setMethod("rstream.sample", c("rstream.sample", "missing"),
            function( stream, n=1, ...) rstream.sample(stream, n))

+ seth