function construction - f[arg1, arg2,...,argN] vs. f[{arg1, arg2,...,argN}]

I am trying to reproduce the API of a function (written in R) that accepts an arbitrary number of arguments and handles it the same way as it would handle a single argument that is a list of different data sets.

Is there a Mathematica idiom that allows a function to be defined so that:

f[ arg1, arg2, ..., argN ]

behaves the same as

f[ {arg1, arg2, ..., argN} ]

?

Answer

As described by Andy Ross in a comment, you can make a definition that preprocesses the argument(s) into a canonical form. Turning his example around simply to illustrate flexibility:

f[{args__}] := f[args]

f[args__] := Multinomial[args] / Plus[args]

f[{12, 7, 3}] == f[12, 7, 3]

True

This method is useful for more complicated preprocessing, but in simple cases such as this it is often easier to use Alternatives:

g[{args__} | args__] := Multinomial[args]/Plus[args]

g[{12, 7, 3}] == g[12, 7, 3]

True

Be aware that when using Alternatives you must manually order the patterns, for they are tried in sequence. The pattern args__ | {args__} would not work as desired because args__ will match {12, 7, 3} as a single argument.