For default values, there are at least two choices. The first one I mentioned -- just create a variable and export it. Users can pass it:

In skein package:

   var DefaultHash = &Args{ ... } // the default args you want

Users will call it like this:

   skein.New(32, &skein.DefaultHash)

Another way, better for some cases, is have methods on arguments struct, like in base32 package from stdlib http://golang.org/pkg/encoding/base32/

  base32.StdEncoding.DecodeToString(...)
  base32.HexEncoding.DecodeToString(...)

In most cases (but now all), there are better ways to design API than stuff them with default arguments, which make functions behave unexpectedly if you forget something.

But maybe we should talk about specific cases? What are the examples of functions which need to accept default arguments? Maybe I can try to convert them into something reasonable in Go?