What do you mean that there is no standard decoder? For what, UTF-8?
I agree on the composability. Accepting Unicode code points is more generic. I guess it depends on your environment. If every caller will combine it with a UTF-8 decoder, you might want to include it.
I meant something like a standard component that everybody uses (or a selection of components that vary here and there but are interchangeable in general). A catalog of such components; a - well, I'll say it - a software factory that builds them, a fast one, a slower but very detailed on errors, one that accepts single bytes, one that takes a pointer to a string, and so on, all different, yet all working in the same way as far as the main task is concerned.
I agree on the composability. Accepting Unicode code points is more generic. I guess it depends on your environment. If every caller will combine it with a UTF-8 decoder, you might want to include it.