That's incorrect. NFKC ONLY merges canonical equivalents, NOT compatibility equivalents. In fixing the issue with Hangul we could have either elevated the Jamo compatibility decompositions to be canonical, or removed them. We chose to remove them, for a variety of reasons.
As you say, this doesn't matter any more, since the algorithm and the decompositions of existing characters are frozen in stone as of Unicode 3.0. However, your original point is good. We need to change the text of Step #1 in a corrigendum, since as it stands it is a NOP.
John Cowan wrote:
> Mark Davis wrote:
> > There was an problem when it came to normalization. Since the compatibility
> > composition of jamo pieces was, in itself, not adequate for keyboard composition
> > (one needs a more sophisticated algorithm in any event) it was felt best to
> > simply remove the compatibility decompositions.
> > The problem with normalization was that NFKC would not have produced canonical
> > syllables had we left them in.
> I'm confused. In KC normalization, the first macro-step converts all characters
> with compatibility decompositions to their equivalents. That would have
> disassembled ks_f to k_f s_f. Step 1 of Hangul composition would then immediately
> undo this. (Obviously a smart KC algorithm would be able to skip both steps in
> simple cases.)
> > While if you dig back in time to 1998-12-16, in the draft version of
> > http://www.unicode.org/unicode/reports/tr15/tr15-9.html, you will see the
> > following [CC was the draft name for KC]:
> > > Normalization Form CC Examples
> > > ...
> > > u' kakk k_i + a_m + k_f + k_f kak + k_f
> > > Hangul syllables are not maintained.
> I see that TR15-9 says that, but I maintain that it is in error relative
> to Unicode 2.1. Not that it matters any more.
> > This is probably more detail than you really wanted, but I thought it might be
> > interesting to you to see some of the progression in this particular case.
> It certainly is interesting. It is important that the reasons for past
> decisions be documented, so that they are not ignorantly reopened when all
> the former decision-makers have dropped out.
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