Re: Creating a coding system

[Stephen J. Turnbull]

David Kastrup writes:

 > > The translation table operates on the decoded contents.
 > 
 > Why would something be decoded twice in a row?

Who can give a complete description?  I can't, but sometimes it's
useful, just as sometimes tr(1) is useful.

 > I am missing the big picture here in some manner.  Does decoding not
 > start from a byte stream but rather from an emacs-utf-8 encoded version
 > of a byte stream?

Either (see Eli's reply).  But the translation table is an add-on, not
part of the bytes-to-character coding system itself.  XEmacs's coding
system stuff is quite different (don't start, I'm trying to be helpful
in lieu of a real expert), so I can't be sure, but your definition

(define-coding-system 'midi
  "This converts Midi note-on events to note names"
  :mnemonic ?M
  :coding-type 'charset
  :eol-type 'unix
  :decode-translation-table 'midi-decode-table
  :mime-text-unsuitable t)

doesn't match the idioms used throughout the rest of the :coding-type
'charset coding systems.  Specifically, I suspect the :charset-list
property is required for anything useful to happen.  Also, I suspect
:coding-type 'charset requires a unibyte charset.  I'm not sure it can
be made to work for you, as your "characters" seem to be multibyte.

Try adding :charset-list '(latin-1) (or maybe '(raw-text)) to your
definition.  That should just translate bytes to internal
representation of the characters with codes 0-255, and then your
translation table should DTRT.  If that doesn't do the trick, you'll
have to ask Ken Handa most likely.