[XeTeX] Whitespace in input

[Philip TAYLOR]

Ross, I do not dispute your arguments : I was answering
Keith's question in an honest way.  I (personally) do not
think of a space in TeX output as a character at all,
because I am steeped in TeX philosophy; but I am quite
willing to accept that /if/ the objective is not to
produce output for the sake of output, but output for
subsequent processing as input by another program, then
there /may/ be an argument for outputting a space as a
variable-width glyph.

However, I do think that what appears in the output stream
is a secondary consideration; far more important (IMHO) is
how we represent that space /within XeTeX/.  There is, I am
sure, not a suggestion on the table that we start to treat
a conventional space in XeTeX other than as TeX has traditionally
treated it, and therefore the real question is (to my mind),
"do we adopt an extension of this traditional TeX treatment
for non-breaking space, thin-space, and any of the other
not-quite-standard spaces that Unicode encompasses, or do
we look for an alternative model which /might/ be glyph-
or character-based ?".

** Phil.