Rendering the em dash on the terminal

[Dave Kemper]

Commit 298a0281f
(http://git.savannah.gnu.org/cgit/groff.git/commit/?id=298a0281f)
modified how the em dash is displayed on UTF-8 terminals.  I have
reservations about this change and want to get other opinions.

The change applies only to UTF-8 output, the only terminal encoding
that includes an em dash character.  The behavior on all other output
encodings remains the same, changing the em dash to "--".  This is the
longstanding typewriter convention for the em dash, and remains the
best option for character sets that lack the character.

The new logic is this:

.ie '\?\*[.T]\?'\?utf8\?' .char \[em] \[em]\[em]
.el                       .char \[em] --

The motivation is given in the commit log: making \[em] look "more
like a true em dash, taking up two character cells."

I'm sympathetic to the aesthetic concern.  An em dash in any monospace
font is hard to distinguish from a hyphen and other dash-like glyphs.

My first concern is that this motivation is communicated only in the
commit log, leaving a bit of a head-scratcher to anyone merely reading
the code.  If this logic is kept, its motive should be commented in
the code.

My larger concern, though, is with keeping this substitution at all.
Two em dashes in a row is part of no typographic convention.  This
will paste very poorly into any text field that uses a proportional
font.  It interferes with greps and other searches: most readers
seeing two hyphen-like characters in a row in a monospace font will
conclude that they are in fact two hyphens, the longstanding
convention, rather than two em dashes.  It will (rightfully) get
flagged as an error by any software that does grammar and punctuation
checking.  (Admittedly, such software is probably rarely deployed on
terminal output.)

My instinct is that these concerns outweigh the aesthetic benefit of
the substitution on the screen.

But even if the aesthetic concern in monospace-land is given more
weight, two em dashes in a row is a less preferable substitution than
the longstanding convention of two hyphens.  Visually, they'll look
nearly the same, but the two hyphens will have fewer of the drawbacks
of two em dashes.

Opinions (concurring and opposing) wanted.