Setting em-dashes with hair spaces

Question

Em-dashes — when used to separate parenthetical clauses — are often used without any spacing, but I rather prefer them with a hair (or perhaps thin) space on either side.

The ‘obvious’ way to do this is probably to write A,---,B rather than A---B (no spaces) or A --- B (unfortunately large spaces). However, this does not allow line breaks around the dash, should they be needed.

The best solution I've been able to come up with is this:

newcommand{mdash}
           {discretionary{}{}{kern 0.1em}---discretionary{}{}{kern 0.1em}}

It looks like that works, but:

Does this look like good practice? 
Is there a standard way to do this?
Is there any way to, say, call this command ---- rather than mdash{}?

barbara beeton · Accepted Answer

tugboat has been setting em-dashes with thin spaces for years.  (the editors are
not fond of the dash nearly crashing into whatever is on either side.)

here is a somewhat simplified version of the tugboat definitions.
Observe that it wipes out preceding and following spaces so results are uniform.

DeclareRobustCommand{thinskip}{hskip 0.16667emrelax}
defemdash{---}
defd@sh#1#2{unskip#1thinskip#2thinskipignorespaces}
defDash{d@shnobreakemdash}
defLdash{d@shempty{hbox{emdash}nobreak}}
defRdash{d@shnobreakemdash}

a line break is permissible on either side of the basic Dash.  (the use of
thinskip with hskip rather than thinspace is what allows this.)

if a line break should be permitted on only the right or left of the dash,
Rdash and Ldash are provided to accommodate that.

lukeuser · Answer

First off I need to note this method works only with pdfTeX (≥1.40).
I used the microtype package as such:
usepackage[activate=false,kerning]{microtype}
SetExtraKerning[unit=character]{encoding=*}{textemdash={100,100}}

As far as I can tell, this is pretty much the sort of thing microtype kerning is designed for, and if nothing else has the advantage that you can still just type ---.
This adds space (but not a space), equal to a tenth of the dash itself, to the left and right (this is half a 'thin space'). The unit is actually a thousandth of what is assigned to unit; unit=1em would make the unit 0.001em.
This package adds some very nice automatic features if you omit the activate=false as well.
A documented (extremely minor in my view) limitation of microtype kerning is that the word following the dash will not be automatically hyphenated—but you can still insert a manual breakpoint. I haven't tested this.
I don't believe this changes the breaking behaviour immediately before or after the dash; I've certainly confirmed that it will still break immediately after, and it still does this in preference to hyphenating a word.

Setting em-dashes with hair spaces

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