Re: [Groff] \[-+] not available? why?

[Ralph Corderoy]

I wrote:
>     $ echo '\[-+]' | nroff | grep .
>     ∓
>     $ echo '\[-+]' | nroff -Tascii | grep .
>     -+
>     $ echo '\[-+]' | nroff -Tiso-8859-1 | grep .
>     ∓
>     $

I made a mistake with `nroff -Tiso-8859-1';  that's not a valid -T
value.  nroff(1) says

    If neither the GROFF_TYPESETTER environment variable nor the -T
    command line option (which overrides the environment variable)
    specifies a (valid) device, nroff checks the current locale to
    select a default output device.

Why is it useful to have nroff ignore a duff -T value and fall back as
if it was unspecified?

Switching to the valid equivalent(?):

    $ echo '\[-+]' | nroff -Tlatin1 | grep .
    -+
    $

latin1(7) here says

    Oct   Dec   Hex   Char   Description
    ───────────────────────────────────────────
    ...
    261   177   B1     ±     PLUS-MINUS SIGN

and that seems defined in the device's fonts

    $ fgrep +- font/devlatin1/R
    +-      24      0       0261
    t+-     "
    $

so why isn't grotty outputing the single byte for it?

> I'm guessing it's to do with the type of output you're producing.

As Carsten's said.

    $ echo '\[-+]' | groff >/dev/null
    <standard input>:1: warning: can't find special character `-+'
    $

The PostScript glyph name is `plusminus'.

    $ curl -sS 
http://partners.adobe.com/public/developer/en/opentype/glyphlist.txt |
    > grep plusminus
    plusminus;00B1
    $

It's used in the Times Roman font definition.

    $ grep plusminus font/devps/TR
    t+-     564,506 0       177     plusminus
    $

But for some reason, it's called `t+-'.  Using that works, with the
appropriate single byte being printed.

    $ echo '\[t+-]' | groff | grep /F0
    /F0 10/Times-Roman@0 SF<b1>72 12 Q 0 Cg EP
    $ echo $((0xb1))
    177
    $

Why it's not known as \[+-] too, I don't know.

So, I haven't answered the "whys".  Someone else can, I'm sure.  :-)

Cheers, Ralph.