An hbox-vbox-halign permeable macro, is it possible?

Question

I currently have a Plain TeX file which does something like this in a multitude of places:

\indent\hbox{\vbox{\halign{#\hfil\quad&#\hfil\cr
... table content here ...
}}}

I would love to abstract things out so that this could instead read as:

\mySpecialHAlign{
... table content here ...
}

However, such a macro is not easy for a relative newbie such as myself to define, for reasons that will be obvious to anybody who gives it a try.

Is this sort of macro technically possible to create with the right sort of TeX incantations, or am I doomed to repeat the whole hbox-vbox-halign preamble each and every time?

(Note again that I'm using Plain TeX)

Answer 1

\long\def\mytable#1{%
  \leavevmode
  \vbox{%
    \halign{%
      ##\hfil\quad&##\hfil\cr
      #1\crcr
    }%
  }%
}

\mytable{a&b\cr cc&dd\cr}% or
\mytable{a&b\cr cc&dd}

Remarks:

• \long: The argument of \mytable may use paragraphs (\par).
• I do not think, it belongs to a table macro to decide, whether the current paragraph starts with an indent, if the table macro is used as start of a paragraph. IMHO, \leavevmode is the better choice and it is also used in LaTeX.
• The additional \hbox serves no purpose, thus I have removed it in the macro.
• \crcr helps to catch the case, the user forgets the final \cr. \crcr does nothing, if it is following a \cr or \noalign{...}, but it becomes \cr otherwise to close the final table row.
• # in \halign needs doubling inside definitions.

Variant without argument:

It is also possible to avoid the argument. This is useful, if the arguemnt might contain verbatim code that requires catcode changes:

\def\BeginTable{%
  \leavevmode
  \vbox\bgroup
    \halign\bgroup
      ##\hfil\quad&##\hfil\cr
}
\def\EndTable{%
      \crcr
    \egroup
  \egroup
}

\BeginTable a&b\cr cc&dd\cr\EndTable % or
\BeginTable a&b\cr cc&dd\EndTable

Answer 2

Try

\def\mySpecialHAlign#1{\indent\hbox{\vbox{\halign{##\hfil\quad&##\hfil\cr #1 }}}}

Note the doubling of # since the definition is inside a macro.