Re: Metafont optional parameters (Owen Lamb)

[Christopher Heckman]

On Sun, May 31, 2020 at 12:50 AM <lilypond-devel-request@gnu.org> wrote:
>
> Date: Sat, 30 May 2020 15:41:31 -0700
> From: Owen Lamb <owendlamb@gmail.com>
> To: Werner LEMBERG <wl@gnu.org>, lilypond-devel@gnu.org
> Subject: Metafont optional parameters
> Message-ID:
>         <CAHpoxV-+XVN3x5QU6gh0yMkGXp6tSu4t-y7H4QVGSWdx=UFnSA@mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Hi all,
>
> I'd like to add an optional parameter for smuflcode to fet_beginchar, so
> that I don't have to take two lines redeclaring the variable in every
> glyph. Ideally, it won't have to be optional once every character has it,
> but in the meantime, it would help with testing individual characters' new
> encodings.
>
> I wasn't able to find whether MF supports optional parameters, only that
> certain built-in functions have them. Does anyone with MF experience know
> if it does, and, if so, how to put one in?
>
> Thanks,
> Owen

The METAFONTbook shows how to handle a variable number of arguments to
a macro. It gives examples about how to write max() with a variable
number of parameters.

If you want to allow at least one parameter, then look at Appendix D
(Dirty Tricks), trick #2 (pp. 290-291 in my version):

vardef max(text t) =
  let switch_ = firstset_;
  for u = t: switch_ u > u_: u_ := u ;fi endfor
  u_ enddef;

def firstset_ primary u =
 setu_ u; let switch_ = if; if false: enddef.

If you want to allow at least two parameters (but not one), it's a bit
easier; the code is on page 266 of Appendix B (Basic Operations):

vardef max(expr u)(text t) = % t is a list of numerics, pairs or strings
  save u_; set u_ u; for uu = t: if uu > u_: u_ := uu; fi endfor
  u_ enddef;

def setu_ primary u =
  if pair u: pair u_ elseif string u: string u_ fi;
  u_ = u enddef;

Any typos are my fault, not DEK's.

--- Christopher Heckman