LaTeX: to define a macro that needs only one expansion to get the result

## ixquickmacro.sty
%%
%% This is file 'ixquickmacro.sty'.
%%
%% Copyright (c) 2019 Takayuki YATO (aka. "ZR")
%%   GitHub:   https://github.com/zr-tex8r
%%   Twitter:  @zr_tex8r
%%
%% This package is distributed under the MIT License.
%%

%% package declaration
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{ixquickmacro}[2019/12/31 v0.2]
\def\ixzqm@pkgname{ixquickmacro}

%--------------------------------------- environment check

% engine
\RequirePackage{iftex}
\RequireLuaTeX

\unless\ifprimitive\luadef
  \PackageError\ixzqm@pkgname
   {Your LuaTeX does not support \string\luadef}%
   {Package loading is aborted.}%
\expandafter\endinput\fi\relax

%--------------------------------------- general

%% packages
\RequirePackage{etoolbox}[]

%% unique tokens
\def\ixzqm@uniq{\@gobble\ixzqm@uniq@}% vanishing

%% constants
\chardef\ixzqm@iconst=42

%% variables
\newif\ifixzqm@ok
\newluafunction\ixzqm@lua@setglobal
\newluafunction\ixzqm@lua@prepare
\newluafunction\ixzqm@lua@declare
\let\ixzqm@probe\relax
\let\ixzqm@body\relax
\let\ixzqm@lua@target\relax

%--------------------------------------- Lua part

\begingroup
\escapechar\m@ne
\catcode10=12 \endlinechar=10 %
\directlua{
local M = {}; xx = M

local functable = lua.get_functions_table()
local rnumber = luatexbase.registernumber
local create, put_next = token.create, token.put_next
local insert, unpack = table.insert, table.unpack

functable[rnumber('ixzqm@lua@setglobal')] = function()
  M.global = (token.scan_int() > 0)
end

functable[rnumber('ixzqm@lua@prepare')] = function()
  -- save prefix info
  -- TODO: what to do with 'global'?
  local probe = token.create("ixzqm@probe")
  M.protected = probe.protected
  -- check if the target token (CS) is definable
  M.targetcs = token.get_next()
  put_next(create('let'), M.targetcs, create('ixzqm@uniq'),
      create('ifx'), M.targetcs, create('ixzqm@uniq'),
      create('ixzqm@next'), M.targetcs, create('fi'),
      create('ixzqm@fail'))
end

functable[rnumber("ixzqm@lua@declare")] = function()
  local id = rnumber('ixzqm@lua@target')
  local drive = create('ixzqm@drive')
  local delegate = create('ixzqm@D/'..tostring(id))
  functable[id] = function()
    put_next(drive, delegate)
    token.scan_int() -- trigger full expansion
  end
  -- write out luadef statement
  local t = {create('luadef'), M.targetcs, create('ixzqm@lua@target')}
  if M.global then insert(t, 1, create('global')) end
  if M.protected then insert(t, 1, create('protected')) end
  put_next(unpack(t))
end
}\endgroup%

%--------------------------------------- TeX part

%%<*> [\protected]\defquickmacro\CS{<body>}
\newrobustcmd*{\defquickmacro}{%
  \luafunction\ixzqm@lua@setglobal\z@
  \ixzqm@def@quick@macro@a
}
\newrobustcmd*{\gdefquickmacro}{%
  \luafunction\ixzqm@lua@setglobal\@ne
  \ixzqm@def@quick@macro@a
}
\def\ixzqm@def@quick@macro@a#1#{%
  % once pass through TeX processor
  \ixzqm@def@quick@macro@b#1%
}
\def\ixzqm@def@quick@macro@b{%
  \def\ixzqm@probe{}% dummy def for picking up macro prefixes
  \let\ixzqm@next\ixzqm@def@quick@macro@c
  \luafunction\ixzqm@lua@prepare
}
% invoked as: \ixzqm@def@quick@macro@c \CS \fi \ixzqm@fail
\@gobbletwo\if\if \def\ixzqm@def@quick@macro@c#1\fi#2{\fi
  % pick up the macro body
  \afterassignment\ixzqm@def@quick@macro@d
  \def\ixzqm@body
}
\def\ixzqm@def@quick@macro@d{%
  \newluafunction\ixzqm@lua@target
  \global\cslet{ixzqm@D/\number\ixzqm@lua@target}\ixzqm@body
  \luafunction\ixzqm@lua@declare
}

%% \ixzqm@fail
% Gobbles the macro body.
\def\ixzqm@fail{%
  \afterassignment\ixzqm@fail@a
  \gdef\ixzqm@probe
}
\def\ixzqm@fail@a{%
  \global\let\ixzqm@probe\@empty
}

%% \ixzqm@drive\CS
% A execution of scan_int triggers the full expansion.
\def\ixzqm@drive#1{%
  \expandafter\ixzqm@iconst\expanded{#1}%
}

%--------------------------------------- done
\endinput
%% EOF

## test-ixquickmacro.tex
\documentclass{article}
\usepackage{ixquickmacro}
\makeatletter %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code BEGIN
% to show the expanded forms
\def\xInspect#1{\typeout{%
  once->\unexpanded\expandafter{#1}^^J%
  twice->\unexpanded\expandafter\expandafter\expandafter{#1}^^J%
  full->#1}}

%% \todayYMD *once-expands* to the desired token list.
\defquickmacro\todayYMD{%
  % this code is "only fully-ecpandable"
  \the\year/\two@digits\month/\two@digits\day}

%% \todayYMDp is truly unexpandable.
\protected\defquickmacro\todayYMDp{%
  \the\year/\two@digits\month/\two@digits\day}

\makeatother  %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code END
\begin{document}

\xInspect{\todayYMD}
% once->2019/12/31
% twice->2019/12/31
% full->2019/12/31

\stop
	%%
	%% This is file 'ixquickmacro.sty'.
	%%
	%% Copyright (c) 2019 Takayuki YATO (aka. "ZR")
	%% GitHub: https://github.com/zr-tex8r
	%% Twitter: @zr_tex8r
	%%
	%% This package is distributed under the MIT License.
	%%

	%% package declaration
	\NeedsTeXFormat{LaTeX2e}
	\ProvidesPackage{ixquickmacro}[2019/12/31 v0.2]
	\def\ixzqm@pkgname{ixquickmacro}

	%--------------------------------------- environment check

	% engine
	\RequirePackage{iftex}
	\RequireLuaTeX

	\unless\ifprimitive\luadef
	\PackageError\ixzqm@pkgname
	{Your LuaTeX does not support \string\luadef}%
	{Package loading is aborted.}%
	\expandafter\endinput\fi\relax

	%--------------------------------------- general

	%% packages
	\RequirePackage{etoolbox}[]

	%% unique tokens
	\def\ixzqm@uniq{\@gobble\ixzqm@uniq@}% vanishing

	%% constants
	\chardef\ixzqm@iconst=42

	%% variables
	\newif\ifixzqm@ok
	\newluafunction\ixzqm@lua@setglobal
	\newluafunction\ixzqm@lua@prepare
	\newluafunction\ixzqm@lua@declare
	\let\ixzqm@probe\relax
	\let\ixzqm@body\relax
	\let\ixzqm@lua@target\relax

	%--------------------------------------- Lua part

	\begingroup
	\escapechar\m@ne
	\catcode10=12 \endlinechar=10 %
	\directlua{
	local M = {}; xx = M

	local functable = lua.get_functions_table()
	local rnumber = luatexbase.registernumber
	local create, put_next = token.create, token.put_next
	local insert, unpack = table.insert, table.unpack

	functable[rnumber('ixzqm@lua@setglobal')] = function()
	M.global = (token.scan_int() > 0)
	end

	functable[rnumber('ixzqm@lua@prepare')] = function()
	-- save prefix info
	-- TODO: what to do with 'global'?
	local probe = token.create("ixzqm@probe")
	M.protected = probe.protected
	-- check if the target token (CS) is definable
	M.targetcs = token.get_next()
	put_next(create('let'), M.targetcs, create('ixzqm@uniq'),
	create('ifx'), M.targetcs, create('ixzqm@uniq'),
	create('ixzqm@next'), M.targetcs, create('fi'),
	create('ixzqm@fail'))
	end

	functable[rnumber("ixzqm@lua@declare")] = function()
	local id = rnumber('ixzqm@lua@target')
	local drive = create('ixzqm@drive')
	local delegate = create('ixzqm@D/'..tostring(id))
	functable[id] = function()
	put_next(drive, delegate)
	token.scan_int() -- trigger full expansion
	end
	-- write out luadef statement
	local t = {create('luadef'), M.targetcs, create('ixzqm@lua@target')}
	if M.global then insert(t, 1, create('global')) end
	if M.protected then insert(t, 1, create('protected')) end
	put_next(unpack(t))
	end
	}\endgroup%

	%--------------------------------------- TeX part

	%%<*> [\protected]\defquickmacro\CS{<body>}
	\newrobustcmd*{\defquickmacro}{%
	\luafunction\ixzqm@lua@setglobal\z@
	\ixzqm@def@quick@macro@a
	}
	\newrobustcmd*{\gdefquickmacro}{%
	\luafunction\ixzqm@lua@setglobal\@ne
	\ixzqm@def@quick@macro@a
	}
	\def\ixzqm@def@quick@macro@a#1#{%
	% once pass through TeX processor
	\ixzqm@def@quick@macro@b#1%
	}
	\def\ixzqm@def@quick@macro@b{%
	\def\ixzqm@probe{}% dummy def for picking up macro prefixes
	\let\ixzqm@next\ixzqm@def@quick@macro@c
	\luafunction\ixzqm@lua@prepare
	}
	% invoked as: \ixzqm@def@quick@macro@c \CS \fi \ixzqm@fail
	\@gobbletwo\if\if \def\ixzqm@def@quick@macro@c#1\fi#2{\fi
	% pick up the macro body
	\afterassignment\ixzqm@def@quick@macro@d
	\def\ixzqm@body
	}
	\def\ixzqm@def@quick@macro@d{%
	\newluafunction\ixzqm@lua@target
	\global\cslet{ixzqm@D/\number\ixzqm@lua@target}\ixzqm@body
	\luafunction\ixzqm@lua@declare
	}

	%% \ixzqm@fail
	% Gobbles the macro body.
	\def\ixzqm@fail{%
	\afterassignment\ixzqm@fail@a
	\gdef\ixzqm@probe
	}
	\def\ixzqm@fail@a{%
	\global\let\ixzqm@probe\@empty
	}

	%% \ixzqm@drive\CS
	% A execution of scan_int triggers the full expansion.
	\def\ixzqm@drive#1{%
	\expandafter\ixzqm@iconst\expanded{#1}%
	}

	%--------------------------------------- done
	\endinput
	%% EOF
	\documentclass{article}
	\usepackage{ixquickmacro}
	\makeatletter %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code BEGIN
	% to show the expanded forms
	\def\xInspect#1{\typeout{%
	once->\unexpanded\expandafter{#1}^^J%
	twice->\unexpanded\expandafter\expandafter\expandafter{#1}^^J%
	full->#1}}

	%% \todayYMD once-expands to the desired token list.
	\defquickmacro\todayYMD{%
	% this code is "only fully-ecpandable"
	\the\year/\two@digits\month/\two@digits\day}

	%% \todayYMDp is truly unexpandable.
	\protected\defquickmacro\todayYMDp{%
	\the\year/\two@digits\month/\two@digits\day}

	\makeatother %!!!!!!!!!!!!!!!!!!!!!!!!! TeX code END
	\begin{document}

	\xInspect{\todayYMD}
	% once->2019/12/31
	% twice->2019/12/31
	% full->2019/12/31

	\stop