Can I make a macro behave like \numexpr?

Question

For example I define \fpeval in Plain LuaTeX:

\begingroup
\catcode`\%=12
\directlua{
   function math.round_int ( x )
     return x>=0 and math.floor(x+0.5) or math.ceil(x-0.5)
   end
   function math.round ( x , n )
     return math.round_int ( x*10^n ) / 10^n 
   end
   function gobblezero(x)
     local y = math.round ( x , 8 )
     if y == math.floor(y) then
       return string.format ( "%.0f", y )
     else
       return math.round(y, 7)
     end
   end}
\endgroup
\def\fpeval#1{\directlua{tex.print(gobblezero(#1))}}
\directlua{%
    p = 68
    P = 80}
You have \fpeval{p} of \fpeval{P} points, this makes $\fpeval{p * 100/P},%$.
\bye

But I would like to have a macro called \fpexpr that behaves like \numexpr, so that I can write:

You have \fpexpr p of \fpexpr P points, this makes $\fpexpr p * 100/P \,\%$.

In the first case \fpexpr should take only p as the argument because of has never been defined to be a number or a function like \directlua{of = ...}. The second and the third case are similar as points and \, are undefined in the Lua interpreter.

So \fpexpr should add everything to the lua code until something would be added that would cause an error.

Is this possible?

Answer 1

This is far from complete but it does some cases.

It is very chatty on the terminal:

token is p
variable found 68 p
token is of
stop at of collected: 68
2: eval result is 68 x
token is P
variable found 80 P
token is points,
stop at points, collected: 80
2: eval result is 80 x
token is p
variable found 68 p
token is *
token is 100
token is /
token is P
variable found 80 P
token is ?
1: eval result is 85.0

code is

\begingroup
\catcode`\%=12
\directlua{
   function math.round_int ( x )
     return x>=0 and math.floor(x+0.5) or math.ceil(x-0.5)
   end
   function math.round ( x , n )
     return math.round_int ( x*10^n ) / 10^n 
   end
   function gobblezero(x)
     local y = math.round ( x , 8 )
     if y == math.floor(y) then
       return string.format ( "%.0f", y )
     else
       return math.round(y, 7)
     end
   end}
\endgroup
\def\fpeval#1{\directlua{tex.print(gobblezero(#1))}}
{\catcode`%=12
\xdef\pc{%}
}
\directlua{
function zz (s)
local t = token.scan_string()
print ('token is ' .. (t or '?'))
if (t == nil) then
local f = loadstring('r= ' .. s)
f()
print('1: eval result is ' .. (r or '?'))
tex.print(r)
else
% t is local variable or operator or number (inexact pattern)
if (_G[t] \string~= nil) then
% recurse with value
print ('variable found ' .. _G[t] .. ' ' .. t)
 zz(s .. _G[t])
else if(string.find(t,'^[+-/*().]?\pc d*$')) then
% recurse
 zz(s .. t)
else
% evaluate string so far and retrun result and the rejected token
print('stop at ' .. t ..  ' collected: ' .. s)
local f = loadstring('r= ' .. s)
f()
print('2: eval result is ' .. (r or '?') .. ' x')
tex.print(r .. ' ' .. t )
end
end
end
end
}
\def\fpzz{\directlua{zz('')}}
\directlua{%
    p = 68
    P = 80}
You have \fpzz p of \fpzz P points, this makes $\fpzz p * 100 / P ,%$.
\bye