How to implement token.get_char

Question

There is a token.set_char but no token.get_char. Starting from

\chardef\MyNumber=1

I can change the value with

\directlua{
  token.set_char('MyNumber', 421)
}

and retrieve the value on the TeX side

\the\MyNumber

but how to retrieve the value on the lua side?

I have one solution in mind but it is not satisfying: with tex.runtoks, lua can ask tex to copy MyNumber to a dedicated count register, and then retrieve the value using tex.count.

Answer 1

The fastest way is to extract the value from the token using .index

\directlua{
  local char_given = token.command_id'char_given'
  local function get_char(name)
    local tok = token.create(name)
    if not (tok.command == char_given) then
      error'Not a chardef'
    end
    return tok.index
  end
  token.set_char('MyNumber', 421)
  local restored = get_char'MyNumber'
}
\bye

but if you want it to be more generic and also e.g. accepts count registers you can also add the token to the input stream and than scan it as a number:

  local function get_char(name)
    token.put_next(token.create(name))
    return token.scan_int()
  end
  token.set_char('MyNumber', 421)
  local restored = get_char'MyNumber'
}
\bye

Answer 2

If you are using \directlua (i.e. no external .lua file) then you can get the value from TeX world to the Lua world by expanding it inside the Lua code, because \directlua expands its argument before the Lua code is read. For example

\directlua{... ... myvariable = \the\MyNumber ... ...}