On Mon, 9 Aug 2021, Hans van der Meer via ntg-context wrote:

In the TeX to Lua interface with

\def\Macro#1{\directlua{luafunction("#1")}}

the transfer from TeX to Lua is by string. However what to do at the Lua end when calling \Macro{}? As I understand it, then on the Lua side an empty argument and an empty string are indistinguishable.

That is not the case: $ echo 'print("" == nil)' | lua false

This is not always wanted, for example when one needs to place either a nil or an empty string into a Lua table. I am running in trouble when to t.key set to \Macro{} should become nil or when it should become "".

If you call luafunction(""), the string is empty and not nil. \startluacode userdata = userdata or {} function userdata.macro(str) if str == nil then context("Argument is nil") elseif str == "" then context("Argument is empty") else context("Argument is %s", str) end end \stopluacode \define[1]\Macro{\ctxlua{userdata.macro([==[#1]==])}} \starttext \startlines \Macro{} \Macro{ABC} \ctxlua{userdata.macro()} \stoplines \stoptext Aditya