[NTG-context] Checking existence of a macro (control sequence) by Lua

[Hans Hagen]

On 5/28/2020 11:24 PM, context at vivaldi.net wrote:
> Hello,
> 
> one more question - why macros \Undefined and \DoNothing show "defined" 
> although I (tried to) undefine them?
> 
> ----
> \starttext
>    \def\MyMacro{Ahoj}
>    \def\MyMac#1#2{Something}
> 
>    \def\Undefined{}
>    \let\Undefined\undefined
> 
>    \def\DoNothing{}
>    \let\DoNothing\donothing
> 
>    \startluacode
>      local str = { [true] = "defined", [false] = "undefined" }
> 
>      local function whatever(s)
>        context.type("\\" .. s)
>        context(" is " .. (tokens.defined(s) and "defined" or 
> "undefined")) --% Hans' way
>        context(" and is " .. str[token.is_defined(s)] .. ".") --% 
> Henri's way
>        context.par()
>      end
> 
>      whatever("MyMacro")
>      whatever("MyMacroD")
>      whatever(" ")
>      whatever("-")
>      whatever("Undefined")
>      whatever("DoNothing")
>    \stopluacode
> 
>    %\Undefined % Causes "! Undefined control sequence"
> \stoptext
> ----
> 
> Gives:
> 
> "
> \MyMacro is defined and is defined.
> \MyMacroD is undefined and is undefined.
> \ is defined and is defined.
> \- is defined and is defined.
> \Undefined is defined and is defined.
> \DoNothing is defined and is defined.
> "
Because the macro actually *is* defined: as soon as tex sees

\foo

it reserves the name and gives it the meaning undefined, so even 
\undefined is defined.

Anyway, that is why we have \ifdefined that does a different kind of 
checking. In retrospect, that is a better one, so I'll adapt that in lmtx.

\starttext
   \let\MyMacroA\undefined

   \startluacode
     local function whatever(s)
         context.type("\\" .. s)
         context(" is " .. (tokens.defined(s,true) and "defined" or 
"undefined")) --% Hans' way
         context(" and has meaning " .. (tokens.defined(s) and "defined" 
or "undefined")) --% Hans' way
         context.par()
     end

     whatever("MyMacroA")
     whatever("MyMacroB")
   \stopluacode

\stoptext

\MyMacroA is defined and has meaning undefined
\MyMacroB is undefined and has meaning undefined


You can do this in current luatex/mkiv:

     if CONTEXTLMTXMODE == 0 then

         local d = tokens.defined
         local c = tokens.create

         function tokens.defined(s,b)
             if b then
                 return d(s)
             else
                 return c(s).cmd_name == "undefined_cmd"
             end
         end

     end

Hans


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                                           Hans Hagen | PRAGMA ADE
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