Re: Checking for a macro in a string without expanding it

[Hans Hagen via ntg-context <ntg-context@ntg.nl>]

On 11/16/2021 8:22 PM, Joey McCollum via ntg-context wrote:
> As the subject of this question suggests, this is really more of a 
> question about expansion control (a topic that is still a bit obscure to 
> me). Suppose I have a macro \inner that expects a single argument or an 
> assignment of parameters in brackets. For my purposes, I don't want this 
> macro to do anything when it is typeset, so I'll just define it as empty:
> 
> ```
> \def\inner[#1]\empty
> ```
> 
> Now suppose I have another macro \outer that invokes this macro with 
> some specific input and sets some plain text after it:
> 
> ```
> \def\outer{\inner[123] etc.}
> ```
> 
> What I'd like to do is parse the argument of \inner in \outer. I was 
> hoping that a string search in Lua would work, but I'm not having any 
> luck. A minimal (non)-working example is included below:
> 
> ```
> 
> \def\inner[#1]\empty
> 
> \def\outer{\inner[123] etc.}
> 
> 
> \startluacode
> 
> local userdata = userdata or {}
> 
> function userdata.parseinner(str)
> 
> local innerparams = ""
> 
>      if string.find(str, "\\inner(%b[])") then
> 
>        i, j = string.find(str, "\\inner(%b[])")
> 
>        innerparams = string.sub(str, i+1, j-1) -- we just want the 
> content inside the brackets
> 
>      end
> 
>      context(innerparams)
> 
>      return
> 
> end
> 
> \stopluacode
> 
> \def\parseinner#1{\ctxlua{userdata.parseinner([==[#1]==])}}
> 
> 
> \starttext
> 
> Testing:\blank
> 
> \parseinner{\outer}
> 
> \stoptext
> 
> ```
> 
> My problem is that when I pass \outer to the \parseinner macro, it gets 
> fully expanded, so there isn't anything left to match "\\inner%b[]". Is 
> there a way to expand \outer when I pass it to the \parseinner macro 
> without also expanding the \inner macro inside it? Or is there some 
> other preferred way of doing this?

One can always abuse native tex:

\starttext

\def\MyOuter#1%
   {\beginlocalcontrol % hides the next
    \let\Indeed\empty
    \def\MyInner[##1]{\gdef\Indeed{##1}}%
    \setbox\scratchbox\hpack{#1}%
    \endlocalcontrol  % but only if needed
    \Indeed}

whatever: \MyOuter{\MyInner[oeps]}%

\edef\ItWorks{\MyOuter{\MyInner[oeps]}}

whatever: \meaningless\ItWorks

\stoptext



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