All right, I think I've solved the expansion problem I described before: to
ensure that the first input is expanded when it is passed to Lua, I have to
pass it as [==[#1]==], not "#1". But the updated MWE below still does not
seem to work, as the \DoIfPrefixElse macro is printing "NOP" instead of
"YES":

```

\starttexdefinition loc [#1]

\doifassignmentelse{#1} {

% if an assignment, then parse and format accordingly

\getparameters[loc][#1]

% Was a section number specified?

\doifdefined{locsec} {

§ \locsec

}

} {

% otherwise, just print the input as-is

#1

}

\stoptexdefinition


\def\DoIfPrefixElse#1#2{\ctxlua{commands.doifelse(string.find([==[#2]==],"^"..[==[#1]==]))}}


\def\currentbtxloctext{\loc[sec=31]}


\starttext

\currentbtxloctext\blank

\DoIfPrefixElse{§}{\currentbtxloctext}{YES}{NOP}

\stoptext
```

Indeed, if I add a simple \doifelse equality check, it looks like the value
I expect is not the same as what the macro produces, even though they look
identical:

```

\starttexdefinition loc [#1]
    \doifassignmentelse{#1} {
        % if an assignment, then parse and format accordingly
        \getparameters[loc][#1]
        % Was a section number specified?
        \doifdefined{locsec} {
            § \locsec
        }
    } {
        % otherwise, just print the input as-is
        #1
    }
\stoptexdefinition

\def\DoIfPrefixElse#1#2{\ctxlua{commands.doifelse(string.find([==[#2]==],"^"..[==[#1]==]))}}

\def\currentbtxloctext{\loc[sec=31]}

\starttext
    § 31\blank%the raw text we expect
    \currentbtxloctext\blank%the text as produced by the macro
    \doifelse{\currentbtxloctext}{§ 31}{YES}{NOP}\blank% should output YES,
but doesn't
    \DoIfPrefixElse{§}{\currentbtxloctext}{YES}{NOP}\blank% should output
YES, but doesn't
\stoptext
```

What am I missing here?

Joey

On Fri, Nov 26, 2021 at 11:57 AM Joey McCollum <jmccollum20140511@gmail.com>
wrote:

> Thanks! Invoking string.find(str,"^"..pre) is certainly much easier. This
> sort of thing would be a nice general-purpose helper function. Denis Maier
> and I should have a "wish list" of desired helpers and features (mostly
> related to bibliographies/publication support) ready for you (and Alan)
> sometime soon.
>
> But for now, it looks like I still need to work out errors that arise when
> I pass a macro as the "str" input to be searched. I've defined a \loc macro
> that accepts an assignment and outputs a formatted string, and later, I
> want to check if this macro (after it has been fully expanded) starts with
> a certain prefix. A minimal (non-)working example follows:
>
> ```
>
> \starttexdefinition loc [#1]
>
> \doifassignmentelse{#1} {
>
> % if an assignment, then parse and format accordingly
>
> \getparameters[loc][#1]
>
> % Was a section number specified?
>
> \doifdefined{locsec} {
>
> § \locsec\btxcomma
>
> }
>
> } {
>
> % otherwise, just print the input as-is
>
> #1
>
> }
>
> \stoptexdefinition
>
>
>
> \def\DoIfPrefixElse#1#2{\ctxlua{commands.doifelse(string.find("#2","^".."#1"))}}
>
>
> \def\currentbtxloctext{\loc[sec=31]}
>
>
> \starttext
>
> \DoIfPrefixElse{§}{\currentbtxloctext}{YES}{NOP}
>
> \stoptext
> ```
>
> Lua is throwing an "invalid escape sequence near '"\l'" error, which is
> presumably because the macro \loc[sec=31] is not being expanded. How do I
> fix this?
>
> Thanks!
>
> Joey
>
> On Fri, Nov 26, 2021 at 3:46 AM Hans Hagen via ntg-context <
> ntg-context@ntg.nl> wrote:
>
>> On 11/26/2021 7:42 AM, Joey McCollum via ntg-context wrote:
>> > I wasn't aware of a general-purpose "doifstartswith" macro in ConTeXt
>> > (the \doifnextcharelse macro only works one character at a time, and
>> the
>> > \doifinstring macros may capture substrings that are not prefixes), and
>> > I'd like to develop one for something I'm working on. I've been trying
>> > to do this in Lua, as that seemed like the most natural approach.
>> > Normally, something like this would work fine as a foundation:
>> >
>> > ```
>> >    function isprefix(prefix, str)
>> >      if string.sub(str, 1, string.len(prefix)) == prefix then
>> >        return true
>> >      end
>> >      return false
>> >    end
>> > ```
>>
>> how about
>>
>> if string.find(str,"^"..prefix) then
>>
>> in:
>>
>> \starttext
>>
>>
>> \def\DoIfPrefixElse#1#2{\ctxlua{commands.doifelse(string.find("#2","^".."#1"))}}
>>
>> \DoIfPrefixElse{pre}{prefix}{YES}{NOP}
>> \DoIfPrefixElse{pre}{suffix}{YES}{NOP}
>>
>> \stoptext
>>
>> utf strings are just sequences of bytes so matching works
>>
>> when you want to do more in lua you can decide for
>>
>> \startluacode
>> interfaces.implement {
>>      name      = "DoIfPrefixElse",
>>      arguments = { "argument", "argument" },
>>      actions   = function(pre,str)
>>          commands.doifelse(string.find(str,"^"..pre))
>>      end
>> }
>> \stopluacode
>>
>> \DoIfPrefixElse{pre}{prefix}{YES}{NOP}
>> \DoIfPrefixElse{pre}{suffix}{YES}{NOP}
>>
>> but in any case: make sure that you don't clash with built in ...
>>
>> if needed i can make a set of fast(er) ones but someone has to collect a
>> list of 'handy helpers' first
>>
>> Hans
>>
>>
>> -----------------------------------------------------------------
>>                                            Hans Hagen | PRAGMA ADE
>>                Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>>         tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
>> -----------------------------------------------------------------
>>
>> ___________________________________________________________________________________
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>