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

}

\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

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