# [NTG-context] \ctxlua and \startluacode ... \stopluacode

Hans Hagen j.hagen at xs4all.nl
Tue Oct 15 10:15:34 CEST 2019

```On 10/15/2019 10:04 AM, Taco Hoekwater wrote:
> Hi Rudolf,
>
>> On 15 Oct 2019, at 00:50, Rudolf Bahr <quasi at quasi.de> wrote:
>>
>> Now to your suggestion not to use "assert ()". Indeed it's a solution! My program
>> works! I append again its output (again just for fun!). Did you really try it or
>> has it been just an idea?
>
> First, let me say that embedded lua versions are typically a little bit different
> from standalone. Just how much different depends on the embedding program. Luatex
> is actually pretty close to standalone lua. Luatex adds a bunch of extension libraries,
> but it changes very little of the core language. And all those changes are documented
> in the luatex manual. None of those changes affect your minimal example, except
> that lua errors are handled a little differently.
>
> Note that standalone lua *also* produces an error, as that is what assert() is
> supposed to do. And when lua runs into an error, it ignores the rest of the current
> chunk:
>
>    “... whenever an error occurs, Lua ends the current chunk and returns
>    to the application.” (from lua.org)
>
> Standalone lua typically sees only one chunk (the file you pass on the command
> line) but embedded lua implementation often see (sometimes many) more chunks.
> In luatex’s case, each \directlua is a separate chunk (in ConTeXt, that means every
> \startluacode block and every \ctxlua call is a separate chunk).
>
> The only unusual thing here is that standalone lua silently quits and returns a
> non-zero exit code to the shell, whereas luatex gives you the typical TeX-style
> error prompt. The rationale for that is: lua errors can happen in many places in
> your input file, and if they were silently ignored, your typeset pages could be
> wrong without you realising it.
>
>
> And to answer your question above: I did not have to try or guess. I know about
> how assert() works because it is documented in the lua manual (and as it closely
> mimics the assert() C function, that is easy for me to remember).
as you say, it's all normal lua behaviour:

if you do:

print(io.open("crap.crap"))

one gets

nil	crap.crap: No such file or directory	2

and i think that assert then returns the second returned argument

but if one does

context(io.open("crap.crap") and "yes" or "no")

then the first argument is checked against

Hans

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