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Message-ID:  <e2e84d22-bf7b-c88f-9ef0-6ae7246b135e@morningstar2.co.uk>
Date:         Wed, 19 Feb 2020 08:15:50 +0000
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From:         Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: Request for Clarification on Boxes and Functions
To:           LATEX-L@LISTSERV.UNI-HEIDELBERG.DE
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Status: R

On 19/02/2020 00:50, Phelype Oleinik wrote:
> Hi Kelly,
> 
>> - Are all boxes and coffins made so that their content is wrapped in a TeX group?
>>    More specifically, is it safe for code to assume that all box/coffin content is
>>    implicity grouped?
> 
> Yes, because the underlying TeX box makes a group:
>      \count0=1 \hbox{\count0=2 }\the\count0 \bye
> so you can assume the grouping.

All boxes are also explicitly documented as colour-safe, so have an 
additional group level.

>> - Many functions in expl3 are of the form `a := func(b, c)` (e.g. `\str_concat:NNN`,
>>    `\seq_set_map:NNn`) or `a := func(b)` (e.g. `\int_set:Nn`, `\tl_set:Nn`).
>>    Sometimes, one will want to write code like `a := func(a, b)` or `a := func(a)`,
>>    that is, directly assign a new value to a variable based on its current value.
>>    Is it ever safe to write such code in expl3?
> 
> Yes, because first `func(a, b)` is evaluated, and only then
> the assignment is performed. Something like:
>      \def\x{b} \edef\x{a\x c}
> makes `\x=macro:->abc`.
> 
> Though this last one may depend a bit on the macro
> implementation (if the function needs intermediate steps
> to do its job, for example), but I'd expect to able to rely
> on that behaviour.

This is safe for all expl3 functions.

Joseph