Re: Walk and deep copy in Lua filters

[BPJ <melroch-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Why not use the clone method to get a copy? That said I like that walk
modifies a copy.

The only builtin Lua function which alters its argument in place I can
think of OTTOMH is table.sort, which is a gotcha because strings are
immutable so I always forget that table.sort modifies in place.

Den sön 12 juni 2022 13:18Bastien DUMONT <bastien.dumont-VwIFZPTo/vqsTnJN9+BGXg@public.gmane.org> skrev:

> Hi all,
>
> I just discovered by chance that the method walk returns a deep copy of
> the passed object, leaving the original object untouched. See the following
> test case:
>
> ```deep-copy.lua
> local orig = pandoc.Emph({ pandoc.Str('one'), pandoc.Space(),
> pandoc.Str('two') })
> local copy = orig:walk({})
> table.insert(copy.content, pandoc.Space())
> table.insert(copy.content, pandoc.Str('three'))
> assert(#orig.content == 3)
> assert(#copy.content == 5)
> ```
> `pandoc -L deep-copy.lua <<< ''` should return nothing (i.e. no error).
>
> However, in the documentation, it is only stated that the “Result” is the
> “filtered block/inline element”: from this indication alone, it seems not
> obvious to me whether the passed object is changed (as it is customary with
> functions operating on Lua tables or userdata) or the method returns a new
> object.
>
> To be clear, I am very happy with the current behaviour, for it gives a
> very simple means to get several deep copies of an objet and to manipulate
> them separately without having to regenerate the original object (e.g. via
> rather expensive calls to `pandoc.read()`). Nevertheless, since it is not
> really documented, can it be considered to be deliberate and stable? If
> this is the case, could it be possible to make explicit in the
> documentation, under the header “Result”, that the original block or inline
> element is left unchanged?
>
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