digraph edges and improper lists

[Mikael Pettersson]

On Mon, Dec 20, 2021 at 6:12 PM Fred Youhanaie <fly@REDACTED> wrote:
>
> Hi Dmytro
>
> Many thanks for the explanation and the links.
>
> So, we save memory when using an improper list instead of proper list of 2 elements.
>
> How about 2-element tuples? It seems to be similar to the improper list, but I haven't read text properly yet!

A 2-element tuple consumes 2+1 = 3 words, so it's not as compact as a
single list cell. It's also faster to determine the shape of that
single list cell than of the 2-element tuple (for the list cell we
only need to inspect the handle, while for the 2-element tuple we also
have to fetch its header and inspect that).

As long as this is hidden behind appropriate ADTs no-one should care
what the representation is.


>
> Cheers,
> Fred
>
>
> On 20/12/2021 16:28, Dmytro Lytovchenko wrote:
> > A list of 2 elements will consume:
> > 1 list cell (2 words of memory) to store $n and pointer to second cell
> > 1 list cell (2 more words) to store N and [] (that's nil, end of proper list)
> > total: 4 words of memory, on a 64 bit machine that's 32 bytes
> >
> > An improper list of 2 elements will consume:
> > 1 list cell (2 words of memory) to store $n and tail is stored as N
> > total 16 bytes of memory on 64 bit machine
> >
> > http://beam-wisdoms.clau.se/en/latest/indepth-memory-layout.html#lists-cons <http://beam-wisdoms.clau.se/en/latest/indepth-memory-layout.html#lists-cons>
> > http://beam-wisdoms.clau.se/en/latest/indepth-data-sizes.html#list <http://beam-wisdoms.clau.se/en/latest/indepth-data-sizes.html#list>
> >
> > On Mon, 20 Dec 2021 at 17:23, Fred Youhanaie <fly@REDACTED <mailto:fly@REDACTED>> wrote:
> >
> >     Hi
> >
> >     The digraph module uses the term ['$e'|N] to represent an edge.
> >
> >     Is there an advantage in using an improper list, as opposed to the more intuitive tuple format {'$e', N}?
> >
> >     I recently came across a situation where the following crashed because logger tried to flatten the list of improper lists!
> >
> >     logger:notice(#{ edges => digraph:edges(G) }).
> >
> >     However, these two were OK:
> >
> >     logger:notice(#{ edges => { digraph:edges(G) } }). %% Note the extra braces
> >     logger:notice("edges = ~w", [digraph:edges(G)] ).
> >
> >     For me this was a temporary annoyance, but I'm just curious about the choice of list structure. Is this explained anywhere, such as docs or a blog?
> >
> >     Cheers,
> >     Fred
> >