digraph edges and improper lists

[Fred Youhanaie]

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!

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
>