ets:match/2 with maps

[Fred Youhanaie]

Thanks, Peti.

I was hoping that I wouldn't have to resort to select.

In my use case the ets:match is called from within a gen_server, and the match pattern, which can be any tuple, is supplied by the client. So, hand-crafted ets:select will not help!

I have now opted for a different data/tuple format.

Cheers,
Fred


On 13/04/2020 19:31, Peti Gömöri wrote:
> Hi Fred,
> 
> Unfortunately just with pattern matching it is not possible to exactly match a map, but adding a map_size match condition in the match-spec could work:
> 
> 7> ets:select(T, ets:fun2ms(fun({K, #{one := 11}} = O) when map_size(element(2, O)) =:= 1 -> K end)).
> [gamma]
> 
> On Mon, Apr 13, 2020 at 2:34 PM Fred Youhanaie <fly@REDACTED <mailto:fly@REDACTED>> wrote:
> 
>     Hi
> 
>     I couldn't find any details on map based match patterns!
> 
>     ETS allows maps as elements of the tuples.
> 
>     ets:match/2 allows map based match patterns, and returns a result if the pattern map is a subset of the a matched element.
> 
>     Is there a way of matching a tuple that is not a superset of the map pattern, i.e. only contains the keys in the map pattern?
> 
>     To clarify, in the following example I would like ets:match/2 to only return [[gamma]].
> 
>     Many thanks
>     Fred
> 
>     1> T = ets:new(aaa, []).
>     #Ref<0.3164011085.1783496708.202417>
> 
>     2> ets:insert(T, {alpha, #{one=>1, two=>2}}).
>     true
> 
>     3> ets:insert(T, {beta, #{one=>11, two=>12}}).
>     true
> 
>     4> ets:insert(T, {gamma, #{one=>11}}).
>     true
> 
>     5> ets:tab2list(T).
>     [{gamma,#{one => 11}},
>        {alpha,#{one => 1,two => 2}},
>        {beta,#{one => 11,two => 12}}]
> 
>     6> ets:match(T, {'$1', #{one=>11}}).
>     [[gamma],[beta]]
>