[erlang-questions] Merging maps with a fun like dict:merge/3

[Alexander Petrovsky]

Hi!

I've make some little benchmarks, and noticed that the most effective way
is:

merge(F, [Head | Tail]) ->
    lists:foldl(fun(Y, X) -> maps:merge(X, maps:map(fun(Key, Value) ->
F(maps:get(Key, X, 0), Value) end, Y)) end, Head, Tail).


2015-08-23 20:05 GMT+03:00 Ivan Uemlianin <ivan@REDACTED>:

> Dear All
>
> The maps module does not have a merge/3 like dict:merge/3, which calls a
> function fun(Key, Val1, Val2) when both dicts have the same key.  From the
> documentation, "If a key occurs in both dictionaries then Fun is called
> with the key and both values to return a new value."
>
> The dict documentation says that dict:merge/3 is faster than an equivalent
> implemented in erlang.
>
> http://www.erlang.org/doc/man/dict.html#merge-3
>
> Why does the maps module not have a similar merge/3?  Is it because
> writing our own erlang implementation is at fast as it gets?
>
> Below is an implementation which seems to work.  Can it be made faster?
>
>
> merge_maps(Fun, M1, M2) ->
>     maps:fold(fun(K, V1, Acc) ->
>                       case maps:is_key(K, Acc) of
>                           false ->
>                               maps:put(K, V1, Acc);
>                           true ->
>                               V2 = maps:get(K, Acc),
>                               maps:put(K, Fun(K, V1, V2), Acc)
>                       end
>               end,
>               M1,
>               M2).
>
> merge_maps_test() ->
>     merge_maps(fun(_K, V1, V2) -> V1 + V2 end,
>                #{a => 1, b => 2, c => 3},
>                #{b => 2, c => 3, d => 4}) =:=
>         #{a => 1, b => 4, c => 6, d => 4}.
>
>
> With thanks and best wishes
>
> Ivan
>
> --
> ============================================================
> Ivan A. Uemlianin PhD
> Llaisdy
> Speech Technology Research and Development
>
>                     ivan@REDACTED
>                         @llaisdy
>                          llaisdy.wordpress.com
>               github.com/llaisdy
>                      www.linkedin.com/in/ivanuemlianin
>
>                         festina lente
> ============================================================
>
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-- 
Петровский Александр / Alexander Petrovsky,

Skype: askjuise
Phone: +7 914 8 820 815
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