[erlang-questions] optimal way to append an element in a list inside a map

Jesper Louis Andersen <>
Tue Aug 29 21:34:40 CEST 2017


If you measure and find the map to be too slow for you, the use case you
have where the contents are ephemeral suggest you should look into using an
ETS table of type bag or ordered_bag. You can get some really fast code by
using this if you don't need the persistence the map provides.

I'd measure first though. Chances are the maps are plenty fast. You can
always encapsulate the implementation so you can subst in ETS later if need
be.

On Tue, Aug 29, 2017 at 9:16 PM Caragea Silviu <> wrote:

> Hello,
>
> Thanks for clarification. I thought that in background erlang can see when
> there is no other reference to the original map/list (for example in case
> of accumulator inside a recursive fun) and will append to the original
> object while updating the reference instead of making partial copy.
>
> Silviu
>
> On Tue, Aug 29, 2017 at 10:01 PM, Sverker Eriksson <
> > wrote:
>
>> Maps have two implementations depending on number of keys.
>>
>> Small maps (<= 32 keys) are basically a key-tuple and a value tuple.
>> Updating the value of an existing key will copy the value-tuple
>> while reusing the key-tuple.
>>
>> Large maps (> 32 keys) are basically a tree (HAMT).
>> All nodes from the root to the one containing the key must be copied
>> while all other nodes can be reused.
>>
>> /Sverker
>>
>> On 08/29/2017 08:41 PM, Caragea Silviu wrote:
>>
>> Hmm,
>>
>> Basically what you are saying is that any update over a map requires
>> rebuilding of the entire map ? I doubt as time implementation seems so
>> lame..
>>
>> Any way that we con confirm or not this theory ?
>>
>> Silviu
>>
>> On Tue, Aug 29, 2017 at 9:34 PM, Dmytro Lytovchenko <> wrote:
>>
>>
>>
>> 2017-08-29 20:19 GMT+02:00 Dmitry Kolesnikov <> <>:
>>
>>
>> Hello,
>>
>> Premature optimisation is an evil ;-)
>>
>> I would use the following syntax:
>> ```
>> append(Key, Value, Map) ->
>>    List = case Map of
>>       #{Key := Tail} ->
>>          [Value | Tail];
>>       _ ->
>>          [Value]
>>    end,
>>    Map#{Key => List}.
>> ```
>>
>> Lists are not copied they are referenced. Maps… Hmm, I am not sure. I
>> hope the implementation is smart enough to keep reference as well.
>>
>>
>> In BEAM any object which does not fit into a machine register (such as a
>> list or a map) will be placed on heap and a pointer will be used instead.
>> But prepending to a list will change the list pointer value (the pointer to
>> list head will change, the remaining tail elements will not move and will
>> not change). This requires writing the new head into the map value. And
>> this will incur a map update, which most likely will rebuild the map. I'm
>> almost sure that the optimization possibilities for this are very limited
>> and are similar to tuple optimizations (i.e. works only at creation time).
>>
>>
>>
>> - Dmitry
>>
>>
>>
>> On 29 Aug 2017, at 20.34, Caragea Silviu <> <> wrote:
>>
>> Hello,
>>
>> Having a map where the value of each element it's a list :
>>
>> #{ 1 => [], 2 => [], ... n => []}
>>
>> and you need to append elements in the list for a specific key, what's
>>
>> the most optimal way to do this without copying the lists and the map
>> inside the VM lot of times ?
>>
>> Anything better than the following solution:
>>
>> append_element(Key, Value, Map) ->
>>     case maps:find(Key, Map) of
>>         {ok, V} ->
>>             maps:put(Key, [Value | V], Map);
>>         _ ->
>>             maps:put(Key, [Value], Map)
>>     end.
>>
>> Kind regards,
>> Silviu
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