[erlang-questions] analogue of lists:keyfind and other lists:key*** functions for lists which contain maps

Sergej Jurecko <>
Sun Nov 23 16:04:18 CET 2014


I just converted a ~5k lines of code project into using maps. I wrote myself mapfind(Key,Value,L) and mapstore(Key,Val,L,Map). That was all I needed. I did not care about compatibility with other types in list. 

Is worrying about mixing tuples/records with maps in a list really necessary? In the 6+ years of writing erlang, I don’t think I’ve had a list of mixed types more than a couple of times. 


Sergej

On 23 Nov 2014, at 15:43, Lukas Larsson <> wrote:

> Hello,
> 
> I agree with you that in certain situations they would be very convenient. I've myself already written these functions in a couple of modules.
> 
> How do you imagine the semantics would work? It is fairly straight forward when the list only contains maps or tuples.
> 
> lists:keyfind(you, name, [#{ name => me, from => here }, #{ name => you, from => there}]) returns #{ name => you, from => there}.
> 
> But if we start to mix them some interesting corner cases pop up:
> 
> lists:keyfind(2, 1,[#{ 2 => 3 },{2, 4},#{ 1 => 2 } ]) returns {2,4} or #{ 1 => 2 }?
> lists:keyfind(1, id, [{1,2}, #{ id => 2}, #{ id => 1}) returns #{ id => 1 } or crashes as id is not a valid key for {1,2}?
> 
> Or error cases of today:
> 
> lists:keyfind(2, id, []) today gives badarg, but should return false for a map list?
> 
> The safest way to introduce these functions would be to create new ones called something like lists:mkeyfind, but if possible it would be very neat to use the same API. It really is unfortunate that lists:keyfind is so forgiving today, you can mix and match whatever you want in the list and keyfind will quietly ignore it.
> 
> Also we have to consider which of the lists:key* functions we want a map counterpart? Does it make sense for all of them?
> 
> Thoughts? Ideas? Suggestions?
> 
> Lukas
> 
> 
> On Sat, Nov 22, 2014 at 7:33 PM, John Doe <> wrote:
> It would be nice to extend lists module with a set of optimized functions like lists:key*** for lists containing maps. 
> 
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