[erlang-questions] Nested match specifications?

Diego Llarrull diego.llarrull@REDACTED
Tue Mar 5 17:21:48 CET 2013

Ivan, Patrik:

Thank you both for the swift replies! Both provided functions that yield 
the desired results. The functions are:

  (I) ets:match(table, {"Peter", [{"children", '$1'},'_'] }).         
(II) ets:select(xxx,ets:fun2ms(fun({"Peter",[{"children",Data}|_]}) -> 
Data end)).   (Patrick's)

(II) is effectively rendered to


which is pretty similar to Ivan's suggestion. My final question (and I 
so promise) is: it appears to me that (I) performs better than (II) 
because of the lack of need of a transformation function such as 
ets:fun2ms(). However, I don't known for sure whether ets:match() 
outperforms ets:select() or not. I'm about to run a series of tests on 
big datasets to check this, but I imagine that there is already an 
answer to this question.

Once again, thank you very much.


El 05/03/13 11:34, Patrik Nyblom escribió:
> Hi!
> On 03/04/2013 08:58 PM, Diego Llarrull wrote:
>> Hello everyone,
>> I would like to insert in an ETS table a tuple with the following 
>> type signature:
>> /{string(), [{string(), [string()]}]}/
>> As an example:
>> /
>> //{"Peter", [{"children", ["Bob", "Paul"]}, {"father", ["Mike"]}]}//
>> /
>> My question is the following: is it possible to solve, using match 
>> specifications, nested queries like "Retrieve the name of Peter's 
>> children" ?
>> That is, a query where I would need to
>> a) Fetch all ("the", since its a set) tuples of size 3 where "Peter" 
>> is located in the first position (doable with MS)
>> b) Let '$2' be the value in the second position of the tuple fetched 
>> in a). Then, fetch the value corresponding to the key "children" in 
>> '$2', if interpreted as a key-value list (i.e. lists:keyfind() should 
>> work on '$2').
>> If I understood correctly, lists:keyfind can't be used inside a match 
>> specification because they only allow the BIFs described in 
>> http://www.erlang.org/doc/apps/erts/match_spec.html. My question is: 
>> is there any low-level mechanism to operate on lists inside match 
>> specifications, or am I trying to push the boundaries of match 
>> specifications?
> Yes, you're trying to push the boundaries of match specifications :)
> You'll have to do a lists:keysearch/keyfind on the results of the 
> ets:select (when you're back in proper Erlang code), only constant 
> time BIF's can be added to the ms language, so it's unfortunately not 
> even an option to add this particular function... Without a huge 
> rewrite that is...
> So...
>   [lists:keyfind("children",1,P) || P <- 
> ets:select(xxx,ets:fun2ms(fun({"Peter",Data}) -> Data end))].
> would be the solution. Except of course if the list has a particular 
> order, or you instead use some record'ish data structure, so that you 
> can match directly:
>   ets:select(xxx,ets:fun2ms(fun({"Peter",[{"children",Data}|_]}) -> 
> Data end)).
> But looping over the list inside the ms is not possible.
>> In case anyone wonders "Why not use Query Lists Comprehensions?" the 
>> answer is: "Because of performance issues: in our platform, we need 
>> to dynamically build QLCs based on the number of arguments that 
>> arrive, which forces us to build them as strings and then use 
>> qlc:string_to_handle() which is SLOW".
>> Any help of insight will be greatly appreciated. Thank you very much 
>> in advance.
>> Diego Llarrull
> Cheers,
> /Patrik
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