[erlang-questions] Nested match specifications?

Ivan Carmenates García <>
Mon Mar 4 22:53:09 CET 2013

Hi Diego,


I don’t know exactly if this could work for you but you can do complex
patterns like this to take what you want


1>  ets:new(table, [named_table]),

2>  ets:insert("Peter", [{"children", ["Bob", "Paul"]}, {"father",

3>  ets:match(table, {"Peter", [{"children", '$1'},'_'] }).



If you always match against the key, you do really faster.







[mailto:] En nombre de Diego Llarrull
Enviado el: lunes, 04 de marzo de 2013 14:58
Asunto: [erlang-questions] Nested match specifications?


Hello everyone, 

I would like to insert in an ETS table a tuple with the following type

{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

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

Diego Llarrull


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