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<div class="moz-cite-prefix">Hi!<br>
On 03/04/2013 08:58 PM, Diego Llarrull wrote:<br>
</div>
<blockquote cite="mid:5134FCDF.30607@tecso.coop" type="cite">
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Hello everyone, <br>
<br>
I would like to insert in an ETS table a tuple with the following
type signature: <br>
<br>
<i><big><tt>{string(), [{string(), [string()]}]}</tt></big></i><br>
<br>
As an example: <br>
<i><br>
</i><i><big><tt>{"Peter", [{"children", ["Bob", "Paul"]},
{"father", ["Mike"]}]}</tt></big></i><i><br>
<br>
</i><br>
My question is the following: is it possible to solve, using match
specifications, nested queries like "Retrieve the name of Peter's
children" ?<br>
<br>
That is, a query where I would need to <br>
<br>
a) Fetch all ("the", since its a set) tuples of size 3 where
"Peter" is located in the first position (doable with MS)<br>
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'). <br>
<br>
If I understood correctly, lists:keyfind can't be used inside a
match specification because they only allow the BIFs described in
<a moz-do-not-send="true"
href="http://www.erlang.org/doc/apps/erts/match_spec.html">http://www.erlang.org/doc/apps/erts/match_spec.html</a>.
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?<br>
<br>
</blockquote>
Yes, you're trying to push the boundaries of match specifications :)<br>
<br>
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... <br>
<br>
So...<br>
[lists:keyfind("children",1,P) || P <-
ets:select(xxx,ets:fun2ms(fun({"Peter",Data}) -> Data end))].<br>
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:<br>
ets:select(xxx,ets:fun2ms(fun({"Peter",[{"children",Data}|_]})
-> Data end)).<br>
But looping over the list inside the ms is not possible.<br>
<blockquote cite="mid:5134FCDF.30607@tecso.coop" type="cite"> 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". <br>
<br>
Any help of insight will be greatly appreciated. Thank you very
much in advance. <br>
<br>
Diego Llarrull<br>
<br>
<br>
</blockquote>
Cheers,<br>
/Patrik<br>
<blockquote cite="mid:5134FCDF.30607@tecso.coop" type="cite"> <br>
<br>
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<br>
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