[erlang-questions] Nested match specifications?
Diego Llarrull
diego.llarrull@REDACTED
Mon Mar 4 20:58:23 CET 2013
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?
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
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