Elixir with Erlang fun2ms -> ETS

amnu3387

Hi, does anyone know how I can create a matchspec for this condition?

matchspec = :ets.fun2ms(fn({key, test}) when key != :counter -> test end)

[error] exited in: :ets.fun2ms(:function, :called, :with, :real, :fun, :should, :be, :transformed, :with, :parse_transform, :or, :called, :with, :a, :fun, :generated, :in, :the, :shell)
        ** (EXIT) :badarg

I can get the result I want with:

test = :ets.foldl(fn({key, obj}, acc) -> case key != :counter do
                                           true -> [obj | acc]
                                           _ -> acc
                                         end
 end, [], :queue_cache)

 test = :ets.foldl(fn({key, obj}, acc) when key != :counter -> [obj | acc]
                   (_, acc) -> acc
 end, [], :queue_cache)

But fun2ms seems a bit cleaner to write - I’m not sure I understand the restraints in fun2ms, perhaps is my when condition that’s not valid?

19 comments

#erlang #ets #troubleshooting

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2020-05-02 14:03:48 UTC

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kokolegorille

I had a similar question…

I think it’s safer to use ex2ms (Elixir to match spec) for the moment.

Post #1

amnu3387

thank you! I ended up going to the console and see what match specification it output and I got this:

[{{:"$1", :"$2"}, [{:"/=", :"$1", :counter}], [:"$2"]}]

Which seems to be correct to my very thin knowledge of Erlang,
So I tried:
test = :ets.match(:queue_cache, [{{:"$1", :"$2"}, [{:"/=", :"$1", :counter}], [:"$2"]}])

but it won’t match anything, I’ve tried with :"counter" as well

Post #2

Nicd

In your second example, /= has turned into \=?

Post #3

amnu3387

ups, that was after trying a couple variations, it’s :"/="

Post #4

rvirding

Creator of Erlang

The reason :ets.fun2ms/1 works like this is because it is basically a macro. To use this in Erlang you need to include a -include_lib("stdlib/include/ms_transform.hrl"). in your Erlang file. (it’s in the docs) This runs a parse transform which at compile-time transforms the fun into the match spec. If this transformation is not done at compile time then the :ets.fun2ms/1 function is called at run-time and it gives that rather clumsy error message.

This include also the :dbg.fun2ms/1 “macro” which is used to build match specs for tracing. They are very similar to the ones for tables but not quite the same.

Parse transforms are like macros on steroids. The parse transform’s module/2 function is called with the whole Erlang module AST as an argument with which it can do whatever it likes. It just has to return a new complete module AST. Quite fun once you get the hand of it.

Post #5

amnu3387

Thank you for replying Robert - I did read the other thread and the docs as well. The error message was quite explicit (although presented in sort of a lispy fashion) I was just unsure how to include the matchspecs transform file when in elixir (actually I thought it would be somehow baked in).

I definitively need to dig a bit more into erlang.

Is that why you named these functions as “fun2…”? (j/k)

I still haven’t figured out why the match spec that I got from the terminal won’t work? I imagine that the one we retrieve in the shell will be equal to the one that is parse-transformed at compile-time right? Or am I missing something? I did also check the match specification in the erlangs docs

[{{:"$1", :"$2"}, [{:"/=", :"$1", :counter}], [:"$2"]}]

[{ { MatchHead }, [{ Guard Function }], [ MatchBody#$2_bound_in_matchhead ] }]

But it doesn’t return me any records and I haven’t figured yet what I’m doing wrong (I haven’t looked much more into it because :ets.foldl solves it right now)

Post #6

rvirding

Creator of Erlang

Well it should only return the test value. I get it to work from the shell:

iex(8)> :ets.new(:qc, [:named_table,:bag])
:qc
iex(9)> :ets.insert(:qc, [counter: 1, foo: 2, counter: 3, bar: 4])
true
iex(10)> :ets.tab2list(:qc)                                        
[counter: 1, counter: 3, foo: 2, bar: 4]
iex(11)> ms = :ets.fun2ms(fn({k,t}) when k != :counter -> t end)   
[{{:"$1", :"$2"}, [{:"/=", :"$1", :counter}], [:"$2"]}]
iex(12)> :ets.select(:qc, ms)
[2, 4]

The name isn’t a joke, unfortunately. He who wrote the module like the shorter 2 instead of _to_. It does crop up in a few other places in the libraries as well.

Post #7

Nicd

Maybe the problem is that you’re giving the match spec to :ets.match, when it needs to be given to :ets.select? match accepts a pattern instead of match spec, if I’m not mistaken.

Post #8

amnu3387

Thank you @rvirding and @Nicd - indeed the problem was simply using a matchspec with match instead of with select (also known as "overlooking the obvious"TM)

for my defence though a match spec would seem like the right fit for a match function

Post #9