amnu3387

Elixir with Erlang fun2ms -> ETS

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

0 5465 19

2020-05-02 14:03:48 UTC

Marked As Solved

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

Also Liked

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

Nicd

Maybe it should be called a select spec.

Post #10

rvirding

Creator of Erlang

Well, how tracing uses match specs is a bit different so calling them a select spec would not fit in there. And they are so close that giving them different names would be stupid. The main difference is what you can do in the body: in tables you can basically just return data while in tracing you can control how tracing works.

Post #11

Last Post!

NobbZ

Works for me:

$ nix run nixpkgs.elixir_1_10 -c mix new foo
* creating README.md
* creating .formatter.exs
* creating .gitignore
* creating mix.exs
* creating lib
* creating lib/foo.ex
* creating test
* creating test/test_helper.exs
* creating test/foo_test.exs

Your Mix project was created successfully.
You can use "mix" to compile it, test it, and more:

    cd foo
    mix test

Run "mix help" for more commands.
$ cd foo
$ emacs mix.exs
Waiting for Emacs...
$ nix run nixpkgs.elixir_1_10 -c mix deps.get
Resolving Hex dependencies...
Dependency resolution completed:
New:
  ex2ms 1.6.0
* Getting ex2ms (Hex package)
$ nix run nixpkgs.elixir_1_10 -c iex -S mix
Erlang/OTP 22 [erts-10.7] [source] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:1] [hipe]

==> ex2ms
Compiling 1 file (.ex)
Generated ex2ms app
==> foo
Compiling 1 file (.ex)
Generated foo app
Interactive Elixir (1.10.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> import Ex2ms
Ex2ms

Post #19