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.
rvirding
Creator of Erlang
amnu3387
I definitively need to dig through the docs as I’ll be using quite a bit of ETS (btw I really like them, but until now I have only used them for unique key->value, with no need to do other stuff with them).
2
bodgix
I’ve run into the same issue.
When I run:
iex(1)> :ets.fun2ms(fn x -> x end)
[{:"$1", [], [:"$1"]}]
iex(2)>
In the iex shell, it works. But when I try to use it in my module:
def list() do
:mnesia.transaction(fn ->
:mnesia.select(mnesia_table_name(), :ets.fun2ms(fn x -> x end))
end)
end
I get the error
{:aborted,
{:badarg,
{:ets, :fun2ms,
[:function, :called, :with, :real, :fun, :should, :be, :transformed, :with,
:parse_transform, :or, :called, :with, :a, :fun, :generated, :in, :the,
:shell]}}}
What’s strange and why in my case the solution proposed earlier doesn’t work is that when I replace the :ets.fun2ms with the MatchSpec which I get in the iex shell, like so:
def list() do
:mnesia.transaction(fn ->
:mnesia.select(mnesia_table_name(), [{:"$1", [], [:"$1"]}])
end)
end
it works.
0
NobbZ
The :ets.fun2ms is a placeholder function. In erlang code a “parse transform” is run over your sourcecode, which replaces all calls to the :ets.fun2ms with the matchspec at compiletime. It comes close to macros in elixir, but not quite.
There is a package that can be used from within elixir code:
0
bodgix
Thanks for the reply. All clear now.
I’m going to give ex2ms a shot, because I’d rather avoid more complex matchspecs in my code because I find them rather hard to read and maintain.
0
bodgix
Added
{:ex2ms, "~> 1.6"},
to my mix.exs but something isn’t working:
iex(1)> import Ex2ms
** (CompileError) iex:1: module Ex2ms is not loaded and could not be found
0
NobbZ
bodgix
Yes, I have
$ grep ex2ms mix.exs ‹ruby-2.4.4›
{:ex2ms, "~> 1.6"},
$ mix deps.get
Resolving Hex dependencies...
Dependency resolution completed:
Unchanged:
amqp 1.4.2
amqp_client 3.8.3
cachex 3.2.0
cowboy 2.7.0
cowlib 2.8.0
credentials_obfuscation 1.1.0
eternal 1.2.1
ex2ms 1.6.0
goldrush 0.1.9
jsx 2.9.0
jumper 1.0.1
lager 3.8.0
mime 1.3.1
plug 1.10.0
plug_cowboy 2.2.1
plug_crypto 1.1.2
poison 3.1.0
rabbit_common 3.8.3
ranch 1.7.1
recon 2.5.0
sleeplocks 1.1.1
telemetry 0.4.1
unsafe 1.0.1
All dependencies are up to date
$ iex -S mix
Erlang/OTP 22 [erts-10.7.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe] [dtrace]
iex(1)> import Ex2ms
** (CompileError) iex:1: module Ex2ms is not loaded and could not be found
0
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
1