Searching for an ets entry with a tuple as the key

Questions / Help
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1

I’ve been following a small tutorial around Registries and ETS Tables and i’m attempting to select an entry from a Registry.

{:ok, pid} = Registry.register(:user_registry, {:hello, :world}, "world")
fun = :ets.fun2ms(fn {product_id, product_name} = product when product_name == :hello -> product end)
Registry.select(:user_registry, fun)

This returns an error:

** (ArgumentError) invalid match specification in Registry.select/2: [{{:"$1", :"$2"}, [{:==, :"$2", :hello}], [:"$_"]}]
    (elixir 1.10.2) lib/registry.ex:1210: anonymous fn/3 in Registry.select/2
    (elixir 1.10.2) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
    (elixir 1.10.2) lib/registry.ex:1204: Registry.select/2

I’m not sure how to write the argument in such a way to search for tuple based keys and i think i need to do this so I can search for specific supervisors.

2

Strange as that matchspec works on an ETS table created in the shell and try to select on it.

3

Registry can have unexpected effects if you try use the special ets selector :"$_", which appears as a result of your fun2ms. https://hexdocs.pm/elixir/Registry.html#select/2 (section “do not use special match variables”)

Also, I think you’ll need a third bit in your select to match on the registry “value”.

concretely:

fn {{product_id, product_name}, _, _} when product_name == :hello -> {product_id, product_name} end
3 Likes
4

Answering my own question. It turns out that the select function demands a three tuple argument as the first parameter all the time in the form {key, pid, value}.

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5

yeah actually my fun is wrong, editing…

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