[ruby-core:113533] [Ruby master Feature#19634] Pattern matching dynamic key

19 May 2023

      Issue #19634 has been updated by austin (Austin Ziegler).

I count 44 instances of this in our production code (~100k lines of Elixir), but I don’t think I’ve ever used key and value pinning as shown in the examples above.

But dynamic key matching is precisely what is required when it is required, although it’s rare enough.

Here's a sample GraphQL response parser or the Shopify Customers API:

```elixir
  def parse_customer_response(%{} = body, resource, action) do
    gql_action = resource <> String.capitalize(action)

    case body do
      %{status: 200, body: %{^gql_action => %{^resource => %{} = result, "userErrors" => []}}} ->
        {:ok, result}

      %{status: 200, body: %{^gql_action => %{"userErrors" => []} = result}} ->
        {:ok, result}

      %{errors: errors} ->
        {:error, Enum.map(errors, &Map.get(&1, "message"))}

      %{body: %{^gql_action => %{"userErrors" => errors}}} ->
        {:error, Enum.map(errors, &Map.get(&1, "message"))}

      %{body: %{^gql_action => nil}} ->
        {:error, [%{message: "#{gql_action} access denied"}]}

      %{body: nil} ->
        {:error, [%{message: "#{gql_action} access denied"}]}

      _ ->
        {:error, [%{message: "Unknown error"}]}
    end
  end
```

I haven’t started using pattern matching in Ruby, but I could see some simplified code with dynamic key matching.

(As a separate note, it appears that email replies to redmine aren’t necessarily working, as this should have come in by email.)

----------------------------------------
Feature #19634: Pattern matching dynamic key
https://bugs.ruby-lang.org/issues/19634#change-103160

* Author: baweaver (Brandon Weaver)
* Status: Open
* Priority: Normal
----------------------------------------
I found myself in a situation ([stable marriage problem](https://rosettacode.org/wiki/Stable_marriage_problem#top-page)) where I would like to match against a dynamic key, like so:

```ruby
mentor_proposals = { mentor_name: ['mentee_1', 'mentee_2'] }
mentor_name = :mentor_name
mentee_name = 'mentee_1'

mentor_proposals in ^key: [*, ^mentee_name, *] # SyntaxError
```

Currently this is not supported syntax, but there are some use cases in which I might see myself wanting to use it including this one. As `deconstruct_keys` currently accepts an `Array` of keys this would not break compatibility but would introduce syntactic complexity in capturing keys on hash-like matches.

I believe the tradeoff is worthwhile, but would like to hear others opinions on the matter.

Granted this case has some oddities of `Symbol` and `String` interchangeability as an implementation detail, and I will not be arguing for key irreverence in this issue as that's a [much more involved topic](https://dev.to/baweaver/the-case-for-pattern-matching-key-irreverence-in-rub...).

-- 
https://bugs.ruby-lang.org/