Issue #21358 has been updated by matz (Yukihiro Matsumoto). I prefer pattern matching. Matz. ---------------------------------------- Feature #21358: Advanced filtering support for #dig https://bugs.ruby-lang.org/issues/21358#change-113622 * Author: lovro-bikic (Lovro Bikić) * Status: Feedback ---------------------------------------- Currently, `#dig` can be used to access nested data structures using "simple" keys, such as array indices or hash keys. Real-world applications sometimes require non-trivial data access, for example, finding an item in an array based on some criteria, then returning some property of that item. This feature request is to add support for such non-trivial access to `#dig`, concretely by allowing `Proc`s as dig keys. ## Introductory example Given the following data structure (similar to one from [dig docs](https://docs.ruby-lang.org/en/3.4/dig_methods_rdoc.html)): ```ruby item = { id: '0001', batters: { batter: [ { id: '1001', type: 'Regular' }, { id: '1002', type: 'Chocolate' }, { id: '1003', type: 'Blueberry' }, { id: '1004', type: 'Devils Food' } ] } } ``` and the requirement "find ID of batter with type 'Chocolate'" (assuming we don't know its position in the array), currently the datum can be retrieved like so: ```ruby item.dig(:batters, :batter)&.find { it[:type] == 'Chocolate' }&.[](:id) # => "1002" ``` If `#dig` supported `Proc` as keys, the solution could be: ```ruby item.dig(:batters, :batter, -> { it[:type] == 'Chocolate' }, :id) # => "1002" ``` ## Implementation Here's a monkey patch which adds `Proc` support to `Array#dig`, to play around with: ```ruby class Array alias_method :original_dig, :dig def dig(key, *identifiers) case key when Proc val = find(&key) identifiers.any? ? val&.dig(*identifiers) : val else original_dig(key, *identifiers) end end end ``` This code also shows how I would define `Proc` argument behavior for arrays. The proc is called with each array item, and the *first* one for which a truthy value is evaluated is returned. ## Precedence I see this feature similar to [JSONPath's filter selector](https://www.rfc-editor.org/rfc/rfc9535.html#name-filter-selector), where the [equivalent path](https://jsonpath.com/#eJx1jlFrwjAQx79KOAQnlFKnPliQjTn2LHs1PsT2rIEs0eQilNLvvt...) for the introductory example would be: ``` $.batters.batter[?(@.type == 'Chocolate')].id ``` I am not familiar with similar implementations in other programming languages, so I cannot draw parallels there. ## Real-world examples Here's a [GitHub code search](https://github.com/search?q=%2F(%5Cw%5C%5B%5Cw%2B%3F%5C%5D%7C%5C.(fetch%7Cdig%7Cat%7C%5C%5B%5Cw%2B%3F%5C%5D)%5C(.%2B%3F%5C))%26%3F%5C.(find%7Cdetect)%20%5C%7B%2F%20lang%3Aruby&type=code&p=1) with potential candidates that could be refactored with this new feature. There are some false positives on this link, sorry about that. The regex is also probably incomplete, so there could be more results. I will highlight two examples from the results and how they could be refactored: ```ruby # https://github.com/moraki-finance/ruby-experian/blob/84f7def9987b6377f4718a0... value_section&.find { |d| d["Tipo"] == value_name }&.dig("ListaValores", "Valor")&.find { |v| v["Periodo"] == period.to_s }&.dig("Individual")&.to_i value_section&.dig(-> { it["Tipo"] == value_name }, "ListaValores", "Valor", -> { it["Periodo"] == period.to_s }, "Individual")&.to_i # https://github.com/cyfronet-fid/marketplace/blob/f84947777aa79d02fb987092416... datasource_data&.dig("identifiers", "alternativeIdentifiers")&.find { |id| id["type"] == "PID" }&.[]("value") datasource_data&.dig("identifiers", "alternativeIdentifiers", -> { it["type"] == "PID" }, "value") ``` ## Final thoughts The main benefit of this feature would be to not have to insert other methods in-between `#dig`, for example `#find` in the above examples. This could lead to cleaner code when handling complex data structures (such as returned from JSON APIs), with fewer usages of safe navigation. On the other hand, `Proc` argument could be misinterpreted by developers to mean something else (e.g. return _all_ items for which the proc returns a truthy value). `Proc` arguments could also be seen as non-idiomatic Ruby. It should also be clearly defined how many results such `dig`s return, just the first one for which the Proc is truthy (`#find` behavior), or all for which it's truthy (`#select` behavior). I admit at this point I'm not entirely sure, though I'm leaning towards the former (`#find`). Final note: in this feature request I considered only `Array#dig`, but `Hash#dig` could be considered as well (the proc receives each key/value pair). -- https://bugs.ruby-lang.org/