From: tyler@... Date: 2019-01-01T23:14:21+00:00 Subject: [ruby-core:90847] [Ruby trunk Feature#15492] Let #dig take a "default value" block like Hash#fetch does Issue #15492 has been reported by TylerRick (Tyler Rick). ---------------------------------------- Feature #15492: Let #dig take a "default value" block like Hash#fetch does https://bugs.ruby-lang.org/issues/15492 * Author: TylerRick (Tyler Rick) * Status: Open * Priority: Normal * Assignee: * Target version: ---------------------------------------- [fetch](https://ruby-doc.org/core-2.6/Hash.html#method-i-fetch) provides multiple ways to handle the case where a key can't be found: > If the key can't be found, there are several options: With no other arguments, it will raise a KeyError exception; if `default` is given, then that will be returned; if the optional code block is specified, then that will be run and its result returned. ~~~ ruby {a: 'a'}.fetch(:d) { 'default' } #=> "default" {a: 'a'}.fetch(:d) {|key| key } #=> :d ~~~ [dig](https://ruby-doc.org/core-2.6/Hash.html#method-i-dig) obviously can't let you specify a `default` value as a positional argument like `fetch` does, but couldn't it at least let you specify a default value by passing a block? The fact that it currently just silently *ignores* the block that you pass to `dig` could be misleading, as one might assume (as I did at first) that it's going to return that in case any of the key lookups fail. Current/desired behavior: ~~~ ruby {a: {b: {c: 'c'}}}.dig(:a, :b, :d) {|key| key } #=> nil # wish it returned :d {a: {b: {c: 'c'}}}.dig(:a, :b, :d) {|key| 'default' } #=> nil # wish it returned 'default' ~~~ There isn't currently a nice way to do this (that I can think of). You could mix `dig` and `fetch` (or chain a bunch of `fetch`s), but that loses the simple elegance of `dig` and looks awful: ~~~ ruby object = {a: {b: {c: 'c'}}} (object.dig(:a, :b) || {}).fetch(:d) { 'default' } #=> "default" ~~~ Note, of course, that if we added default-block behavior to `dig`, its behavior would be slightly different: it would return the result of the default block if _any_ intermediate step were `nil`, not just if the _last_ lookup were nil. ~~~ ruby object = {a: {b: {c: 'c'}}} object.dig(:x, :y, :z) { 'default' } #=> "default" ~~~ ## Example use case Sometimes you start out with a simple Hash but over time you may end up moving keys into sub-hashes. You might have started out with something like ~~~ ruby config.fetch(:something) { 'default' } ~~~ But after you move `:something` under `:settings`, you have to use `dig` instead of `fetch`: ~~~ ruby config.dig(:settings, :something) ~~~ The problem is, how do you keep the default value now that it's in a sub-hash? Can't just use `|| 'default'` because the stored value might be `false`. Currently I'm working around this with an explicit `nil?` check... ~~~ ruby value = config.dig(:settings, :something) value.nil? ? 'default' : value ~~~ (Even *that* may not be good enough if one wanted to distinguish between a `nil` value _stored_ in the object and the case where the key can't be found (a "cache miss"). Having a default block would let you distinguish between a missing key and a nil value stored at the key, in case that distinction were important...) -- https://bugs.ruby-lang.org/ Unsubscribe: