Feature #15492
openLet #dig take a "default value" block like Hash#fetch does
Description
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.
{a: 'a'}.fetch(:d) { 'default' }
#=> "default"
{a: 'a'}.fetch(:d) {|key| key }
#=> :d
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:
{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). I guess you could mix dig
and fetch
(or chain a bunch of fetch
s), but that loses the simple elegance of dig
:
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.
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
config.fetch(:something) { 'default' }
But after you move :something
under :settings
, you have to use dig
instead of fetch
:
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...
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...)