Ruby

The following is unexpected for me.

Hash.new { }.transform_values { }.default
# => #<Proc:0x000055eecda01510 -e:1>
Hash.new { }.transform_values { }[:any]
# => #<Proc:0x0000563a12e35510 -e:1>
Hash.new { :default }.transform_values { }[:any][]
# => :default

I did not think a default_proc would turn into a default. The previous behaviour in 2.6 was that the default would be nil, so this is new in 2.7. But I didn't see this mentioned in the 2.7 NEWS. May I learn of the proper use for this and/or whether it is intentional?

If you wonder when we might see something like this, I had found it with code similar in idea to the following:

weights = Hash.new { |h, k| h[k] = [] }
weights[:apple] << 1
weights[:apple] << 2

prices = weights.transform_values { :arbitrary }

puts (prices[:mango] || [])[0] || "We're all out of mangoes today"

in `block in <main>': undefined method `[]=' for 0:Integer (NoMethodError)

That was quite a confusing error message. It took a while to realise why []= was called on 0.

We also cannot fix this by using dig (puts prices.dig(:mango, 0) || "We're all out of mangoes today") because then, we would get:

in `dig': Proc does not have #dig method (TypeError)