Feature #19069
Updated by sawa (Tsuyoshi Sawada) about 2 years ago
This is a spin-out from #19063, and is a recapture of my comment https://bugs.ruby-lang.org/issues/19063#note-15.
I propose to change the behavior of `Hash.new` when it takes a block with its parameter signature absent. In such case, the evaluated value of the block should be assigned to the hash with the missing key in question (`h3` below). When the block does take parameters, then the behavior should remain as is now (`h1`, `h2` below).
```rb
h1 = Hash.new{|h, k| h[k] = "foo"}
h2 = Hash.new{|h, k| "foo"}
h3 = Hash.new{"foo"}
h1[:a] # => "foo"
h2[:a] # => "foo"
h3[:a] # => "foo"
h1 # => {:a=>"foo"}
h2 # => {}
h3 # => {:a=>"foo"}
```
This will solve a few problems. First, as discussed in #19063, many users make the mistake of writing `Hash.new([])` when they actually mean `Hash.new{|h, k| h[k] = []}`, and I suspect this is partially due to the fact that the block `{|h, k| h[k] = []}` is too long, and the users are tempted to avoid writing so. With the proposed feature introduced, the users will be encouraged to naturally write the correct form `Hash.new{[]}`.
Second, some of the more advanced users than those mentioned above still sill make the mistake of writing `Hash.new{foo}` when they actually mean `Hash.new{|h, k| h[k] = foo}`, and the current proposal is to let them actually be equivalent.
Third, this will resemble a similar construct `Array.new(5){[]}` and they will make a good parallel.
Indeed, there are situations where the intended behavior is to just run a routine without assigning a new key-value pair to the hash. Examples of current code may be as follows:
```rb
h4 = Hash.new{some_routine_to_take_care_of_the_missing_key_situation}
h5 = Hash.new{raise WrongKeyBlahBlahError}
```
Such blocks would need to be prepended by a parameter signature `|h, k|` in order to avoid unwanted results. I do not think such transition would be a huge pain.