Strange, unexpected behavior (disappearing/changing values) when using Hash default value, e.g. Hash.new([])

Question

Consider this code:

h = Hash.new(0)  # New hash pairs will by default have 0 as values
h[1] += 1  #=> {1=>1}
h[2] += 2  #=> {2=>2}

Answer 1

The operator += when applied to those hashes work as expected.

[1] pry(main)> foo = Hash.new( [] )
=> {}
[2] pry(main)> foo[1]+=[1]
=> [1]
[3] pry(main)> foo[2]+=[2]
=> [2]
[4] pry(main)> foo
=> {1=>[1], 2=>[2]}
[5] pry(main)> bar = Hash.new { [] }
=> {}
[6] pry(main)> bar[1]+=[1]
=> [1]
[7] pry(main)> bar[2]+=[2]
=> [2]
[8] pry(main)> bar
=> {1=>[1], 2=>[2]}

This may be because foo[bar]+=baz is syntactic sugar for foo[bar]=foo[bar]+baz when foo[bar] on the right hand of = is evaluated it returns the default value object and the + operator will not change it. The left hand is syntactic sugar for the []= method which won't change the default value.

Note that this doesn't apply to foo[bar]<<=bazas it'll be equivalent to foo[bar]=foo[bar]< and << will change the default value.

Also, I found no difference between Hash.new{[]} and Hash.new{|hash, key| hash[key]=[];}. At least on ruby 2.1.2 .