Mutable Objects as Hash Keys

m.rb

a = [1, 2]    # [1, 2]
a.object_id   # 70364895085700
a << 3        # [1, 2, 3]
a.object_id   # 70364895085700

h =  {}       # {}
h[a] = true   # true
h             # {[1, 2, 3]=>true}
h[a]          # true

a << 4        # [1, 2, 3, 4]
h             # {[1, 2, 3, 4]=>true}
h[a]          # nil

h[a] = false  # false
h             # {[1, 2, 3, 4]=>true, [1, 2, 3, 4]=>false}

h[[1,2,3,4]]  # false

h[[1,2,3]]    # nil

l = [1, 2, 3, 4] # [1, 2, 3, 4]
l.instance_eval { def hash; [1,2,3].hash; end }
h[l]             # true