I thought {} was synonymous with Hash.new

Therefore

{{} => true}[{}]

generates 2 different hashes with different object_ids and so will return nil

It is Hash.new yes - {{} => true} is a Hash which has one key/value pair - the key is {} (an empty Hash) and the value true.

[] is then the lookup method for that Hash - so we’re looking up the value corresponding to the key {}. We expect to get true, but it doesn’t find the value, due to {}.hash not being fixed for different instances of an empty hash.

h = Hash.new
empty_hash_as_key = {}
another_empty_hash = {}
h[empty_hash_as_key] = true

h[another_empty_hash]
=> nil

-Matt

From my naive non-software enginneering perspective the first three code examples behave exactly as I would expect:

I thought {} was synonymous with Hash.new

Therefore

{{} => true}[{}]

generates 2 different hashes with different object_ids and so will return nil

and its the same with

{{}=>true, {}=>true}

the two hashes have two different object_ids so are different keys

where as

{} == {}

compares just the contents of both hashes and returns true.

There is no === operator for Hashes which I would expect to assert that both objects have the same object_id and the same contents.