What is the behaviour if maxSize is not specified?

[singpolyma]

Is it just the same as a Data.Map in that case?

[chowells79]

Mostly, except it still retains LRU ordering. You'd use it without a max size if you care about being able to use pop to get rid of the oldest entry or use toList to get insertion order, but don't care about automatically evicting the oldest entries.