It was wonderful to discover the functional utilities available when working with hashmaps, like assoc, get, update, and their nested friends assoc-in, get-in, and update-in. Just today I discovered an alternative to get-in made possible by the thread-first (->) macro and the fact that map keys are functions on maps.
Some data:
user=> (def stuff {:intro {:hello 42}})
Using get-in:
user=> (get-in stuff [:intro :hello])
42
Using ->:
user=> (-> stuff :intro :hello)
42
Depending on whether you continue threading the results to additional operations you can consider making this even safer using the some-> macro (and the corresponding some->>).
user=> (-> stuff :intro :not-there pos?)
Execution error (NullPointerException) at user/eval158 (REPL:1).
null
user=> (some-> stuff :intro :not-there pos?)
nil
-Michael Whatcott