Some explanations from https://clojure.atlassian.net/browse/CLJ-2961:

> Clojure’s idiomatic use of maps has proven valuable, but missing required keys, misspelled keys, and invalid values can lead to failures that do not connect to the actual source of the problem (e.g. NPEs) making diagnosis difficult. At the same time, Clojure lacks a simple inline mechanism for functions to document and check the keys they require and accept. Existing tools either separate those expectations from the function itself or couple data shape and data provision.