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Lazy maps for Clojure

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This library provides a new Clojure data type, the lazy map. Lazy maps act just like regular (persistent) maps, except that their values are not computed until they are requested.


Start by requiring the namespace:

user> (require '[lazy-map.core :as lm])

You can then construct a lazy map using the lazy-map macro.

user> (def m (lm/lazy-map {:a (do (println "resolved :a") "value :a")
                           :b (do (println "resolved :b") "value :b")}))
user> m
{:a <unrealized>, :b <unrealized>}

When you request a value from the map, it will be evaluated and its value will be cached:

user> (:a m)
resolved :a
"value :a"
user> (:a m)
"value :a"

You can assoc values onto lazy maps just like regular maps. If you assoc a delay, it will be treated as an unrealized value and not forced until necessary:

user> (assoc (lm/lazy-map {}) :a 1 :b (delay 2))
{:a 1, :b <unrealized>}

Lazy maps are very lazy. In practice, this means they probably will not compute their values until absolutely necessary. For example, taking the seq of a lazy map does not force any computation, and map entries have been made lazy as well:

user> (def m (lm/lazy-map {:a (do (println "resolved :a") "value :a")
                           :b (do (println "resolved :b") "value :b")}))
lazy-map.core> (dorun m)
lazy-map.core> (keys m)
(:a :b)
lazy-map.core> (key (first m))
lazy-map.core> (val (first m))
resolved :a
"value :a"

You can also initialize a lazy map from a regular map, where delays are taken as unrealized values:

user> (lm/->LazyMap {:a 1 :b (delay 2)})
{:a 1, :b <unrealized>}

You might prefer to use ->?LazyMap instead of ->LazyMap. The only difference is that ->?LazyMap acts as the identity function if you pass it a map that is already lazy. This prevents nested lazy maps, which are not inherently wrong but which could be bad for performance if you nest them thousands of layers deep.

There are also some utility functions for dealing with lazy maps. You can use force-map to compute all of the values in a lazy map. Alternatively, you can use freeze-map to replace all the unrealized values with a placeholder. Here is an illustration:

user> (lm/force-map
        (lm/->LazyMap {:a (delay :foo)
                       :b :bar}))
{:a :foo, :b :bar}
user> (lm/force-map
          (lm/->LazyMap {:a (delay :foo)
                         :b :bar})))
{:a :quux, :b :bar}

Finally, lazy maps will automatically avoid computing their values when they are converted to strings using str, pr-str, and print-dup. To accomplish the same for pprint, you must use a special pretty-print dispatch function:

user> (pp/with-pprint-dispatch lm/lazy-map-dispatch
        (pp/pprint (lm/lazy-map {:a (println "lazy")})))
{:a <unrealized>}

Check out the unit tests for more information on the exact behavior of lazy maps.


All the code is currently in the lazy-map.core namespace, and the unit tests are in the lazy-map.core-test namespace.

See also

Malabarba's implementation of lazy maps in Clojure.

Features unique to malabarba/lazy-map:

  • ClojureScript support
  • Transform Java classes into lazy maps (methods become keys)

Features unique to raxod502/lazy-map:

  • More robust handling of laziness: all possible operations on maps are supported correctly (e.g. seq and reduce-kv)
  • Pretty string representation and support for pretty-printing

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