The Maps Module: A Compendium of Useful Functions

LFE provides access to all of Erlang’s maps module functions. Here are the essential ones, annotated for your convenience:

maps:new/0

Creates an empty map. Philosophically equivalent to a blank canvas or an unwritten novel.

(maps:new)
#M()

maps:size/1

Returns the number of key-value pairs, useful for determining if your map has achieved critical mass.

(maps:size '#M(a 1 b 2 c 3))
3

maps:get/2

Retrieves a value or raises an exception, rather like a particularly strict librarian.

(maps:get 'a '#M(a 1 b 2))
1

(maps:get 'z '#M(a 1 b 2))
;; ** exception error: bad_key

maps:find/2

The polite version of get, returning (tuple 'ok value) or the atom error.

(maps:find 'a '#M(a 1 b 2))
#(ok 1)

(maps:find 'z '#M(a 1 b 2))
error

maps:is_key/2

A simple existential check that returns a boolean without drama.

(maps:is_key 'a '#M(a 1 b 2))
true

(maps:is_key 'z '#M(a 1 b 2))
false

maps:keys/1

Returns all keys in ascending order, neatly arranged like soldiers on parade.

(maps:keys '#M(c 3 a 1 b 2))
(a b c)

maps:values/1

Returns all values in key-sorted order, which is to say, the values corresponding to the sorted keys.

(maps:values '#M(c 3 a 1 b 2))
(1 2 3)

maps:to_list/1

Converts a map to a list of two-tuples, preserving key order.

(maps:to_list '#M(a 1 b 2))
(#(a 1) #(b 2))

maps:from_list/1

The inverse operation, constructing a map from tuple pairs. If duplicate keys exist, the first value wins—first come, first served, as it were.

(maps:from_list '(#(a 1) #(b 2) #(a 99)))
#M(a 1 b 2)

maps:remove/2

Removes a key-value pair, if it exists. Idempotent and unbothered.

(maps:remove 'b '#M(a 1 b 2 c 3))
#M(a 1 c 3)

maps:merge/2

Combines two maps, with the second map’s values taking precedence in key conflicts. Excellent for establishing defaults:

(let ((defaults '#M(timeout 5000 retry 3 verbose false))
      (user-opts '#M(timeout 1000 verbose true)))
  (maps:merge defaults user-opts))
;; #M(retry 3 timeout 1000 verbose true)

maps:map/2

Applies a function to all values while preserving keys, rather like redecorating without changing the floor plan.

(maps:map (lambda (k v) (* v 2)) '#M(a 1 b 2 c 3))
#M(a 2 b 4 c 6)

maps:filter/2

Keeps only the key-value pairs satisfying a predicate, in the finest tradition of selective attention.

(maps:filter (lambda (k v) (> v 2)) '#M(a 1 b 2 c 3 d 4))
#M(c 3 d 4)