Membership and Keys

proplists:is_defined/2

A simple existential check that returns a boolean without drama:

lfe> (proplists:is_defined 'timeout '((timeout 5000) (retry 3)))
true

lfe> (proplists:is_defined 'verbose '((timeout 5000) (retry 3)))
false

lfe> (proplists:is_defined 'binary '(read write binary))
true

This answers the question “does this key exist?” which is simpler than “what is this key’s value?” and thus requires less cosmic contemplation.

proplists:get_keys/1

Returns an unordered list of all keys used in the proplist, without duplicates:

lfe> (proplists:get_keys '((a 1) (b 2) (c 3) (a 99)))
(c b a)

lfe> (proplists:get_keys '(read write (mode binary) read))
(mode write read)

The keys are returned in whatever order the implementation finds convenient, which is to say, don’t count on any particular ordering. If you need ordering, you’ve wandered into map territory.