Inconsistent use of booleans in various parts of Elektra.

needs to be string

only check presence or absence (no cascading override of already present key possible)
use booleans as in CMake, which allows on/off, true/false, ... (would need convenience across the code)
do not accept a specification with type = boolean without a default

Only the strings 0 and 1 are allowed in the KeySet for type = boolean, for both values and defaults. Everything else should lead to errors in checkers (in kdbSet).

A spec with type = boolean without a specified default should be interpreted as default = 0.

Example for an implementation in C in an application:

if (k != NULL && strcmp(keyString(k), "1") == 0) {/*true*/} else {/*false*/}

Storage plugins are allowed any representation as suitable, e.g., a JSON plugin might render 1 as true.

The type checker plugin should allow

non-presence
the string "0"
the string "1"

most easy to implement
allows non-presence to be false
plugins allow us to convert to any other behavior

Storage plugins are only allowed to emit 0 or 1 as key values
Applications either get 0 or 1, or (without a key) can safely assume that false is meant