LaTeX3: Defining key to set boolean variable

Question

I want to define a key, which, if set, sets a boolean variable to false.

The documentation is not clear on that point. It states (source3.pdf, sect. 87, p. 134):

Defines <key> to set <bool> to <value> (which must be either true or false).

However, I cannot see where to put the false value in the key definition. <value> is not present in the synopsis. It seems that the default behaviour is to set <bool> to true.

Answer 1

The .bool_set:N key property is intended to work as

\keys_define:nn { module }
  {
    key-name .bool_set:N = \l_my_bool
  }

which will be used as key-name = true or key-name = false. If you want to have the key name alone to be equivalent to setting false, you need a default:

\keys_define:nn { module }
  {
    key-name .bool_set:N = \l_my_bool ,
    key-name .default:n  = false
  }

However, I'd probably use a raw .code:n approach, as this seems to be 'backward' compared to what I'd expect

\bool_new:N \l_my_bool
\keys_define:nn { module } 
  {
    key-name .code:n = { \bool_set_false:N \l_my_bool } 
  }