src/swipl-devel/packages/xpce/man/refmanual/md/bool.md

In rswipl: Embed 'SWI'-'Prolog'

class bool {#class-bool}

Class bool defines represents PCE's two boolean constants: @on and @off. These objects are protected (see ->protect). No other boolean constants can be created.

@tbd Methods for logical expressions are still lacking.

@see class constant

Send methods {#class-bool-send}

• bool->initialise Generates the error !cannot_create_instances.

• bool->unlink The two instances of class bool cannot be unlinked as they are ->protect'ed.

Get methods {#class-bool-get}

• bool<-convert: any -> bool The following values are converted to booleans:

  | 0 | @off | | any other integer | @on | | '@on' | @on | | '@off' | @off | | true, yes, @on <-name | @on | | false, no, @off <-name | @off |

• bool<-negate: -> bool Maps @on to @off and visa versa. The recogniser object below will toggle the graphical ->inverted:

  ?- new(@toggle_inverted, click_gesture(left, '', single, message(@receiver, inverted, @receiver?inverted?negate))).

rswipl documentation built on June 16, 2026, 5:07 p.m.