REBOL 3 Datatypes: Logic!

Contents

Concept

The logic! datatype consists of two states representing true and false. They are often returned from comparisons such as:

age: 100
probe age = 100
true

time: 10:31:00
probe time < 10:30
false

str: "this is a string"
probe (length? str) > 10
true

The logic! datatype is most commonly used as parameters to conditional functions such as if, while, and until:

if age = 100 [print "Centennial human"]
Centennial human

while [time > 6:30] [
    send person "Wake up!"
    wait [0:10]
]

The complement of a logic value is obtained from the not function:

there: place = "Ukiah" 
if not there [...]

Format

Normally, logic values are retrieved from the evaluation of comparison expressions. However, words can be set to a logic value and used to turn the word on or off:

print-me: false
print either print-me ["turned on"]["turned off"]
turned off

print-me: true
print either print-me ["turned on"]["turned off"]
turned on

The false value is not equivalent to integer zero or none!. However, in conditional expressions false and none! have the same effect:

print-me: none
print either print-me ["turned on"]["turned off"]
turned off

Just about any value assigned to a word has the same effect as true:

print-me: "just a string"
print either print-me ["turned on"]["turned off"]
turned on

print-me: 11-11-1999
print either print-me ["turned on"]["turned off"]
turned on

The following words are predefined to hold logic values:

true
on     ;same as true
yes    ;same as true
false
off    ;same as false
no     ;same as false

So, instead of true and false, when it makes sense, the words on and off, or yes and no can be used instead:

print-me: yes
print either print-me ["turned on"]["turned off"]
turned on

print-me: no
print either print-me ["turned on"]["turned off"]
turned off

print-me: on
print either print-me ["turned on"]["turned off"]
turned on

print-me: off
print either print-me ["turned on"]["turned off"]
turned off

Creation

The to-logic function converts integer! or none! values to the logic! datatype:

probe to-logic 0
false

probe to-logic 200
true

probe to-logic none
false

probe to-logic []
true

probe to-logic "a"
true

probe to-logic none
false

Related

Use logic? to determine whether a value is a logic! datatype.

probe logic? 1
false

probe logic? on
true

probe logic? false
true

Use the functions form, print, and mold to print a logic value:

probe form true
true

probe mold false
false

print true
true