Booleans and Conditions

Booleans

• true is the unique instance of class True
• false is the unique instance of class False

In Pharo, booleans have nothing special

• & | not
• or: and: (lazy)
• xor:
• ifTrue:ifFalse:
• ifFalse:ifTrue:
• ...

Eager and Lazy Logical Operators

   false & (1 error: 'crazy')
   -> an error

• the argument (1 error: 'crazy') is executed because this is a non lazy operator

   false and: [ 1 error: 'crazy' ]
   -> false "no error!"

• the argument [1 error: 'crazy'] is not executed because it is not necessary

Conditionals

In Pharo, traditional conditional (if, else, while) are messages sent to boolean or block objects

Yes ifTrue:ifFalse: is a message!

Weather isRaining
   ifTrue: [ self takeMyUmbrella ]
   ifFalse: [ self takeMySunglasses ]

• Conceptually ifTrue:ifFalse: is a message sent to an object: a boolean!
• Heavily optimised by the compiler

Boolean Implementation

• true is the unique instance of the class True
• false is the unique instance of the class False

More details in a future lecture (The Essence of Dispatch)

Conditionals: ifTrue: and ifTrue:ifFalse:

ifTrue: [ ] and ifTrue: [ ] ifFalse: [ ] are two different messages

forceItalicOrOblique
   self slantValue = 0 
      ifTrue: [ slantValue := 1 ]

fullName isEmptyOrNil
   ifTrue: [ 'FirstnameLastname' translated ]
   ifFalse: [ fullName ].

Conditionals: ifFalse: and ifFalse:ifTrue:

ifFalse: [ ] and ifFalse: [ ] ifTrue: [ ] are two different messages

Conditionals: ifEmpty: ifNotEmpty:

myProtocol 
   ifEmpty: [ 'As yet unclassified' ]

self listItems 
   ifNotEmpty: [ :aList | aList at: index ]

• Notice that when the receiver is not empty we get it as argument
• No need to ask it again

Summary

• Booleans are real objects
• Some conditionals are messages sent to Booleans