Parentheses Vs. Square Brackets

( ) vs. [ ]

• ( ) just changes the priority of an execution but the program is executed
• [ ] blocks program execution: the program is NOT executed

Use [ ] for 'Unknown' Execution Occurrence

Use [ expressions ] when:

• expressions may not be executed at all (if, and, ...)
• expressions may be executed multiple times (while, ...)

Example

n timesRepeat: [ self doSomething ]

timesRepeat: executes a number of times its argument, therefore the argument is a block

( ) vs. [ ] Example

x isNil ifTrue: [ self doSomething ]

ifTrue: may execute or not its argument, therefore the argument is a block

( ) vs. [ ] Example

[ self start ] whileTrue: [ self doSomething ]

whileTrue: may execute both its receiver and argument multiple times, therefore they are both a block

Quiz

1 to: n do: ... self doSomething ...

x ifEmpty: ... self doSomething ...

Quiz

1 to: 100 do: [ :i | self doSomething ]

x ifEmpty: [ self doSomething ]

Summary

• ( ) is about changing the order of a computation
• [ ] is freezing the computation and controlling it