Object subclass: #Square.
Square instanceVariableNames: 'side'.
Square class extend [
    new: square [
        | r |
        <category: 'instance creation'>
        r := super new.
        r init: square.
        ^r
    ]
]
Square extend [
    init: square [
        <category: 'initialization'>
        side := square sqrt.
    ]
    perimeter [
        | p |
        <category: 'perimeter calculation'>
        p := 4 * side.
        ^p
    ]
]
 
Object subclass: #App.
App class extend [
    run [
        | s |
        s := Square new: 4.
        (s perimeter) printNl
    ]
]
App run

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