Defining methods with double greater than sign (>>) in Smalltalk

Question

In the book Smalltalk Best Practice Patterns from Kent Beck, the double greater sign (>>) is used to define methods like this:

Point class>>x: xNumber y: yNumber
    ^self new
        setX: xNumber
        y: yNumber

Point>>setX: xNumber y: yNumber
    x := xNumber.
    y := yNumber.
    ^self

However, I cannot get it run in GNU Smalltalk.

Is it valid syntax in some implementation of Smalltalk? Or is it just kind of pseudo code?

score 5 · Accepted Answer · edited Mar 11 '13 at 12:17

In fact this is Pseudo code.

In other languages you would use the . to tell people that the method is in this class but in smalltalk you write >>

What you would do in a Smalltalk like Squeak or Pharo for

Point class>>x: xNumber y: yNumber
    ^self new
        setX: xNumber
        y: yNumber

Open the System Browser
klick on class, a button that will show you the class side of the class.

paste the method in the text area with the source code:

x: xNumber y: yNumber
    ^self new
        setX: xNumber
        y: yNumber

Strg-s to save the code

For

Point>>setX: xNumber y: yNumber
    x := xNumber.
    y := yNumber.
    ^self

You would do the same but not use the class side

score 4 · Answer 2 · answered Mar 10 '13 at 16:06

Also, notice that indeed, #>> is a message that you can send to a class and it basically access the method dictionary for the symbol (selector argument). See, Behavior class, method >>

  >> selector 
"Answer the compiled method associated with the argument, selector (a 
Symbol), a message selector in the receiver's method dictionary. If the 
selector is not in the dictionary, create an error notification."

^self compiledMethodAt: selector

So you can do, for example (inspect that)

  Point class >> #x:y:

Notice however, that here we send #class because #x:y: is a class side method. If you want to access an instance side method, say #normalized then you can do:

  Point >> #normalized

score 2 · Answer 3 · answered Mar 11 '13 at 08:54

The correct syntax for GNU Smalltalk will look something like this:

Point class extend [
    x: xNumber y: yNumber [
        ^self new
            setX: xNumber
            y: yNumber ]
]

Point extend [
    setX: xNumber y: yNumber [
        x := xNumber.
        y := yNumber.
        ^self ]
]

For more information on GNU Smalltalk syntax see here.

Defining methods with double greater than sign (>>) in Smalltalk

3 Answers3