double dispatch example

Hi

If you happen to know a double dispatch situation in Pharo, I'm
interested since I'm revisiting my lecture.


Stef

Hi Stef!

Plenty of examples:

- Converting money
- Paper, Stone, Scissor
- A canvas containing triangle, circle, box has to be printed, on an html canvas or PDF. (Very close to the Visitor design pattern, but simpler)

Here are the lectures I use:
https://dl.dropboxusercontent.com/u/31543901/TMP/DoubleDispatch.zip

(One of them is highly inspired from a lecture from Oscar)

Hope it helps!

Cheers,
Alexandre

Thanks Alex

I was thinking about the code in Pharo.

Plenty of examples:
> - Converting money

can you tell me more about this one?

> - Paper, Stone, Scissor
> - A canvas containing triangle, circle, box has to be printed, on an html canvas or PDF. (Very close to the Visitor design pattern, but simpler)
>
> Here are the lectures I use:
> https://dl.dropboxusercontent.com/u/31543901/TMP/DoubleDispatch.zip

TX
here is what I wrote today



>
> (One of them is highly inspired from a lecture from Oscar)

I will have a look now

http://lists.pharo.org/mailman/listinfo/pharo-users_lists.pharo.org

On Thu, Sep 15, 2016 at 10:04 AM, Melanie Tarr <[hidden email]> wrote:

> I was thinking about the code in Pharo.
>
> Plenty of examples:
>> - Converting money
>
> can you tell me more about this one?

I mean summing and converting different money. Here is the whole idea:

1 EUR = 662 CLP (Chilean pesos)

You have a class Money to which you can sum other money.

Object subclass: #Money instVarNames: ‘value’
“I omit here the accessors of value. Note that generating the accessors of value produces a method value1, you have to rename it"

Money>>+ anotherMoney
        self subclassResponsibility

Money>>sumWithEUR: money
        self subclassResponsibility

Money>>sumWithCLP: money
        self subclassResponsibility

Money>>= anotherMoney
        ^ self class == anotherMoney class and: [ self value = anotherMoney value ]

Money >>printOn: str
        "Useful for debugging"
        super printOn: str.
        str nextPut: $<.
        str nextPutAll: self value asString.
        str nextPut: $>.



You have two subclasses:
Money subclass: #EUR
Money subclass: #CLP

EUR>>+ anotherMoney
        ^ anotherMoney sumWithEUR: self

EUR>>sumWithEUR: money
        ^ EUR new value: self value + money value

EUR>>sumWithCLP: money
        ^ CLP new value: (self value * 662) + money value


CLP>>+ anotherMoney
        ^ anotherMoney sumWithCLP: self

CLP>>sumWithEUR: money
        ^ EUR new value: (self value / 662) + money value

CLP>>sumWithCLP: money
        ^ CLP new value: self value + money value


Here is a test:
TestCase subclass: #MoneyTest

MoneyTest>>testSum
        | clp1 eur1 clp2 eur2 |
        clp1 := CLP new value: 3500.
        eur1 := EUR new value: 10.
        clp2 := CLP new value: 5000.
        eur2 := EUR new value: 20.

        self assert: clp1 + clp2 equals: (CLP new value: 8500).
        self assert: clp1 + eur1 equals: (CLP new value: 3500 + 6620).
       
        self assert: eur1 + eur2 equals: (EUR new value: 30).
        self assert: eur1 + clp2 equals: (EUR new value: 5000 / 662 + 10).


To be really complete, the example also needs to implement hash and the test should also test equality.
I hope it helps!

Cheers,
Alexandre

On 13 September 2016 at 11:33, stepharo <[hidden email]> wrote:
> Hi
>
> If you happen to know a double dispatch situation in Pharo, I'm interested
> since I'm revisiting my lecture.
>

There is a good example in
http://www.lulu.com/shop/andres-valloud/fundamentals-of-smalltalk-programming-technique-volume-1/paperback/product-5299835.html

He uses it in an implementation of Game Of Life.

>
> Stef
>
>

Tx todd

For me it was too complex.

I should check the book and andres and now I used the examples given my alex.

On Sep 13, 2016, at 12:35, Denis Kudriashov <[hidden email]> wrote:

2016-09-13 20:56 GMT+02:00 stepharo <[hidden email]>:

I was thinking about the code in Pharo.

Probably most known and beautiful case is arithmetic operations. Look at #adaptToInteger:andSend: and friends.

That one is my go-to example to explain it.