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Date: Sat, 06 Apr 2024 11:36:40 +0100
To: internals@lists.php.net
Subject: Re: [PHP-DEV] [RFC] [Discussion] Support object type in BCMath
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")



On 5 April 2024 19:30:24 BST, Jordan LeDoux <jordan=2Eledoux@gmail=2Ecom> =
wrote:

>A composed class
>does not somehow prevent the accidental error of mixing currencies, it ju=
st
>moves where that error would occur

It does not prevent someone accidentally *attempting* to mix currencies, b=
ut it absolutely prevents that mistake leading to bogus values in the appli=
cation, because the methods available can all detect it and throw an Error=
=2E=20

If the class has a mixture of currency-aware methods, and methods / operat=
or overloads inherited from Number, you can end up getting nonsensical resu=
lts instead of an Error=2E


>If you want an actual answer about how a Money class would actually work =
in
>practice, it would likely be something like this:
>
>```
>// $val1 and $val2 are instances of the Money class with unknown currenci=
es
>$val1Currency =3D $val1->getCurrency();
>$val2Currency =3D $val2->getCurrency();
>$val1 =3D $val1->convertToCommonCurrency();
>$val2 =3D $val2->convertToCommonCurrency();
>// perform the necessary calculations using operators
>$answer =3D $answer->convertToCurrency($userCurrency);
>```

You have missed out the key section: how do you actually add the two numbe=
rs? The addition MUST enforce the precondition that its operands are in the=
 same currency; any other behaviour is nonsensical=2E So the definition of =
that method must be on the Money class, not inherited from Number=2E=20

If the add() method on Number is final, you'll need to define a new method=
 $val1->addCurrency($val2)=2E The existing add() method and operator overlo=
ad will be inherited unchanged, but calling them won't just be useless, it =
will be dangerous, because they can give nonsensical results=2E=20

That's what makes composition the better design in this case, because the =
Money class can expose only the methods that actually have useful and safe =
behaviour=2E

The fact that the composed class can't add its own operator overloads is u=
nfortunate; but allowing inheritance wouldn't solve that, because the inher=
ited overloads are all wrong anyway=2E

Regards,
Rowan Tommins
[IMSoP]