Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:122993
Feedback-ID: id5114917:Fastmail
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
User-Agent: K-9 Mail for Android
In-Reply-To: <CAMrTa2Geo1XHFxfa1Mj6tXcYC-gVwQeUK+Wk0h11d1fELUyZJQ@mail.gmail.com>
References: <4F094EDA-5058-407D-AF39-06FD934FDE1F@sakiot.com> <68CF373E-6ABF-4471-8992-813B4BA1B508@sakiot.com> <904197f4-afb5-401e-9e17-7a655c5449d0@alec.pl> <CAMrTa2EK6ymBQuxeFj7tMV0M2crqCCCR=mLN-FSfuCgrQHEZ6A@mail.gmail.com> <AB663F3F-A888-45F7-B19A-00530A4EDB0C@sakiot.com> <CAMrTa2HOEUWQcPdZE3FZ9JWCS1DWuRTNhhfqGGZOmr=v1Mctdw@mail.gmail.com> <CANS-=pfVuKKfAiu0LUJTrEPnSRaa_nHwJCgMvSORruAwHMsvQg@mail.gmail.com> <CAMrTa2HXD+62TKbaaA-v-mct5zdR5ortUAbtG1R1D22amAgpNA@mail.gmail.com> <CAMrTa2GaK3AUp0WodhNO8RdvoLbbiCk14Gs2oWF6vEs_+qMYNg@mail.gmail.com> <655FEA80-9AB4-4AAD-A310-70ED968C97A2@sakiot.com> <CAMrTa2Fh_-M_Kezi7s_4P4+a+dTyKq=RExd6ARPRDqZYS175kA@mail.gmail.com> <CAMrTa2FrkvNPK6kETNJFNNDn0mL5dxkV=PmdxsysruhhjY=Uig@mail.gmail.com> <8FB87901-02D7-4934-9119-55B21CEDDA9D@sakiot.com> <CAMrTa2EJf2NK2D3J=q7xkAVuNKsjEBDVv+RDmxp_g9DSsjRgwg@mail.gmail.com> <8035a2b2-f8dc-4a25-98eb-1d19c986bc75@bastelstu.be> <CAMrTa2Geo1XHFxfa1Mj6tXcYC-gVwQeUK+Wk0h11d1fELUyZJQ@mail.gmail.com>
Message-ID: <DD7E3EE0-49B2-4FBB-A99D-8A558CC9F185@rwec.co.uk>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain;
 charset=utf-8
Content-Transfer-Encoding: quoted-printable
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]