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Date: Tue, 3 Mar 2020 09:52:52 +0000
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Content-Type: multipart/alternative; boundary="000000000000192d85059ff0458d"
Subject: Re: [PHP-DEV] Re: [RFC] Userspace operator overloading
From: rowan.collins@gmail.com (Rowan Tommins)
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Hi Terje,
I think both of your examples are compatible with the idea of grouped
operators, as long as we don't constrain a type to implement all operators
with the same right-hand side.
On Mon, 2 Mar 2020 at 17:23, Terje Sletteb=C3=B8 <
terje.slettebo@sandefjordbredband.net> wrote:
>
> Adding and subtracting money makes sense, but it makes no sense to
> multiply
> or divide them, and this is just *one* domain.
>
This is true, but it does make sense to multiply and divide them by
numbers, for instance $totalPrice =3D $unitPrice * $quantity
In general, if $a + $a represents addition, then $a * 2 should logically
give the same result. As has been raised, there is an argument for keeping
/ separate, for types that cannot handle fractional operations; it would be
a matter of choice whether $price / 2 should be supported because it would
lead to fractional pennies/cents.
> $total_discount =3D $total * $discount;
>
> Here, $total_discount and $total is Money, while $discount is a
> Percentage,
> so we should be able to define a method that allows you to multiply Money
> with Percentage, and return a Money object. Adding Money and Percentage
> would
> make no sense, but multiplying them does.
>
If this operator was implemented in isolation, it would again be drifting
into domain-specific language territory. (I don't personally have a problem
with that, but many people vocally object to it.)
However, it could easily be made part of a consistent set of arithmetic
operators:
On class Money:
Money + Money =3D> Money
Money - Money =3D> Money
Money * int =3D> Money
Money / int =3D> Money
int * Money =3D> Money
int / Money =3D> Error
Money * Percentage =3D> Money
Money / Percentage =3D> Money
Percentage * Money =3D> Money
Percentage / Money =3D> Error
Optionally, on class Percentage:
Percentage + Percentage =3D> Percentage
Percentage - Percentage =3D> Percentage
Percentage* int =3D> Percentage
Percentage / int =3D> Percentage
Regards,
--=20
Rowan Tommins
[IMSoP]
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