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Date: Sun, 8 Aug 2021 14:11:07 +0100
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Subject: Re: [PHP-DEV] Revisiting Userland Operator Overloads
From: rowan.collins@gmail.com (Rowan Tommins)

On 08/08/2021 08:41, Jordan LeDoux wrote:
> Off the top of my head here are some of the use cases that I think benefit
> greatly from this:
>
> - Complex number objects
> - Fractions objects
> - Matrix objects (math)
> - Vector objects (math)
> - Time/Date intervals
> - Collections
> - Arbitrary precision numbers (which are usually represented as a string in
> PHP)


I think that although mathematical libraries feel like a natural 
example, they're actually rather a small niche in the world of PHP, so 
not a great advert for the feature. Collections are not necessarily a 
great example either, since defining "+" to mean "combine these two 
collections in some type-specific way" is the kind of broad overloading 
which is often controversial.

An example which I think would be applicable to many more PHP users is a 
"Money" type, with a value, a currency, and perhaps a precision. It's 
also an interesting example where all the mathematical operators can be 
implemented, but restricted to certain combinations of types, and even 
combinations of values.

A real-life example from a C# project has the following overloads 
(syntax tweaked to be more PHP-like):

public static operator - (Money $m1, Money $m2): Money
public static operator + (Money $m1, Money $m2): Money
public static operator / (Money $money, float $divisor): Money // 
fraction of a Money value
public static operator / (Money $m1, Money $m2): float // ratio of two 
Money values
public static operator * (Money $money, float $multiple): Money // note 
[Money * Money] is not defined
public static operator < (Money $m1, Money $m2): bool
public static operator > (Money $m1, Money $m2): bool
public static operator == (Money $m1, Money $m2): bool
public static operator != (Money $m1, Money $m2): bool // defined as 
!($m1 == $m2)

All operators which take two Money values throw an exception if they 
have different currencies.

The [float * Money] case isn't overloaded here, although it would make 
sense; I don't know if that's because it can't be in C#'s overload 
system, or just that it is missing from this implementation.

Since even a single operator interface can't guarantee that all inputs 
will have a valid output, I remain unconvinced that implementing 8 
different interfaces for the above is any better than implementing one 
interface and stubbing the parts that have no valid inputs.

Regards,

-- 
Rowan Tommins
[IMSoP]