either

[KennyTM~]

On Jan 11, 11 17:10, Justin Johansson wrote:
> On 10/01/11 05:42, Andrei Alexandrescu wrote:
>> I wrote a simple helper, in spirit with some recent discussions:
>>
>> // either
>> struct Either(Ts...)
>> {
>> Tuple!Ts data_;
>> bool opEquals(E)(E e)
>> {
>> foreach (i, T; Ts)
>> {
>> if (data_[i] == e) return true;
>> }
>> return false;
>> }
>> }
>>
>> auto either(Ts...)(Ts args)
>> {
>> return Either!Ts(tuple(args));
>> }
>>
>> unittest
>> {
>> assert(1 == either(1, 2, 3));
>> assert(4 != either(1, 2, 3));
>> assert("abac" != either("aasd", "s"));
>> assert("abac" == either("aasd", "abac", "s"));
>> }
>>
>> Turns out this is very useful in a variety of algorithms. I just don't
>> know where in std this helper belongs! Any ideas?
>
> Despite that it may be very useful as you say, personally I think it is
> a fundamental no-no to overload the meaning of "==" in any manner that
> does not preserve the generally accepted semantics of equality which
> include the notions of reflexivity, symmetry and transitivity**.
>
> **See http://en.wikipedia.org/wiki/Equality_%28mathematics%29
>
> The symmetric and transitive properties of the equality relation imply
> that if (a == c) is true and if (b == c) is true then (a == b) is also
> true.
>
> In this case the semantics of the overloaded "==" operator have the
> expressions 1 == either(1, 2, 3) and 2 == either(1, 2, 3) both
> evaluating to true and by implication/expectation (1 == 2).
>
> Clearly though, (1 == 2) evaluates to false in terms of the commonly
> accepted meaning of equality.
>
> Just my 2 cents and I wonder if there some other way of achieving the
> desired functionality of your helper without resorting to overloading
> "==" and the consequential violation of the commonly held semantics of
> equality.
>
> Cheers,
> Justin Johansson

We could use in instead of ==

if (1 in oneOf(1, 2, 3)) { ... }
if (4 !in oneOf(1, 2, 3)) { ... }