How to search for an enum by values and why enum items aren't unique

[Ali Çehreli via Digitalmars-d-learn]

On 07/27/2016 08:42 AM, stunaep wrote:
> On Wednesday, 27 July 2016 at 15:32:59 UTC, Meta wrote:
>> On Wednesday, 27 July 2016 at 13:59:54 UTC, stunaep wrote:
>>> So how would I make a function that takes an enum and an id as a
>>> parameter and returns a member in the enum? I tried for quite some
>>> time to do this but it wont let me pass Test as a parameter unless I
>>> use templates. I finally came up with this but it wont let me return
>>> null when there's nothing found
>>>
>>>> E findEnumMember(E)(int id) if (is(E == enum)) {
>>>>     auto found = [EnumMembers!E].find!(a => a.id == id)();
>>>>     if(!found.empty)
>>>>         return found.front;
>>>>     else
>>>>         ...What do I return? null gives error
>>>> }
>>
>> If you're going to do it like this your only real options are to
>> return a Nullable!E or throw an exception if the id isn't found.
>
> I tried Nullable!E earlier and it didnt work.

import std.traits;
import std.algorithm;
import std.array;
import std.typecons;

Nullable!E findEnumMember(E)(int id) if (is(E == enum)) {
     auto found = [EnumMembers!E].find!(a => a.id == id)();
     if(!found.empty)
         return Nullable!E(found.front);
     else
         return Nullable!E();
}

struct S {
     int id;
}

enum MyEnum : S {
     x = S(42),
     invalid = S()    // Useful for the other alternative
}

void main() {
     auto a = findEnumMember!MyEnum(42);
     assert(!a.isNull);
     auto b = findEnumMember!MyEnum(7);
     assert(b.isNull);
}


> I dont need it to be done
> like this, it just has to be done someway. I'm asking for help because
> that's the only way I could think of.

Another alternative is to require that the enum has a special sentinel:

     else
         return E.invalid;

Ali