Covariance of immutable containers

Hello! I’ve just started to use Language.Ext.Core and I’ve ran into the small inconvenience - Either<L, R> (like all other container types in this library) is not covariant on its type arguments. In some cases it forces me to cast returned value manually to the base class. Does this design have some purpose? If I understand everything correctly we can make immutable containers covariant essentially for free. But maybe I’ve missed something important… Thank you in advance.

[louthy]

@Hinidu Indeed it’s sometimes a little ugly I’m afraid. To reduce the amount of clutter from casting I tend to use the as operator (even if it’s slightly less efficient):

    public static Either<Error, TValue> GetEither<TKey, TValue>(
        this IDictionary<TKey, TValue> dictionary, TKey key)
    {
        return dictionary.TryGetValue(key)
            .ToEither(() => new KeyNotExistError<TKey, TValue>(key) as Error);
    }

Or you could use MapLeft if you want to be more declarative:

    public static Either<Error, TValue> GetEither<TKey, TValue>(
        this IDictionary<TKey, TValue> dictionary, TKey key)
    {
        return dictionary.TryGetValue(key)
            .ToEither(() => new KeyNotExistError<TKey, TValue>(key))
            .MapLeft(x => x as Error);
    }

[Hinidu]

@louthy Thank you for suggestions 👍