enum can contain both concrete methods and abstract methods. If an enum class has an abstract method, then each instance of the enum class must implement it 

We can’t create enum objects explicitly and hence we can’t invoke enum constructor directly.

Since it is static, we can access it by using the enum Name. Since it is final, we can’t create child enums.

enum can implement many interfaces

All enums implicitly extend java.lang.Enum class. As a class can only extend one parent in Java, so an enum cannot extend anything else.

In Java, we can also add variables, methods and constructors to it

enum type can be passed as an argument to switch statement.

It is a compile-time error if an enum declaration has the modifier abstract orfinal.

Arguably, the rails-team's choice of raising ArgumentError instead of validation error is correct in the sense that we have full control over what options a user can select from a radio buttons group, or can select over a select field, so if a programmer happens to add a new radio button that has a typo for its value, then it is good to raise an error as it is an application error, and not a user error. However, for APIs, this will not work because we do not have any control anymore on what values get sent to the server.

liberal_enum :kind

When the controller creates the user, instead of adding a validation error to the record, it raises an exception. How to avoid this?

I really dislike the reasoning as stated in the issue listed above. Since the value is coming over the wire, it should be treated the same as a freetext input where the expectation is to validate in the model and not the controller. This is especially true in APIs where the developers have even less of a say as far as expected input coming from form data (for example).

In case anyone wants a hack, here is what I came up with.

But now the first line throws an exception ArgumentError: '0' is not a valid audience

const renderMapping: { [l in letters]: renderFunction<l>; } = { 'a': (a: 'a') => 'alpha', 'b': (b: 'b') => 'bravo', }; type renderFunction<l extends letters> = (letter: l) => string; function renderLetter<l extends letters>(letter: l): renderFunction<l> { return renderMapping[letter]; }

type-safe enum pattern