Discussion on: Modelling domain with state machines in ReasonML

[Margarita Krutikova]

That's interesting. Could you give a code example of how tagged unions can model values that depend on each other? And also how you can rule out the last two invalid states.

[Iven Marquardt]

I don't know ReasonML but I guess it has an Either type, right? Either<A, B> has two value constructors Left/Right, which can produce values of type Either<SomeProperType, B> and Either<A, SomeProperType> respectively. Consequently you cannot produce the invalid values Either<A, B> (no value at all) or Either<SomeProperType, SomeProperType (both values at the same time, aka the product). These values or states are ruled out by the way you modelled the data by using a tagged union. Values produced by Left and Right depend on each other, because they are mutually exclusive.

[Austin]

That's a pretty good description of the way variants work. OCaml/Reason does not have a built-in Either type, but they do have this "disjoint tagged unions" which can behave the same way.

The most common data type that models the Either type you describe (used in Haskell) is the Result type. It is modeled using the variant constructors Ok and Error. So the Reason code would look like:

module Result = {
  type t('a, 'e) =
    | Ok('a)
    | Error('e);
};

And the type we have there is a bifunctor and a monad, just like Either.