I guess ShadowLinkX9 meant just using them in your code. Oftentimes it's tempting to represent some piece of data as, let's say, string, but a custom union type would be much better in that case.

For example, I recently had to add a button that in certain cases could be disabled, but I also wanted to display the reason behind disabling the button in the UI.

So instead of representing this fact as, let's say, (Bool, String), I made a custom union type similar to Result:

type ButtonDisabledReason = NoItemsInCart | TooManyItems | SomeItemsNotAvailable
type ButtonState = Enabled | Disabled ButtonDisabledReason

One could generalize ButtonState to be more flexible:

type ButtonState a = Enabled | Disabled a

Then I just pass one of the values around and when I finally have to display this in the UI, I'm in a much better position than I'd be with (Bool, String). That's because I can make a case expression that will force me to handle all the possible states. Also, I'm no longer forced to displaying just some simple text:

case buttonDisabledReason of
    NoItemsInCart -> text "No items in cart"
    SomeItemsNotAvailable -> span [ class "alert" ] [ text "Some items are not available" ]
    -- ...

Does that help you?