Members of the ShowSing kind class can have their Sing values converted to Strings in a fashion similar to that of the Show class. (In fact, this class only exists because one cannot write Show instances for Sings of the form instance (forall z. Show (Sing (z :: k))) => Show (Sing (x :: [k])).)

This class should not be confused with the promoted or singled versions of Show from Data.Singletons.Prelude.Show (PShow and SShow, respectively). The output of ShowSing is intended to reflect the singleton type, whereas the output of PShow and SShow reflects the original type. That is, showing SFalse with ShowSing would yield "SFalse", whereas PShow and SShow would yield "False".

Instances of this class are generated alongside singleton definitions for datatypes that derive a Show instance. Moreover, having a ShowSing instances makes it simple to define a Show instance. For instance:

instance ShowSing a => ShowSing [a] where
  showsSingPrec = ...
instance ShowSing a => Show (Sing (x :: [a])) where
  showsPrec = showsSingPrec

As a result, singleton definitions for datatypes that derive a Show instance also get a Show instance for the singleton type as well (in addition to promoted and singled Show instances).

To recap: singletons will give you all of these for a datatype that derives a Show instance:

• A promoted (PShow) instance
• A singled (SShow) instance
• A ShowSing instance for the singleton type
• A Show instance for the singleton type

What a bargain!