This module provides some tools to encode multiple versions of a data model in a single data-type parametrized by version number. The addition or removal of a field can be expressed through the Since and Until type families.

Example:

data Rec v = Rec
    { foo :: Int               -- this field exists in all versions
    , bar :: Since V2 v Bool   -- this field has been introduced in V2
    , baz :: Until V2 v Double -- this field has been removed in V3
    }

Besides reducing the number of data declarations, this approach also has other advantages:

• It makes migrations declarative and self-documenting.
• It allows for less verbose version-upcasting functions, since the fields that have a non-parametric type do not need to be copied.
• It is a foundation on which other useful abstractions can be built.

Please note that some classes may require a separate standalone deriving clause for each version of a data-type or some kind of inductive deriving mechanism.