Documentation

The kind of Rel8able types

This type class allows you to define custom Tables using higher-kinded data types. Higher-kinded data types are data types of the pattern:

data MyType f =
  MyType { field1 :: Column f T1 OR HK1 f
         , field2 :: Column f T2 OR HK2 f
         , ...
         , fieldN :: Column f Tn OR HKn f
         }

where Tn is any Haskell type, and HKn is any higher-kinded type.

That is, higher-kinded data are records where all fields in the record are all either of the type Column f T (for any T), or are themselves higher-kinded data:

Nested

data Nested f =
  Nested { nested1 :: MyType f
         , nested2 :: MyType f
         }

The Rel8able type class is used to give us a special mapping operation that lets us change the type parameter f.

Supplying Rel8able instances

This type class should be derived generically for all table types in your project. To do this, enable the DeriveAnyClass and DeriveGeneric language extensions:

{-# LANGUAGE DeriveAnyClass, DeriveGeneric #-}

data MyType f = MyType { fieldA :: Column f T }
  deriving ( GHC.Generics.Generic, Rel8able )