src/Rel8/Table/Unreify.hs · rel8

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rel8/src/Rel8/Table/Unreify.hs

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1{-# language DataKinds #-}

2{-# language StandaloneKindSignatures #-}

3{-# language TypeFamilies #-}

4{-# language UndecidableInstances #-}

6-- | This module implements some machinery for implementing methods of the

7-- 'Table' class for a particular special (but important) class of polymorphic

8-- @Table@ types.

9--

10-- This special case is characterised by a @newtype@ wrapper around a bare

11-- 'HTable' which is constructed by applying a type family to the polymorphic

12-- type variable.

14-- Examples of this class of @Table@ include @ListTable@ and @NonEmptyTable@.

16-- The tricky part about implementing @Table@ for these types is 'reify' and

17-- 'unreify'. There is no guarantee in general that @'Unreify' a@ is itself

18-- a @Table@, let alone a @Table@ with the same 'Columns' as @a@

19-- (e.g., @Unreify (AColumn Result Bool) = Bool@, and @Bool@ is not a

20-- @Table@)

22module Rel8.Table.Unreify

23 ( Unreifiable

24 )

25where

27-- base

28import Data.Kind ( Constraint, Type )

29import Prelude ()

31-- rel8

32import qualified Rel8.Schema.Kind as K

33import Rel8.Schema.Reify ( Reify )

34import Rel8.Table ( Table, Columns, Unreify )

37type Unreifiable :: K.Context -> Type -> Constraint

38type family Unreifiable context a where

39 Unreifiable (Reify context) a =

40 ( Table context (Unreify a)

41 , Columns a ~ Columns (Unreify a)

42 )

43 Unreifiable _ _ = ()