0532a61 · rel8 · git.iko.soy

git.iko.soy/rel8

Fix Rel8able to work with polymorphic types (#113)

Currently, a declaration such as: ```haskell newtype IdRecord a f = IdRecord { recordId :: Column f a } deriving Generic instance DBType a => Rel8able (IdRecord a) ``` Will fail, producing the error: ``` • Could not deduce (rel8-1.0.0.1:Rel8.Generic.Rel8able.Serialize (a Data.Type.Equality.== Identity a) (Expr a) a) arising from a use of ‘rel8-1.0.0.1:Rel8.Generic.Rel8able.$dmgfromColumns’ from the context: DBType a bound by the instance declaration at lib/CircuitHub/Model/Types.hs:231:10-42 ``` The stuck `==` type family gets in the way, even though we can clearly see that this is `False`. This commit fixes this error by introducing a variation of `==` that anticipates this case, adding an explicit pattern. With this new definition of `==`, things work as expected.

commit0532a6193c97c673583aca93fd4e951b5d7f9fcf authorOllie Charles <oliver.charles@circuithub.com> dateThu 8 Jul 2021, 15:34 (5 years ago) parent032242f

2 files changed+25−1

compare with master

unifiedsplit

src/Rel8/Generic/Rel8able.hs+17−1 tests/Rel8/Generic/Rel8able/Test.hs+8

src/Rel8/Generic/Rel8able.hs+17−1

@@ -6,6 +6,7 @@

66{-# language FlexibleInstances #-}

77{-# language LambdaCase #-}

88{-# language MultiParamTypeClasses #-}

9+{-# language PolyKinds #-}

910{-# language QuantifiedConstraints #-}

1011{-# language ScopedTypeVariables #-}

1112{-# language StandaloneKindSignatures #-}

@@ -25,8 +26,9 @@ module Rel8.Generic.Rel8able

2526where

2627

2728-- base

29+import Data.Functor.Identity ( Identity )

2830import Data.Kind ( Constraint, Type )

29−import Data.Type.Equality ( type (==) )

31+import Data.Type.Bool ( type (&&) )

3032import GHC.Generics ( Generic, Rep, from, to )

3133import Prelude

3234

@@ -58,6 +60,20 @@ type KRel8able :: Type

5860type KRel8able = K.Rel8able

5961

6062

63+-- This is almost 'Data.Type.Equality.==', but we add an extra case.

64+type (==) :: k -> k -> Bool

65+type family a == b where

66+ -- This extra case is needed to solve the equation "a == Identity a",

67+ -- which occurs when we have polymorphic Rel8ables

68+ -- (e.g., newtype T a f = T { x :: Column f a })

69+ a == Identity a = 'False

70+

71+ -- These cases are exactly the same as those in 'Data.Type.Equality.==.

72+ f a == g b = f == g && a == b

73+ a == a = 'True

74+ _ == _ = 'False

75+

76+

6177type Serialize :: Bool -> Type -> Type -> Constraint

6278class transposition ~ (a == Transpose Result expr) =>

6379 Serialize transposition expr a

tests/Rel8/Generic/Rel8able/Test.hs+8

@@ -169,3 +169,11 @@ data NestedTableTestB f = NestedTableTestB

169169 }

170170 deriving stock Generic

171171 deriving anyclass Rel8able

172+

173+

174+

175+newtype IdRecord a f = IdRecord { recordId :: Column f a }

176+ deriving stock Generic

177+

178+

179+instance DBType a => Rel8able (IdRecord a)