minimal viable product

src/GHC/Generics/CoerceTo.hs

@@ -0,0 +1,70 @@
+{-# LANGUAGE Unsafe #-}
+{-# LANGUAGE RoleAnnotations #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# OPTIONS_GHC -Wno-unsafe #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE ImportQualifiedPost #-}
+
+module GHC.Generics.CoerceTo
+(GenericCoerceTo(..))
+where
+
+import Prelude ((.), ($))
+
+import GHC.Generics (Generic(Rep))
+import GHC.Generics.Coerce (GenericCoerce())
+
+import GHC.Generics qualified as Generics
+import GHC.Generics.Coerce qualified as GenericCoerce
+
+import Data.Kind (Type)
+
+import Data.List qualified as List
+import Data.List.NonEmpty (NonEmpty)
+import Data.List.NonEmpty qualified as NonEmpty
+
+import Data.Semigroup (Semigroup ((<>), sconcat))
+import Data.Monoid (Monoid (mempty, mconcat))
+
+type role GenericCoerceTo representational phantom
+
+type GenericCoerceTo :: Type -> Type -> Type
+
+newtype GenericCoerceTo self other = GenericCoerceTo { get :: self }
+
+
+toOther :: (Generic b, Generic a, GenericCoerce (Rep a) (Rep b)) => GenericCoerceTo a b -> b
+toOther = Generics.to . GenericCoerce.gfrom . Generics.from . get
+
+fromOther :: (Generic a, Generic b, GenericCoerce (Rep a) (Rep b)) => b -> GenericCoerceTo a b
+fromOther = GenericCoerceTo . Generics.to . GenericCoerce.gto . Generics.from
+
+
+instance 
+  (Generic this
+  , Generic other
+  , GenericCoerce (Rep this) (Rep other)
+  , Semigroup other) 
+  => Semigroup (GenericCoerceTo this other) where
+
+  (<>) :: GenericCoerceTo this other -> GenericCoerceTo this other -> GenericCoerceTo this other
+  lhs <> rhs = fromOther $ toOther lhs <> toOther rhs
+
+  sconcat :: NonEmpty (GenericCoerceTo this other) -> GenericCoerceTo this other
+  sconcat = fromOther . sconcat . NonEmpty.map toOther
+
+instance
+  (Generic this
+  , Generic other
+  , GenericCoerce (Rep this) (Rep other)
+  , Monoid other)
+  => Monoid (GenericCoerceTo this other) where
+
+  mempty :: GenericCoerceTo this other
+  mempty = fromOther mempty
+
+  mconcat :: [GenericCoerceTo this other] -> GenericCoerceTo this other
+  mconcat = fromOther . mconcat . List.map toOther
+