src/Rel8/Type/ReadShow.hs · rel8

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rel8/src/Rel8/Type/ReadShow.hs

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1{-# language ScopedTypeVariables #-}

2{-# language StandaloneKindSignatures #-}

3{-# language TypeApplications #-}

4{-# language ViewPatterns #-}

6module Rel8.Type.ReadShow ( ReadShow(..) ) where

8-- base

9import Data.Kind ( Type )

10import Data.Proxy ( Proxy( Proxy ) )

11import Data.Typeable ( Typeable, typeRep )

12import Prelude

13import Text.Read ( readMaybe )

15-- rel8

16import Rel8.Type ( DBType( typeInformation ) )

17import Rel8.Type.Information ( parseTypeInformation )

19-- text

20import qualified Data.Text as Text

23-- | A deriving-via helper type for column types that store a Haskell value

24-- using a Haskell's 'Read' and 'Show' type classes.

25type ReadShow :: Type -> Type

26newtype ReadShow a = ReadShow { fromReadShow :: a }

29instance (Read a, Show a, Typeable a) => DBType (ReadShow a) where

30 typeInformation = parseTypeInformation parser printer typeInformation

31 where

32 parser (Text.unpack -> t) = case readMaybe t of

33 Just ok -> Right $ ReadShow ok

34 Nothing -> Left $ "Could not read " <> t <> " as a " <> show (typeRep (Proxy @a))

35 printer = Text.pack . show . fromReadShow