Generic deriving with purescript

generic-deriving.purs

-- an example of how to derive a show instance for a Maybe type
-- not totally sure why `derive instance showMyMaybe :: (Show a) => Show (MyMaybe a)` errors with...
-- error: CannotDerive :
-- Cannot derive a type class instance for Data.Show.Show (MyMaybe a) since instances of this type class are not derivable.
module Main where

import Prelude

import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)
import Data.Generic.Rep (class Generic)
import Data.Generic.Rep.Show (genericShow)

data MyMaybe a
  = Just a
  | Nothing

derive instance genericMyMaybe :: Generic (MyMaybe a) _

instance showMyMaybe :: (Show a) => Show (MyMaybe a) where show = genericShow

main :: forall e. Eff (console :: CONSOLE | e) Unit
main = do
  log $ show $ Just "hello, deriving!"