gread.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MonoLocalBinds #-}

import Generics.Eot
import Data.Char

greadMaybe :: forall a. (HasEot a, EnumType (Eot a)) => String -> Maybe a
greadMaybe = (`lookup` table)
  where
    table :: [(String, a)]
    table = zip (map (map toLower) (enumNames (Proxy @a)))
                enumValues

enumNames :: HasEot a => Proxy a -> [String]
enumNames = map constructorName . constructors . datatype

enumValues :: (HasEot a, EnumType (Eot a)) => [a]
enumValues = map fromEot enumEot

-- | A class for things with several zero-field constructors.
class EnumType eot where
  -- | Get all EOT-representations of constructors of the type.
  enumEot :: [eot]

instance EnumType Void where
  enumEot = []
instance EnumType () where
  enumEot = [()]
instance EnumType eot => EnumType (Either () eot) where
  enumEot = Left () : map Right enumEot