Utils.hs

-- |
-- Module      : PowerDNS.Internal.Utils
-- Description : Assorted utilities for the PowerDNS API

{-# LANGUAGE DefaultSignatures #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE OverloadedStrings #-}
module PowerDNS.Internal.Utils
  ( strip
  , Empty(..)
  , GEmpty(..)
  )
where

import           Data.List (stripPrefix)
import           GHC.Generics

-- | A variant of 'stripPrefix' that defaults to id if the prefix is not found.
strip :: Eq a => [a] -> [a] -> [a]
strip p xs = case stripPrefix p xs of
                  Just ys -> ys
                  Nothing -> xs


-- | Typeclass of things we can generate empty values of.
-- This is used to quickly build values from parameters to PowerDNS, because
-- you often only need a few fields.
-- @
--   empty { someField = Just 1
--         , otherField = Just "foo" }
-- @
class Empty a where
  -- | Produce an empty value
  empty :: a
  default empty :: (Generic a, GEmpty (Rep a)) => a
  empty = to gempty

instance Empty (Maybe a) where
  empty = Nothing

class GEmpty f where
  gempty :: f p

instance GEmpty U1 where
  gempty = U1

instance GEmpty f => GEmpty (M1 i t f) where
  gempty = M1 gempty

instance Empty a => GEmpty (K1 i a) where
  gempty = K1 empty

instance (GEmpty f, GEmpty g) => GEmpty (f :*: g) where
  gempty = gempty :*: gempty