Add Codec's Selective instance

src/Toml/Codec/Types.hs

@@ -35,6 +35,7 @@ module Toml.Codec.Types
 
 import Control.Applicative (Alternative (..), liftA2)
 import Control.Monad.State (MonadState (..))
+import Control.Selective (Selective (..))
 import Data.Bifunctor (first)
 import Validation (Validation (..))
 
@@ -137,6 +138,14 @@ infixl 3 <!>
 f <!> g = \a -> f a <|> g a
 {-# INLINE (<!>) #-}
 
+-- | @since 1.3.4
+instance Selective (Codec i) where
+  select branched onLeft = Codec
+        { codecRead  = \o -> (`either` id)  <$> codecRead  onLeft o <*> codecRead  branched o
+        , codecWrite = \i -> (`either` id) <$> codecWrite onLeft i <*> codecWrite branched i
+        }
+  {-# INLINE select #-}
+
 {- | Mutable context for TOML conversion.
 We are introducing our own implemetation of state with 'MonadState' instance due
 to some limitation in the design connected to the usage of State.

tomland.cabal

@@ -137,6 +137,7 @@ library
                      , megaparsec >= 7.0.5 && < 9.3
                      , mtl ^>= 2.2
                      , parser-combinators >= 1.1.0 && < 1.4
+                     , selective ^>= 0.4.2
                      , text >= 1.2 && < 2.1
                      , time >= 1.8 && < 1.14
                      , transformers >= 0.5 && < 0.7