Add coercible

Author: garyb

src/Data/Codec/Argonaut.purs

@@ -25,6 +25,7 @@ module Data.Codec.Argonaut
   , recordPropOptional
   , fix
   , prismaticCodec
+  , coercible
   , module Exports
   ) where
 
@@ -34,6 +35,7 @@ import Control.Monad.Reader (ReaderT(..), runReaderT)
 import Control.Monad.Writer (Writer, mapWriter, writer)
 import Data.Argonaut.Core as J
 import Data.Array as A
+import Data.Bifunctor (bimap)
 import Data.Bifunctor as BF
 import Data.Codec (BasicCodec, Codec, GCodec(..), basicCodec, bihoistGCodec, decode, encode)
 import Data.Codec (decode, encode, (<~<), (>~>), (~)) as Exports
@@ -51,7 +53,9 @@ import Data.Traversable (traverse)
 import Data.Tuple (Tuple(..))
 import Foreign.Object as FO
 import Partial.Unsafe (unsafePartial)
+import Prim.Coerce (class Coercible)
 import Prim.Row as Row
+import Safe.Coerce (coerce)
 import Type.Proxy (Proxy)
 import Unsafe.Coerce (unsafeCoerce)
 
@@ -413,3 +417,13 @@ prismaticCodec name f g orig =
   basicCodec
     (\json' → note (Named name (UnexpectedValue json')) <<< f =<< decode orig json')
     (encode orig <<< g)
+
+-- | A codec for types that can be safely coerced.
+-- |
+-- | Accepts the name of the target type as an argument to improve error messaging when the inner
+-- | codec fails.
+coercible ∷ ∀ a b. Coercible a b ⇒ String → JsonCodec a → JsonCodec b
+coercible name codec =
+  basicCodec
+    (bimap (Named name) coerce <<< decode codec)
+    (coerce (encode codec))