Const support (#118)

* Const instance.

* Update changelog.

Author: bakhtiyarneyman

CHANGELOG.md

@@ -12,6 +12,7 @@ New features:
 Bugfixes:
 
 Other improvements:
+  - Decoding and encoding of the `Const` functor. (#118 by @bakhtiyarneyman)
 
 ## [v9.0.0](https://github.com/purescript-contrib/purescript-argonaut-codecs/releases/tag/v9.1.0) - 2022-06-23
 

bower.json

@@ -17,6 +17,7 @@
     "purescript-argonaut-core": "^7.0.0",
     "purescript-arrays": "^7.0.0",
     "purescript-bifunctors": "^6.0.0",
+    "purescript-const": "^6.0.0",
     "purescript-effect": "^4.0.0",
     "purescript-either": "^6.0.0",
     "purescript-foldable-traversable": "^6.0.0",

spago.dhall

@@ -8,6 +8,7 @@
   , "assert"
   , "bifunctors"
   , "console"
+  , "const"
   , "effect"
   , "either"
   , "exceptions"

src/Data/Argonaut/Decode/Class.purs

@@ -6,6 +6,7 @@ import Data.Argonaut.Core (Json, toObject)
 import Data.Argonaut.Decode.Error (JsonDecodeError(..))
 import Data.Array.NonEmpty (NonEmptyArray)
 import Data.Bifunctor (lmap)
+import Data.Const (Const)
 import Data.Either (Either(..))
 import Data.Identity (Identity)
 import Data.List (List)
@@ -31,6 +32,9 @@ class DecodeJson a where
 instance decodeIdentity :: DecodeJson a => DecodeJson (Identity a) where
   decodeJson = decodeIdentity decodeJson
 
+instance decodeConst :: DecodeJson a => DecodeJson (Const a b) where
+  decodeJson = decodeConst decodeJson
+
 instance decodeJsonMaybe :: DecodeJson a => DecodeJson (Maybe a) where
   decodeJson = decodeMaybe decodeJson
 

src/Data/Argonaut/Decode/Decoders.purs

@@ -8,6 +8,7 @@ import Data.Array as Arr
 import Data.Array.NonEmpty (NonEmptyArray)
 import Data.Array.NonEmpty as NEA
 import Data.Bifunctor (lmap)
+import Data.Const (Const(..))
 import Data.Either (Either(..), note)
 import Data.Identity (Identity(..))
 import Data.Int (fromNumber)
@@ -35,6 +36,13 @@ decodeIdentity
   -> Either JsonDecodeError (Identity a)
 decodeIdentity decoder json = Identity <$> decoder json
 
+decodeConst
+  :: forall a b
+   . (Json -> Either JsonDecodeError a)
+  -> Json
+  -> Either JsonDecodeError (Const a b)
+decodeConst decoder json = Const <$> decoder json
+
 decodeMaybe
   :: forall a
    . (Json -> Either JsonDecodeError a)

src/Data/Argonaut/Encode/Class.purs

@@ -5,6 +5,7 @@ import Data.Argonaut.Encode.Encoders
 import Data.Argonaut.Core (Json, fromObject)
 import Data.Array.NonEmpty (NonEmptyArray)
 import Data.String.NonEmpty (NonEmptyString)
+import Data.Const (Const)
 import Data.Either (Either)
 import Data.Identity (Identity)
 import Data.List (List)
@@ -29,6 +30,9 @@ class EncodeJson a where
 instance encodeIdentity :: EncodeJson a => EncodeJson (Identity a) where
   encodeJson = encodeIdentity encodeJson
 
+instance encodeConst :: EncodeJson a => EncodeJson (Const a b) where
+  encodeJson = encodeConst encodeJson
+
 instance encodeJsonMaybe :: EncodeJson a => EncodeJson (Maybe a) where
   encodeJson = encodeMaybe encodeJson
 

src/Data/Argonaut/Encode/Encoders.purs

@@ -6,6 +6,7 @@ import Data.Argonaut.Core (Json, fromArray, fromBoolean, fromNumber, fromObject,
 import Data.Array as Arr
 import Data.Array.NonEmpty (NonEmptyArray)
 import Data.Array.NonEmpty as NEA
+import Data.Const (Const(..))
 import Data.Either (Either, either)
 import Data.Identity (Identity(..))
 import Data.Int (toNumber)
@@ -27,6 +28,9 @@ import Foreign.Object as FO
 encodeIdentity :: forall a. (a -> Json) -> Identity a -> Json
 encodeIdentity encoder (Identity a) = encoder a
 
+encodeConst :: forall a b. (a -> Json) -> Const a b -> Json
+encodeConst encoder (Const a) = encoder a
+
 encodeMaybe :: forall a. (a -> Json) -> Maybe a -> Json
 encodeMaybe encoder = case _ of
   Nothing -> jsonNull