Improve backwards-compat for config (manual FromJson)

brittany.cabal

@@ -107,6 +107,7 @@ library {
     , semigroups >=0.18.2 && <0.19
     , cmdargs >=0.10.14 && <0.11
     , czipwith >=1.0.0.0 && <1.1
+    , unordered-containers >=0.2.7 && <0.3
     }
   default-extensions: {
     CPP
@@ -167,6 +168,7 @@ executable brittany
     , semigroups
     , cmdargs
     , czipwith
+    , unordered-containers
     , hspec >=2.4.1 && <2.5
     , filepath >=1.4.1.0 && <1.5
     , ghc-boot-th >=8.0.1 && <8.1
@@ -245,6 +247,7 @@ test-suite unittests
     , semigroups
     , cmdargs
     , czipwith
+    , unordered-containers
     , hspec >=2.4.1 && <2.5
     }
   ghc-options:      -Wall
@@ -318,6 +321,7 @@ test-suite littests
     , semigroups
     , cmdargs
     , czipwith
+    , unordered-containers
     , hspec >=2.4.1 && <2.5
     , parsec >=3.1.11 && <3.2
     }

src/Language/Haskell/Brittany/Config/Types.hs

@@ -13,6 +13,7 @@ where
 
 import Data.Yaml
 import qualified Data.Aeson.Types as Aeson
+import qualified Data.HashMap.Lazy as HML
 import GHC.Generics
 
 import Data.Data ( Data )
@@ -219,10 +220,26 @@ makeToJSONOption(CPreProcessorConfig)
 makeToJSONMaybe(CPreProcessorConfig)
 
 makeFromJSONOption(CConfig)
-makeFromJSONMaybe(CConfig)
 makeToJSONOption(CConfig)
 makeToJSONMaybe(CConfig)
 
+-- This custom instance ensures the "omitNothingFields" behaviour not only for
+-- leafs, but for nodes of the config as well. This way e.g. "{}" is valid
+-- config file content.
+instance FromJSON (CConfig Maybe) where
+  parseJSON (Object v) = Config
+    <$> v .:?  Text.pack "conf_version"
+    <*> v .:?= Text.pack "conf_debug"
+    <*> v .:?= Text.pack "conf_layout"
+    <*> v .:?= Text.pack "conf_errorHandling"
+    <*> v .:?= Text.pack "conf_forward"
+    <*> v .:?= Text.pack "conf_preprocessor"
+  parseJSON invalid    = Aeson.typeMismatch "Config" invalid
+
+-- Pretends that the value is {} when the key is not present.
+(.:?=) :: FromJSON a => Object -> Text -> Parser a
+o .:?= k = o .:? k >>= maybe (parseJSON (Object HML.empty)) pure
+
 data IndentPolicy = IndentPolicyLeft -- never create a new indentation at more
                                      -- than old indentation + amount
                   | IndentPolicyFree -- can create new indentations whereever