Adds a pretty decoder that removes Aeson portion of decoding error

This splits the error message produced by `fromValue` into two:
the Aeson and Fleece error message combined (the original behavior)
and just the Fleece error message (that produced by the schema and
the `Decoder` functions). `decodePretty`, as a result, produces a
slightly easier to understand error message that is more suitable
for displaying to users. Ideally, we would be able to remove the
type from the error messages produced by the `Decoder` functions as
well, but for now, this isn't changing that since it would alter
the current tests' expected results.

json-fleece-aeson/json-fleece-aeson.cabal

@@ -1,6 +1,6 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.35.2.
+-- This file has been generated from package.yaml by hpack version 0.36.0.
 --
 -- see: https://github.com/sol/hpack
 
@@ -41,6 +41,7 @@ library
     , base >=4.7 && <5
     , bytestring ==0.11.*
     , containers ==0.6.*
+    , either ==5.0.2
     , json-fleece-core ==0.5.*
     , shrubbery >=0.1.2 && <0.2
     , text >=1.2 && <2.1

json-fleece-aeson/package.yaml

@@ -57,6 +57,7 @@ library:
     - Fleece.Aeson.Decoder
     - Fleece.Aeson.Encoder
   dependencies:
+    - either == 5.0.2
     - shrubbery >= 0.1.2 && < 0.2
 
 tests:

json-fleece-aeson/src/Fleece/Aeson/Decoder.hs

@@ -3,6 +3,7 @@
 module Fleece.Aeson.Decoder
   ( Decoder (..)
   , decode
+  , decodePretty
   , decodeStrict
   , fromValue
   , toParser
@@ -16,6 +17,7 @@ import qualified Data.Aeson.KeyMap as AesonKeyMap
 import qualified Data.Aeson.Types as AesonTypes
 import qualified Data.ByteString as BS
 import qualified Data.ByteString.Lazy as LBS
+import Data.Either.Combinators (mapLeft)
 import qualified Data.Map.Strict as Map
 import qualified Data.Text.Encoding as Enc
 import qualified Shrubbery
@@ -25,21 +27,26 @@ import qualified Fleece.Core as FC
 data Decoder a
   = Decoder FC.Name (Aeson.Value -> AesonTypes.Parser a)
 
-fromValue :: Decoder a -> Aeson.Value -> Either String a
-fromValue =
-  AesonTypes.parseEither . toParser
+fromValue :: Decoder a -> Aeson.Value -> Either (String, String) a
+fromValue decoder value =
+  mapLeft (\(path, err) -> (AesonTypes.formatError path err, err)) $
+    AesonTypes.iparseEither (toParser decoder) value
 
 toParser :: Decoder a -> Aeson.Value -> AesonTypes.Parser a
 toParser (Decoder _name f) =
   f
 
 decode :: Decoder a -> LBS.ByteString -> Either String a
 decode decoder =
-  fromValue decoder <=< Aeson.eitherDecode
+  mapLeft fst . fromValue decoder <=< Aeson.eitherDecode
+
+decodePretty :: Decoder a -> LBS.ByteString -> Either String a
+decodePretty decoder =
+  mapLeft snd . fromValue decoder <=< Aeson.eitherDecode
 
 decodeStrict :: Decoder a -> BS.ByteString -> Either String a
 decodeStrict decoder =
-  fromValue decoder <=< Aeson.eitherDecodeStrict
+  mapLeft fst . fromValue decoder <=< Aeson.eitherDecodeStrict
 
 instance FC.Fleece Decoder where
   data Object Decoder _object a = Object