err

lens-aeson-lossy.hs:30:27: error:
    • No instance for (Data.String.IsString
                         (Getting
                            (base-4.11.1.0:Data.Semigroup.Internal.Endo [a0]) Value a0))
        arising from the literal ‘"mapset"’
        (maybe you haven't applied a function to enough arguments?)
    • In the second argument of ‘(^..)’, namely ‘"mapset"’
      In the first argument of ‘(^@..)’, namely ‘o ^.. "mapset"’
      In the second argument of ‘($)’, namely
        ‘o ^.. "mapset" ^@.. members . _JSON’
   |
30 |      $ M.fromList $ o ^.. "mapset" ^@.. members . _JSON
   |                           ^^^^^^^^

lens-aeson-lossy.hs:30:41: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘members’
      prevents the constraint ‘(AsValue [a0])’ from being solved.
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instance exist:
        instance AsValue String -- Defined in ‘Data.Aeson.Lens’
    • In the first argument of ‘(.)’, namely ‘members’
      In the second argument of ‘(^@..)’, namely ‘members . _JSON’
      In the second argument of ‘($)’, namely
        ‘o ^.. "mapset" ^@.. members . _JSON’
   |
30 |      $ M.fromList $ o ^.. "mapset" ^@.. members . _JSON
   |                                         ^^^^^^^

lens-aeson-lossy.hs:30:51: error:
    • No instance for (ToJSON Val) arising from a use of ‘_JSON’
    • In the second argument of ‘(.)’, namely ‘_JSON’
      In the second argument of ‘(^@..)’, namely ‘members . _JSON’
      In the second argument of ‘($)’, namely
        ‘o ^.. "mapset" ^@.. members . _JSON’
   |
30 |      $ M.fromList $ o ^.. "mapset" ^@.. members . _JSON
   |                                                   ^^^^^

lens-aeson-lossy.hs

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}

import           Data.Aeson
import           Data.Aeson.Lens
import qualified Data.Set as S
import qualified Data.Map as M
import           Control.Lens hiding ((.=))
import           GHC.Generics
import           Test.Hspec
import           Data.Text

data Key = Key1
         deriving (Generic, Show, Eq, Ord)

instance FromJSON Key
instance FromJSONKey Key

data Val = Val1
         deriving (Generic, Show, Eq, Ord)

instance FromJSON Val

data MapSet = MapSet { unMapSet :: M.Map Key (S.Set Val) }
            deriving (Show, Eq)

instance FromJSON MapSet where
  parseJSON o = pure $ MapSet
     $ (undefined :: M.Map Text (S.Set Val) -> M.Map Key (S.Set Val))
     $ M.fromList $ o ^.. "mapset" ^@.. members . _JSON

emptyOuter = "{}"
emptyInner = "{ \"mapset\": {} }"
wrongKey   = "{ \"mapset\": { \"Key2\": [] } }"
emptyList  = "{ \"mapset\": { \"Key1\": [] } }"
wrongVal   = "{ \"mapset\": { \"Key1\": [ \"Val2\" ] } }"
rightVal   = "{ \"mapset\": { \"Key1\": [ \"Val1\" ] } }"


shouldBe' a b = decode a `shouldBe` Just (MapSet b :: MapSet)

main = hspec $ do
  describe "FromJSON MapSet" $ do
    it "maps the empty outer object to the empty Map"
      $ emptyOuter `shouldBe'` M.empty
    it "maps the empty inner object to the empty Map"
      $ emptyInner `shouldBe'` M.empty
    it "discards non-existant Keys"
      $ wrongKey   `shouldBe'` M.empty
    it "maps empty arrays to the empty Set"
      $ emptyList  `shouldBe'` M.fromList [(Key1, S.empty)]
    it "discards non-existant Vals"
      $ wrongVal   `shouldBe'` M.fromList [(Key1, S.empty)]
    it ("maps objects containing the key mapset containing objects "
     ++ "of Keys containing arrays of Vals to Map Key (Set Val)")
      $ rightVal   `shouldBe'` M.fromList [(Key1, S.fromList [Val1])]