{-# LANGUAGE DeriveAnyClass    #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE NamedFieldPuns    #-}
{-# LANGUAGE OverloadedStrings #-}

-- | a monad that collects warnings, outputs, etc,
module LintWriter where

import           Control.Monad.Trans.Maybe ()
import           Control.Monad.Writer      (MonadTrans (lift),
                                            MonadWriter (tell), WriterT)
import           Data.Aeson                (ToJSON (toJSON))
import           Data.Text                 (Text)
import           GHC.Generics              (Generic)


-- | Levels of errors and warnings, collectively called
-- "Hints" until I can think of some better name
data Level = Warning | Suggestion | Info | Forbidden | Error
  deriving (Show, Generic, ToJSON)

-- | a hint comes with an explanation (and a level)
data Hint = Hint
  { hintLevel :: Level
  , hintMsg   :: Text }
  deriving (Show, Generic, ToJSON)

-- shorter constructor
hint :: Level -> Text -> Hint
hint level msg = Hint { hintLevel = level, hintMsg = msg }

-- | a monad to collect hints. If it yields Left, then the
-- map is flawed in some fundamental way which prevented us
-- from getting any hints at all except whatever broke it
type LintWriter a = WriterT [Hint] (Either Hint) a

-- this is wrapped in a newtype because Aeson is silly and wants
-- to serialise Either as { "Right" : … } or { "Left" : … } ...
type LintResult' a = Either Hint (a, [Hint])
newtype LintResult a = LintResult (LintResult' a)

-- better, less confusing serialisation of an Either Hint (a, [Hint]).
-- Note that Left hint is also serialised as a list to make the resulting
-- json schema more regular.
instance ToJSON a => ToJSON (LintResult a) where
  toJSON (LintResult r) = toJson' r
    where toJson' (Left hint)        = toJSON [hint]
          toJson' (Right (_, hints)) = toJSON hints



-- | write a hint into the LintWriter monad
lint :: Level -> Text -> LintWriter ()
lint level = tell . (: []) . hint level

warn = lint Warning
info = lint Info
forbid = lint Forbidden
suggest = lint Suggestion
complain = lint Error


-- TODO: all these functions should probably also just operate on LintWriter

-- | converts a Maybe to an Either, with a default value for Left
unwrap :: b -> Maybe a  -> Either b a
unwrap hint maybe = case maybe of
  Just a  -> Right a
  Nothing -> Left hint

-- | unwrap and produce a warning if the value was Nothing
unwrapWarn :: Text -> Maybe a -> Either Hint a
unwrapWarn msg = unwrap $ hint Warning msg

-- | same as unwrapWarn, but for booleans
assertWarn :: Text -> Bool -> LintWriter ()
assertWarn msg cond = lift $ if cond then Right () else Left $ hint Warning msg