{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE NamedFieldPuns #-}

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

import Data.Text (Text)
import qualified Data.Text as T
import qualified Data.Text.IO as T
import Data.Maybe (isJust, mapMaybe)
import Control.Monad.Writer
import Control.Monad.Trans.Maybe


-- | 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

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

-- shorter constructor
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

type LintResult a = Either Hint (a, [Hint])

-- | 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