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{-# 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
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