calamity-0.1.9.2: src/Calamity/Internal/LocalWriter.hs

{-# LANGUAGE TemplateHaskell #-}

-- | A Writer monad that supports local writing, reverse reader I guess?
module Calamity.Internal.LocalWriter
    ( LocalWriter(..)
    , ltell
    , llisten
    , runLocalWriter ) where

import qualified Polysemy       as P
import qualified Polysemy.State as P

data LocalWriter o m a where
  Ltell :: o -> LocalWriter o m ()
  Llisten :: m a -> LocalWriter o m (o, a)

P.makeSem ''LocalWriter

runLocalWriter :: Monoid o => P.Sem (LocalWriter o ': r) a -> P.Sem r (o, a)
runLocalWriter = P.runState mempty . P.reinterpretH
  (\case
     Ltell o   -> do
       P.modify' (<> o) >>= P.pureT
     Llisten m -> do
       mm <- P.runT m
       (o, fa) <- P.raise $ runLocalWriter mm
       pure $ fmap (o, ) fa)