LFresh.hs

{-# LANGUAGE BlockArguments             #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TemplateHaskell            #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Disco.Effects.LFresh
-- Copyright   :  disco team and contributors
-- Maintainer  :  byorgey@gmail.com
--
-- SPDX-License-Identifier: BSD-3-Clause
--
-- Polysemy effect for local fresh name generation, compatible with
-- the unbound-generics library.
--
-----------------------------------------------------------------------------

module Disco.Effects.LFresh where

import           Data.Set                              (Set)
import qualified Data.Set                              as S
import           Data.Typeable                         (Typeable)
import           Polysemy
import           Polysemy.ConstraintAbsorber
import           Polysemy.Reader
import qualified Unbound.Generics.LocallyNameless      as U
import           Unbound.Generics.LocallyNameless.Name

-- | Local fresh name generation effect.
data LFresh m a where
  Lfresh    :: Typeable a => Name a -> LFresh m (Name a)
  Avoid     :: [AnyName] -> m a -> LFresh m a
  GetAvoids :: LFresh m (Set AnyName)

makeSem ''LFresh

-- | Dispatch an 'LFresh' effect via a 'Reader' effect to keep track
--   of a set of in-scope names.
runLFresh :: Sem (LFresh ': r) a -> Sem r a
runLFresh = runReader S.empty . runLFresh'

runLFresh' :: Sem (LFresh ': r) a -> Sem (Reader (Set AnyName) ': r) a
runLFresh'
  = reinterpretH @_ @(Reader (Set AnyName)) \case
      Lfresh nm -> do
        let s = name2String nm
        used <- ask
        pureT $ head (filter (\x -> not (S.member (AnyName x) used))
                       (map (makeName s) [0..]))
      Avoid names m -> do
        m' <- runT m
        raise (subsume (runLFresh' (local (S.union (S.fromList names)) m')))
      GetAvoids  -> ask >>= pureT

  -- Much of the above code copied from
  -- https://hackage.haskell.org/package/unbound-generics-0.4.1/docs/src/Unbound.Generics.LocallyNameless.LFresh.html
  -- (see instance Monad m => LFresh (LFreshMT m))

  -- It turns out to make things much simpler to reimplement the
  -- LFresh effect ourselves in terms of a reader effect, since then
  -- we can immediately dispatch it as above.  The alternative would
  -- be to implement it in terms of (Final U.LFreshM) (see the
  -- commented code at the bottom of this file), but then we are stuck
  -- with that constraint.  Given the constraint-absorbing machinery
  -- below, just impementing the 'LFresh' effect itself means we can
  -- then reuse other things from unbound-generics that depend on a
  -- Fresh constraint, such as the 'lunbind' function below.

  -- NOTE: originally, there was a single function runLFresh which
  -- called reinterpretH and then immediately dispatched the Reader
  -- (Set AnyName) effect.  However, since runLFresh is recursive,
  -- this means that the recursive calls were running with a
  -- completely *separate* Reader effect that started over from the
  -- empty set! This meant that LFresh basically never changed any
  -- names, leading to all sorts of name clashes and crashes.
  --
  -- Instead, we need to organize things as above: runLFresh' is
  -- recursive, and keeps the Reader effect (using 'subsume' to squash
  -- the duplicated Reader effects together).  Then a top-level
  -- runLFresh function finally runs the Reader effect.

--------------------------------------------------
-- Other functions

-- | Open a binder, automatically freshening the names of the bound
--   variables, and providing the opened pattern and term to the
--   provided continuation.  The bound variables are also added to the
--   set of in-scope variables within in the continuation.
lunbind
  :: (Member LFresh r, U.Alpha p, U.Alpha t)
  => U.Bind p t -> ((p,t) -> Sem r c) -> Sem r c
lunbind b k = absorbLFresh (U.lunbind b k)

------------------------------------------------------------
-- Machinery for absorbing MTL-style constraint.
-- See https://hackage.haskell.org/package/polysemy-zoo-0.7.0.1/docs/Polysemy-ConstraintAbsorber.html
-- Used https://hackage.haskell.org/package/polysemy-zoo-0.7.0.1/docs/src/Polysemy.ConstraintAbsorber.MonadState.html#absorbState as a template.

absorbLFresh :: Member LFresh r => (U.LFresh (Sem r) => Sem r a) -> Sem r a
absorbLFresh = absorbWithSem @U.LFresh @Action (LFreshDict lfresh avoid getAvoids) (Sub Dict)
{-# INLINEABLE absorbLFresh #-}

data LFreshDict m = LFreshDict
  { lfresh_    :: forall a. Typeable a => Name a -> m (Name a)
  , avoid_     :: forall a. [AnyName] -> m a -> m a
  , getAvoids_ :: m (Set AnyName)
  }

-- | Wrapper for a monadic action with phantom type parameter for reflection.
--   Locally defined so that the instance we are going to build with reflection
--   must be coherent, that is there cannot be orphans.
newtype Action m s' a = Action (m a)
  deriving (Functor, Applicative, Monad)

instance ( Monad m
         , Reifies s' (LFreshDict m)
         ) => U.LFresh (Action m s') where
  lfresh x = Action $ lfresh_ (reflect $ Proxy @s') x
  {-# INLINEABLE lfresh #-}
  avoid xs (Action m) = Action $ avoid_ (reflect $ Proxy @s') xs m
  {-# INLINEABLE avoid #-}
  getAvoids = Action $ getAvoids_ (reflect $ Proxy @s')
  {-# INLINEABLE getAvoids #-}

----------------------------------------------------------------------
-- Old code I don't want to delete because I spent so much time
-- banging my head against it.  It wasn't wasted, though, since I used
-- some of my hard-earned knowledge to write runLFresh' above.

-- -- | Dispatch the local fresh name generation effect in an effect stack
-- --   containing the 'LFreshM' monad from @unbound-generics@.
-- runLFreshR :: Member (Final U.LFreshM) r => Sem (LFresh ': r) a -> Sem r a
-- runLFreshR = interpretFinal @U.LFreshM $ \case
--   Avoid xs m  -> do
--     m' <- runS m
--     pure (U.avoid xs m')
--   Lunbind b k -> do
--     s <- getInitialStateS
--     k' <- bindS k
--     pure (U.lunbind b (k' . (<$ s)))

-- -- The above code took me a long time to figure out how to write.
-- -- lunbind is a higher-order effect, so we have to use more
-- -- complicated machinery.  See my Stack Overflow question,
-- -- https://stackoverflow.com/questions/68384508/how-to-incorporate-mtl-style-cps-style-higher-order-effect-into-polysemy/68397358#68397358

-- -- | Run a computation requiring only fresh name generation.
-- runLFresh :: Sem '[LFresh, Final U.LFreshM] a -> a
-- runLFresh = U.runLFreshM . runFinal . runLFreshR