Haskell / GHC: {-# SPECIALIZE #-} Causes 'RULE left-hand side too complicated to desugar' Warning

Question

I have a body of code that uses a monad to abstract whether the actual implementation runs inside ST or IO. Removing the extra layer of abstraction and just substituting concrete types gives a huge speedup (~4.5x) due to the inlining and missing typeclass function call overhead. I was thinking of getting some of that performance by using a specialize pragma, but I'm getting a rather meaningless warning from the compiler. I can't make a simple reproduction case as the simple example seems to work, and I don't know what's causing the difference in my actual program.

Basically, my program does this:

{-# LANGUAGE FlexibleInstances, RankNTypes #-}

module STImpl (runAbstractST, MonadAbstractIOST(..), ReaderST) where

import Control.Monad.Reader
import Control.Monad.ST

class Monad m => MonadAbstractIOST m where
    addstuff :: Int -> m Int

type ReaderST s = ReaderT (Int) (ST s)

instance MonadAbstractIOST (ReaderST s) where
    addstuff a = return . (a +) =<< ask

runAbstractST :: (forall s. ReaderST s a) -> a
runAbstractST f = runST $ runReaderT f 99

and

module Main (main) where

import STImpl

import Control.Monad

{-# SPECIALIZE INLINE useAbstractMonad :: ReaderST s Int #-}
useAbstractMonad :: MonadAbstractIOST m => m Int
useAbstractMonad = foldM (\a b -> a `seq` return . (a +) =<< (addstuff b)) 0 [1..50000000]

main :: IO ()
main = do
    let st = runAbstractST useAbstractMonad
    putStrLn . show $ st

Now, here everything seems to work fine. But in my program I get

RULE left-hand side too complicated to desugar
  let {
    $dFunctor :: Functor (RSTSim s)
    [LclId]
    $dFunctor =
      Control.Monad.Trans.Reader.$fFunctorReaderT
        @ (MonadSim.SimState s)
        @ (GHC.ST.ST s)
        (GHC.ST.$fFunctorST @ s) } in
  simulate
    @ (Control.Monad.Trans.Reader.ReaderT
         (MonadSim.SimState s) (GHC.ST.ST s))
    (MonadSim.$fMonadSimReaderT
       @ s
       $dFunctor
       (Control.Monad.Trans.Reader.$fMonadReaderT
          @ (MonadSim.SimState s)
          @ (GHC.ST.ST s)
          (GHC.ST.$fMonadST @ s))
       (Control.Monad.Trans.Reader.$fApplicativeReaderT
          @ (MonadSim.SimState s)
          @ (GHC.ST.ST s)
          $dFunctor
          (Control.Applicative.$fApplicativeST0
             @ s (GHC.ST.$fFunctorST @ s))))

I don't understand what 'left-hand side', 'too complicated' and 'desugar' mean ;-)

It seems I have the same problem as described here: http://marc.info/?l=haskell-cafe&m=133242702914511

How do I diagnose this? How do I figure out what's causing the optimization to be disabled in my program?

Thanks!

Answer 1

For what it's worth, on the 7.10 RC1 this error no longer occurs, so it looks like the fix to https://ghc.haskell.org/trac/ghc/ticket/8848 may have helped.