Hi all, The STM monad currently has no MonadFix instance, but wonderful things are possible when it has one. I propose adding the 'MonadFix STM' instance provided by Antoine Latter on the Haskell-Cafe[1] list to the STM package:
{-# LANGUAGE MagicHash, UnboxedTuples, DoRec #-}
import GHC.Exts import GHC.Conc import Control.Monad.Fix
data STMret a = STMret (State# RealWorld) a
liftSTM :: STM a -> State# RealWorld -> STMret a liftSTM (STM m) = \s -> case m s of (# s', r #) -> STMret s' r
instance MonadFix STM where mfix k = STM $ \s -> let ans = liftSTM (k r) s STMret _ r = ans in case ans of STMret s' x -> (# s', x #)
Discussion Period: 1 week -Sebastiaan Visser [1] http://www.haskell.org/pipermail/haskell-cafe/2011-February/089226.html