Proposal: Add MonadFix instance to STM

[Sebastiaan Visser]

On Feb 17, 2011, at 3:24 PM, Edward Kmett wrote:
> +1, but to quibble, even for trivial things like this we usually try to allow at least 2 weeks for discussion.

No problem, 2 weeks is fine with me.

> Why not just import STRet from GHC.ST and exploit that rather than redefine it? Just curious -- I'm not biased one way or the other.

I hope STM maintainers have an opinion on this one, I couldn't really tell.

> -Edward
> 
> On Thu, Feb 17, 2011 at 8:57 AM, Sebastiaan Visser <haskell at fvisser.nl> wrote:
> 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

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