On Sat, Feb 12, 2011 at 8:47 AM, Sebastiaan Visser <hask...@fvisser.nl> wrote: > Hi all, > > During a little experiment I discovered there is no MonadFix instance > available for the STM monad. Is this absence the consequence of some deep > restriction of how STM works or 'just accidental'? Is there some way I could > derive this instance?
If you port `fixST` to the STM monad, it seems to work fine at first glance: http://hpaste.org/43915/fixstm But I would want someone else's opinion on it to make sure I'm not doing something like introducing lazy STM values that violate atomicity, or something. The strict `case` on the return value makes me feel pretty good about it. Antoine > > For those who are interested and may know other ways to solve my problem, I'm > trying to tie the knot in a graph by using transactional variables as the > edges between nodes. I'm trying to do this in a rather generic way: > >> {-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, >> StandaloneDeriving, DoRec #-} >> module Graph where >> >> import Control.Applicative >> import Control.Concurrent.STM >> import Control.Monad.Fix >> import Data.Maybe >> import Data.Foldable >> import Data.Traversable >> import Prelude hiding (mapM) >> import qualified Data.Map as M >> >> instance MonadFix STM where >> mfix = error "I need this instance!" -- What to do? >> >> -- A single node is a graph, has a list of both incoming and outgoing edges. >> >> data Node edge = Node >> { nodeId :: NodeId >> , payload :: String >> , incoming :: [edge] >> , ougoing :: [edge] >> } deriving (Functor, Foldable, Traversable) >> >> type NodeId = String >> >> -- A graph with an index on the nodes by NodeId, parametrized with the type >> we >> -- want to used as the edge pointer. >> >> type Graph edge = M.Map NodeId (Node edge) >> >> -- Type level fixed point combinator with a TVar annotation. >> >> newtype TFix f = TIn { tout :: TVar (f (TFix f)) } >> >> -- Take a graph that uses NodeIds as edges and recursively transform it into >> a >> -- graph with TVars to the neighbouring nodes in the edges. >> >> tieTheKnot :: Graph NodeId -> STM (Graph (TFix Node)) >> tieTheKnot untied = >> do rec tied <- (mapM . mapM) (\nodeid -> TIn <$> newTVar (tryLookup nodeid >> tied)) untied >> return tied >> >> -- Helper function to lookup a pre-tied node from a graph, throws an error >> when >> -- the edge could not be resolved. This should, of course, not happen! >> >> tryLookup :: NodeId -> Graph (TFix Node) -> Node (TFix Node) >> tryLookup i = fromJust (error msg) . M.lookup i >> where msg = "tieTheKnot: Lookup error, input is an incomplete graph." > > Thanks in advance, > > -- > Sebastiaan Visser > http://www.silkapp.com > _______________________________________________ > Haskell-Cafe mailing list > haskell-c...@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
