On Feb 12, 2011, at 6:08 PM, Antoine Latter wrote:
> 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
Interesting! This approach seems to work for my examples as well. Unfortunately
I do not have enough insides into STM to be able to tell if the code is correct.
Does someone else have an opinion on this? When not I might propose adding the
instance to the STM package.
Sebastiaan
>> 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-Cafe@haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
