1 Jan
2007
1 Jan
'07
3:06 p.m.
On 1/1/07, Yitzchak Gale
stToState :: MonadState st m => (STRef s st -> ST s a) -> m a
That would make it very convenient, for example, to use arrays inside a pure state monad.
The type signatures above do ensure (as far as I can see) that the opacity of the ST state thread is not violated. But unfortunately, the protective shield in runST created by the higher-rank polymorphism is too thick.
Probably, stToState :: MonadState st m => (forall s. STRef s st -> ST s a) -> m a E.g. stToState :: MonadState st m => (forall s. STRef s st -> ST s a) -> m a stToState f = do s <- get let (x, s') = runST (do r <- newSTRef s x <- f r s' <- readSTRef r return (x, s')) put s' return x -- Best regerds, Tolik