On Sunday 15 February 2009 9:44:42 pm Louis Wasserman wrote:
Hello all,
I just uploaded stateful-mtl and pqueue-mtl 1.0.1. The ST monad transformer and array transformer have been removed -- I've convinced myself that a heap transformer backed by an ST array cannot be referentially transparent -- and the heap monad is now available only as a basic monad and not a transformer, though it still provides priority queue functionality to any of the mtl wrappers around it. stateful-mtl retains a MonadST typeclass which is implemented by ST and monad transformers around it, allowing computations in the the ST-bound heap monad to perform ST operations in its thread.
Since this discussion had largely led to the conclusion that ST can only be used as a bottom-level monad, it would be pretty uncool if ST computations couldn't be performed in a monad using ST internally because the ST thread was hidden and there was no way to place ST computations 'under' the outer monad. Anyway, it's essentially just like the MonadIO typeclass, except with a functional dependency on the state type.
There was a question I asked that never got answered, and I'm still curious: would an ST *arrow* transformer be valid? Arrows impose sequencing on their operations that monads don't... I'm going to test out some ideas, I think.
Your proposed type: State (Kleisli []) x y = (s, x) -> [(s, y)] is (roughly) isomorphic to: x -> StateT s [] y = x -> s -> [(s, y)] The problem with an ST transformer is that the state parameter needs to be used linearly, because that's the only condition under which the optimization of mutable update is safe. ST ensures this by construction, as opposed to other languages (Clean) that have type systems that can express this kind of constraint directly. However, with STT, whether the state parameter is used linearly is a function of the wrapped monad. You'd have to give a more fleshed out version of your proposed state arrow transformer, but off the top of my head, I'm not sure it'd be any better. -- Dan