Okay, I tested it out and the arrow transformer has the same problem. I
realized this after I sent the last message -- the point is that at any
particular point, intuitively there should be exactly one copy of a State# s
for each state thread, and it should never get duplicated; allowing other
monads or arrows to hold a State# s in any form allows them to hold more
than one, violating that goal.
I'm not entirely convinced yet that there *isn't* some really gorgeous type
system magic to fix this issue, like the type-system magic that motivates
the type of runST in the first place, but that's not an argument that such
magic exists...it's certainly an interesting topic to mull.
Louis Wasserman
wasserman.louis@gmail.com
On Sun, Feb 15, 2009 at 9:20 PM, Dan Doel
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
On Sunday 15 February 2009 9:44:42 pm Louis Wasserman wrote: 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