[Haskell-cafe] ANNOUNCE: pqueue-mtl, stateful-mtl

Dan Doel dan.doel at gmail.com
Sun Feb 15 22:20:08 EST 2009

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

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