[Haskell-cafe] Re: ANNOUNCE: pqueue-mtl, stateful-mtl
dave at zednenem.com
Sat Feb 28 03:03:29 EST 2009
On Fri, Feb 27, 2009 at 11:10 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> It's obvious that anything that accesses the STT constructor will
> potentially not be typesafe; the question I have is that whether you
> can construct something that isn't typesafe just via the use of runSTT
> & lift.
To my surprise, it turns out that you can.
data Foo m a b = V a | K (Foo m a b -> m b)
test = do
v <- lift $ callCC $ \k -> return (K k)
case v of
K k -> do
r1 <- newSTTRef 'a'
lift $ k (V r1) -- *
V r1 -> do
r2 <- newSTTRef (65 :: Int)
The call to k in the marked line undoes the heap changes since the
call to callCC, and we end up with r1 and r2 both pointing to the same
Dave Menendez <dave at zednenem.com>
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