id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
5073	Add blockST for nested ST scopes	simonpj		"GHC lacks the `blockST` and `importVar` primitives introduced by Launchbury/Sabry:
 * http://www.cs.indiana.edu/~sabry/papers/monadic-state-ax.pdf (section 10).  
 * See also Fluet & Morrisset http://www.cs.rit.edu/~mtf/research/rgn-monad/JFP06/jfp06.pdf (page 7ff)

The two new combinators have these types:
{{{
blockST :: (forall r. ST (s,r) a) -> ST s a
importVar :: STRef s a -> STRef (s,r) a
}}}
They are useful, and have no runtime cost; I don't think it'd be hard to provide them.  We need these primitives, I think:
{{{
promoteState# :: State# s -> State# (s,r) a
demoteState#  :: State# (s,r) -> State# s
promoteVar# :: MutVar# s a -> MutVar# (s,r) a
}}}
All three are implemented as no-ops.  (Compare with `newMutVar#` etc in http://darcs.haskell.org/ghc/compiler/prelude/primops.txt.pp.)

Now the implementations look (something) like this. (For background see the current impementations of `ST` and `STRef` in http://darcs.haskell.org/packages/base/GHC (`ST.lhs` and `STRef.lhs` resp.)
{{{
blockST (ST thing) = ST (\s -> case thing (promoteState# s) of
                                 (s', r) -> (demoteState# s', r) )
promoteVar (STRef r) = STRef (promoteVar# r)
}}}
Things to think about:
 * Can we use coercions instead of primops?   (I think yes for `promote/demoteState` but it's unsound to demote variables.)
 * I'm hazy about what type safety guarantees we still have in Core, where the representation of the state monad is exposed.
 * Nomenclature. I'm not attached to these particular names.
 * Is there other relevant background material?"	bug	new	low	7.6.2	Compiler	7.0.3			wadler@… Matthew.Fluet@…	Unknown/Multiple	Unknown/Multiple	None/Unknown