#5073: Add blockST for nested ST scopes
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Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.0.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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.
Now the implementations look (something) like this:
{{{
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?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5073>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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