Generalize type of runRW#

Currently, we have

runRW# :: (State# RealWorld -> (# State# RealWorld, o #)) -> (# State# RealWorld, o #)

where o is open-kinded. This type is a bit annoying, because runRW# is very often used in contexts where we want to discard the state token. I think it should be easy to generalize, however:

runRW# :: (State# RealWorld -> o) -> o

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Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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changed milestone to %8.2.1

changed weight to 5

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assigned to @treeowl

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Differential revisions	- → D3012

Can you give an examle of where this is helpful?

Yes. Consider runST for lazy ST. We currently have

runST :: (forall s. ST s a) -> a
runST st = case st of ST the_st -> let (r,_) = the_st (S# realWorld#) in r
{-# NOINLINE runST #-}

If we want to switch to runRW# using today's runRW#, we need to do something like this:

runST (ST st) =
  case runRW# (\s -> case st (S# s) of
                       (r, _s') -> (# s, r #)) of
    (# _, r #) -> r

That is, we need to pack up the result with a state token to satisfy the type of runRW#. Note that we'd use the token we got from runRW# because opening up the one st returned would force actions that may not be necessary. Then once we have the result from runRW#, we have to match on it again to remove the unneeded state token. With the more general type, we can write this as

runST (ST st) = runRW# (\s -> case st (S# s) of (r, _) -> r)

Much simpler, no?

I see. Yes, ok.

mentioned in commit c344005b

closed

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Resolution	Unresolved → ResolvedFixed

added Pnormal label