ST with an early exit

Posted on 2024-03-17 by Oleg Grenrus


I wish there were an early exit functionality in the ST monad. This need comes time to time when writing imperative algorithms in Haskell.

It's very likely there is a functional version of an algorithm, but it might be that ST-version is just simply faster, e.g. by avoiding allocations (as allocating even short lived garbage is not free).

But there are no early exit in the ST monad.

Recent GHC added delimited continuations. The TL;DR is that delimited continuations is somewhat like goto:

  • newPromptTag# creates a label (tag)
  • prompt# brackets the computation
  • control# kind of jumps (goes to) the end of enclosing prompt bracket, and continues from there.

So let's use this functionality to implement a version of ST which has an early exit. It turns out to be quite simple.

The ST monad is define like:

newtype ST s a = ST (State# s -> (# State# s, a #)

and we change it by adding an additional prompt tag argument:

newtype EST e s a = EST
    { unEST :: forall r. PromptTag# (Either e r)
            -> State# s -> (# State# s, a #) 

(Why forall r.? We'll see soon).

It's easy to lift normal ST computations into EST ones:

liftST :: ST s a -> EST e s a
liftST (ST f) = EST (\_ -> f)

so EST is a generalisation of ST, good.

Now we need a way to run EST computations, and also a way to early exit in them.

The early exit is the simpler one. Given that tag prompt brackets the whole computation, we simply jump to the end with Left e. We ignore the captured continuation, we have no use for it.

earlyExitEST :: e -> EST e s any
earlyExitEST e = EST (\tag -> control0## tag (\_k s -> (# s, Left e #)))

Now, the job for runEST is to create the tag and prompt the computation:

runEST :: forall e a. (forall s. EST e s a) -> Either e a
runEST (EST f) = runRW#
    -- create tag
    (\s0 -> case newPromptTag# s0 of {
    -- prompt
    (# s1, tag #) -> case prompt# tag
         -- run the `f` inside prompt,
         -- and once we get to the end return `Right` value
         (\s2 -> case f tag s2 of (# s3, a #) -> (# s3, Right a #)) s1 of {
    (# _, a #) -> a }})

runRW# and forgetting the state at the end is the same as in runST, for comparison:

runST :: (forall s. ST s a) -> a
runST (ST st_rep) = case runRW# st_rep of (# _, a #) -> a
-- See Note [runRW magic] in GHC.CoreToStg.Prep

With all the pieces in place, we can run few simple examples:

-- | >>> ex1
-- Left 'x'
ex1 :: Either Char Bool
ex1 = runEST $ earlyExitEST 'x'

-- | >>> ex2
-- Right True
ex2 :: Either Char Bool
ex2 = runEST (return True)

#Comments & wrinkles

Early exit is one of the simplest "effect" you can implement with delimited continuations. This is the throwing part of the exceptions, with only top-level exception handler. It's a nice exercise (and a brain twister) to implement catch blocks.

One wrinkle in this implementation is the control0## (not control0#) function I used. The delimited continuations primops are made to work only with RealWorld, not arbitrary State# tokens.

I think this is unnecessary specialization GHC issue #24165, I was advice to simply use unsafeIOToST, so I did:

    :: PromptTag# a
    -> (((State# s -> (# State# s, b #)) -> State# s -> (# State# s, a #))
                                         -> State# s -> (# State# s, a #))
    -> State# s -> (# State# s, b #)
control0## = unsafeCoerce# control0#

This still feels silly, especially realizing that the (only) example in the delimited continuations proposal goes like

type role CC nominal representational
newtype CC ans a = CC (State# RealWorld -> (# State# RealWorld, a #))
  deriving (Functor, Applicative, Monad) via IO

runCC :: (forall ans. CC ans a) -> a
runCC (CC m) = case runRW# m of (# _, a #) -> a

but if you look at that, it's just a ST monad done weirdly:

newtype ST s a = ST (State# RealWorld -> (# State# RealWorld, a #))
-- not using `s` argument !?

There might be a good reason why CC should be done like that (other than than primops are RealWorld specific), but the proposal doesn't explain that difference. To me having phantom ans instead of using nominally it as in ST is suspicious.


Delimited continutations are fun and could be very useful.

But surprisingly, at the moment of writing I cannot find any package on Hackage using them for anything! Search for newPromptTag returns only false positives (ghc-lib etc) right now. I wonder why they are unused?

Please try them out!

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