Re: Compact Normal Form and ST

Actually, this is more interesting than I thought! Let me explain. Let's take the compact API and make a hypothetical ST style interface. Here's one possibility: data Compact s a compact :: a -> ST s (Compact s a) getCompact :: Compact a -> a runST :: (forall s. ST s a) -> a Let's consider the following code: let x1 = runST (fmap getCompact (compact big_value)) x2 = runST (fmap getCompact (compact big_value)) GHC may CSE x1 and x2 into a single expression, which means that what was previously two compact expressions turns into a single one. "But Edward", you may say, "That's fine, I only cared about the compact region being distinct within a single ST block. If you really wanted x1 and x2 to be distinct, you should have written it this way:" let (x1, x2) = runST $ do x1 <- fmap getCompact (compact big_value) x2 <- fmap getCompact (compact big_value) return (x1, x2) which is indeed true! So I suppose it depends on what your expectations are. Edward Excerpts from Andrew Martin's message of 2017-06-19 09:06:39 -0400:

I too am struggling to find a scenario in which this causes something to go wrong. Ending up with only one block instead of two copies of it seems like it could only be a problem, as Edward says, "if you were planning to use these blocks as separate allocation buffers for subsequent modifications". But, mutable byte arrays allocated by newByteArray# cannot escape runST, so I don't see how this could happen. Well, I guess if you returned a frozen array, and then you thawed it for subsequent modification outside of runST, that would be problematic, but CSE already makes that unsafe, even without involving compact regions. It's good to know that, for my purposes, I'm in the clear, but I would also like to hear from Edward clarifying the original statement.

-Andrew Martin

On Mon, Jun 19, 2017 at 4:41 AM, David Feuer wrote:

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Can you explain how things could go wrong in ST, perhaps with an example? It's hard to see the potential problem.

On Jun 18, 2017 11:49 PM, "Edward Z. Yang" wrote:

There are two senses of referential transparency here which should be considered. First is whether or not you will get the same value results if you use the compact functionality in ST. Here, the answer is yes. Compact normal form has very trivial semantics in this domain, and it would have been OK even to make compact normal forms be pure functions.

Second is whether or not the performance characteristics are preserved. Here, the situation is different. Most notably, pure expressions and invocations of the same runST block may be commoned up (via an optimization pass like CSE.) In that case, what was previously two separate compact blocks may be commoned up into a single one. This could be disaster if you were planning to use these blocks as separate allocation buffers for subsequent modifications.

This motivated specializing compact to IO. It won't segfault if you put it in ST, but the performance characteristics might change.

Edward

Excerpts from Andrew Martin's message of 2017-06-18 10:24:09 -0400:

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In the primops file where the compact normal form functions are documented (https://github.com/ghc/ghc/blob/adcd1c62b6d372f100ccf1d5d7c d94f79aaac194/compiler/prelude/primops.txt.pp#L2487), I noticed that all of the functions have type signatures that constrain them to only being used in IO. For example:

compactAdd# :: Compact# -> a -> State# RealWorld -> (# State# RealWorld, a #)

I would like to know if generalizing these to allow them to work in ST would be sound. That is, changing the type signature to:

compactAdd# :: Compact# -> a -> State# s -> (# State# s, a #)

I'm not requesting that this change actually be made. I only want to know if using unsafeCoerce to create the second function for my own project would actually be sound.

For those interested in knowing why I want this, it's because there are situation where I'm interested in building up a giant structure in a compact region, but in a way that doesn't actually require IO. I think it's a pity to have to use a type signature with IO and then call unsafePerformIO at the end instead of using the more constrained ST, and runST, which makes it clear that I'm not doing anything observable form the outside.

As a minor bonus, the ST version of the Compact data type shouldn't need the lock that the IO version does, since concurrent calls to compactAdd are not possible.

-Andrew Martin

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