Re: [Haskell-cafe] How to ensure optimization for large immutable vectors to be shared w.r.t. Referential Transparency

On a second thought, maybe GHCi's silence is a bad thing here? Maybe it should complain loudly as GHC does? ```hs λ> :set -package vector package flags have changed, resetting and loading new packages... λ> λ> import Prelude λ> λ> import qualified Data.Vector.Storable as VS λ> λ> :{ λ| λ| newtype SomeVector = SomeVector (VS.Vector Int) λ| λ| isSameVector :: SomeVector -> SomeVector -> Bool λ| isSameVector (SomeVector x) (SomeVector y) = λ| x'offset == y'offset && x'fp == y'fp λ| where λ| (x'fp, x'offset, _x'len) = VS.unsafeToForeignPtr x λ| (y'fp, y'offset, _y'len) = VS.unsafeToForeignPtr y λ| λ| :} λ> λ> let (v :: VS.Vector Int) = VS.fromList [3,2,5] in isSameVector (SomeVector v) (SomeVector v) True λ> λ> λ> :set -XMonomorphismRestriction λ> λ> let v = VS.fromList [3,2,5] in isSameVector (SomeVector v) (SomeVector v) True λ> λ> :set -XNoMonomorphismRestriction λ> λ> let v = VS.fromList [3,2,5] in isSameVector (SomeVector v) (SomeVector v) False λ> ``` Further more, my intuition about GHC's type inference here is proved wrong by it, right. But I still think that with a single piece of `let-in` construct, types are better to be inferred as specific as possible, then the result would not be affected by some extension's semantics modification. Here v's type can obviously be inferred to `VS.Vector Int` according to its usage in the `SomeVector` data constructor, I wonder why GHC is not doing this?

On 2021-04-06, at 22:19, YueCompl via ghc-devs wrote:

Thanks very much for the diagnostic and explanation!

I was wrong in assuming the `in isSameVector (SomeVector v) (SomeVector v)` part is enough to have type of v in `let !v = VS.fromList [3,2,5]` inferred as monomorphic, totally unaware about "NoMonomorphismRestriction" before, I've learned it today :D

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On 2021-04-06, at 21:51, Viktor Dukhovni wrote:

On Tue, Apr 06, 2021 at 07:12:51PM +0800, YueCompl via ghc-devs wrote:

...

λ> import Control.Monad.ST λ> import qualified Data.Vector.Storable as VS λ> λ> :{ λ| λ| newtype SomeVector = SomeVector (VS.Vector Int) λ| λ| isSameVector :: SomeVector -> SomeVector -> Bool λ| isSameVector (SomeVector !x) (SomeVector !y) = runST $ do λ| mx@(VS.MVector !x'offset !x'fp) <- VS.unsafeThaw x λ| my@(VS.MVector !y'offset !y'fp) <- VS.unsafeThaw y λ| _ <- VS.unsafeFreeze mx λ| _ <- VS.unsafeFreeze my λ| return $ x'offset == y'offset && x'fp == y'fp λ| λ| :} λ> λ> let !v = VS.fromList [3,2,5] in isSameVector (SomeVector v) (SomeVector v) False λ> λ> let !v = SomeVector (VS.fromList [3,2,5]) in isSameVector v v True

In GHCi, but not in compiled programs, by default the `NoMonomorphismRestriction` extension is enabled. If I compile your code with that restriction, I can reproduce your results (the values are not shared).

If I either skip the extension, or add an explicit type annotation to for the vector, then the values are shared.

{-# LANGUAGE BangPatterns #-} {-# LANGUAGE NoMonomorphismRestriction #-} import Control.Monad.ST import qualified Data.Vector.Storable as VS

newtype SomeVector = SomeVector (VS.Vector Int)

isSameVector :: SomeVector -> SomeVector -> Bool isSameVector (SomeVector !x) (SomeVector !y) = runST $ do mx@(VS.MVector !x'offset !x'fp) <- VS.unsafeThaw x my@(VS.MVector !y'offset !y'fp) <- VS.unsafeThaw y _ <- VS.unsafeFreeze mx _ <- VS.unsafeFreeze my return $ x'offset == y'offset && x'fp == y'fp

main :: IO () main = let !v = VS.fromList [0..1023] -- :: VS.Vector Int in print $ isSameVector (SomeVector v) (SomeVector v)

Since newtypes are always strict in their argument, I don't think the BangPattern does what you'd like it to do, it just makes "main" strict in v. As defined with `NoMonomorphismRestriction` v is a polymorphic function, and I guess it is specialised at the call site.

-- Viktor. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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