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

[YueCompl]

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

> On 2021-04-06, at 21:51, Viktor Dukhovni <ietf-dane at dukhovni.org> 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.
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