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-d...@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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