How to ensure optimization for large immutable vectors to be shared w.r.t. Referential Transparency

[YueCompl]

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 <ghc-devs at haskell.org> 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
> 
>> 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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> 
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