StdGen: constructor accessible via Internal only

[curiousleo]

Makes StdGen not an instance of Read.

Relates to haskell#59 (comment):

Removing the broken Read instance for StdGen is a reasonable solution and deleting the associated documentation would be a reasonable fix.

• Benchmark to make sure this doesn't slow things down.

[lehins]

This effectively removes RandomGen SM.SMGen, which is not what we want.

We either need to add another instance or simply use GeneralizeNewtypeDeriving to derive RandomGen for StdGen

[curiousleo]

Thanks @lehins. I shouldn't have pushed this between meetings - silly mistakes.

I've updated this so we keep RandomGen SM.SMGen and use GeneralisedNewtypeDeriving for RandomGen StdGen.

[lehins]

This is not a problem for us anymore: haskell#59 (comment) , because SMGen defines a sensible Read instance.

λ> read "abcdef" :: StdGen
*** Exception: Prelude.read: no parse
λ> read "SMGen 1 3" :: StdGen
SMGen 1 3

That being said, I personally think Read is wart in Haskell, so getting rid of that instance for StdGen is fine with me. newtype wrapper is not a bad idea either, now that we have an internal module that can expose the constructor for those who need it

[curiousleo]

That being said, I personally think Read is wart in Haskell, so getting rid of that instance for StdGen is fine with me.

👍

newtype wrapper is not a bad idea either, now that we have an internal module that can expose the constructor for those who need it

That was one of my initial motivations for introducing Internal.

[lehins]

@curiousleo I was a bit too slow to comment. We also could benefit from Eq instance. See haskell#45 for such request. As well as NFData, which can be derived.

[lehins]

Ended up adding Eq and NFData in #125

[curiousleo]

@curiousleo I was a bit too slow to comment. We also could benefit from Eq instance. See haskell#45 for such request. As well as NFData, which can be derived.

Sorry. I think I was a bit too quick to merge!