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StdGen: constructor accessible via Internal only#123

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curiousleo merged 1 commit intointerface-to-performancefrom
stdgen-newtype
May 8, 2020
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StdGen: constructor accessible via Internal only#123
curiousleo merged 1 commit intointerface-to-performancefrom
stdgen-newtype

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@curiousleo curiousleo commented May 7, 2020 •
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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
lehins reviewed May 7, 2020
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System/Random/Internal.hs Outdated
genWord32 = SM.nextWord32
genWord64 = SM.nextWord64
split = SM.splitSMGen
next = second StdGen . SM.nextInt . unStdGen
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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

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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.

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lehins commented May 7, 2020

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

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curiousleo commented May 8, 2020

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.

@curiousleo curiousleo marked this pull request as ready for review May 8, 2020 11:41
@curiousleo curiousleo requested a review from idontgetoutmuch May 8, 2020 11:41
@curiousleo
StdGen: constructor accessible via Internal only
6ba26b8 
Fixes haskell#59 by making 'StdGen' not
an instance of 'Read'.
@curiousleo curiousleo merged commit 9ee79a7 into interface-to-performance May 8, 2020
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lehins commented May 8, 2020

@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.

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lehins commented May 8, 2020

Ended up adding Eq and NFData in #125

@idontgetoutmuch idontgetoutmuch deleted the stdgen-newtype branch May 8, 2020 14:45
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curiousleo commented May 11, 2020

@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!

@idontgetoutmuch idontgetoutmuch mentioned this pull request May 13, 2020
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curiousleo added a commit that referenced this pull request May 13, 2020
@curiousleo
StdGen: constructor accessible via Internal only (#123)
a1276fe 
Fixes haskell#59 by making 'StdGen' not
an instance of 'Read'.
curiousleo added a commit that referenced this pull request May 13, 2020
@curiousleo
StdGen: constructor accessible via Internal only (#123)
55a31b3 
Fixes haskell#59 by making 'StdGen' not
an instance of 'Read'.
lehins pushed a commit that referenced this pull request May 18, 2020
@curiousleo @lehins
StdGen: constructor accessible via Internal only (#123)
28272cb 
Fixes haskell#59 by making 'StdGen' not
an instance of 'Read'.
curiousleo added a commit that referenced this pull request May 19, 2020
@curiousleo
StdGen: constructor accessible via Internal only (#123)
616e31b 
Fixes haskell#59 by making 'StdGen' not
an instance of 'Read'.
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