[ghc-steering-committee] Proposal #209: Levity polymorphic lift. Recommendation: accept

[Eric Seidel]

It’s not that lift has become levity-polymorphic, rather both lift and liftTyped are back in Lift with mutually recursive default implementations. 

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> On Mar 5, 2019, at 03:13, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> 
> | > data Foo
> | >   deriving Data
> | > instance Lift Foo
> | 
> | But after this proposal, it will still be accepted **but will loop**. That
> | is, the instance silently becomes a bomb waiting to maim poor clients with
> | an obscure compile-time hang.
> 
> Can you remind us why making Lift levit-polymorphic causes this change in looping behaviour?
> 
> Simon
> 
> | -----Original Message-----
> | From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org>
> | On Behalf Of Richard Eisenberg
> | Sent: 05 March 2019 03:30
> | To: Eric Seidel <eric at seidel.io>
> | Cc: ghc-steering-committee at haskell.org
> | Subject: Re: [ghc-steering-committee] Proposal #209: Levity polymorphic
> | lift. Recommendation: accept
> | 
> | Thanks, Eric.
> | 
> | So, if we restore lift (and have it be defined in terms of liftTyped) to
> | the Lift class, then we potentially have a problem. This is allowed today
> | (and it works well):
> | 
> | > data Foo
> | >   deriving Data
> | > instance Lift Foo
> | 
> | But after this proposal, it will still be accepted **but will loop**. That
> | is, the instance silently becomes a bomb waiting to maim poor clients with
> | an obscure compile-time hang. Because of the MINIMAL pragma, there will be
> | a warning. So perhaps we decide that the warning is enough of a deterrent
> | and to allow this strange back-compat story.
> | 
> | On the other hand, if we remove lift from the class, then the above code
> | fails with a "liftTyped is not defined" error. That's quite easy to
> | pinpoint.
> | 
> | I'm not wholly against this proposal at all -- indeed, it's a nice
> | application of levity polymorphism -- but I think there is a real drawback
> | here worth debating.
> | 
> | Richard
> | 
> | > On Mar 4, 2019, at 10:19 PM, Eric Seidel <eric at seidel.io> wrote:
> | >
> | > I believe you're thinking of
> | https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.co
> | m%2Fghc-proposals%2Fghc-
> | proposals%2Fpull%2F175&data=02%7C01%7Csimonpj%40microsoft.com%7C179fc89
> | 805fa4e1a3f6008d6a11adfa7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C6368
> | 73534124463833&sdata=WUd8z511ysFVYxvhdpYCtw0useZOtOq37VANULn%2BHd8%3D&a
> | mp;reserved=0. The PR has been marked accepted, but it seems it didn't get
> | merged.
> | >
> | > On Mon, Mar 4, 2019, at 22:16, Richard Eisenberg wrote:
> | >> I recall a discussion in another proposal about the Lift class and
> | >> removing the lift function. This was for a good reason (I think it
> | >> stopped silent, terrible breakage). Does anyone remember where that
> | >> conversation took place? A quick search didn't find an accepted
> | >> proposal about the Lift class.
> | >>
> | >> Thanks,
> | >> Richard
> | >>
> | >>> On Mar 2, 2019, at 4:41 PM, Eric Seidel <eric at seidel.io> wrote:
> | >>>
> | >>> Hi everyone,
> | >>>
> | >>> This proposal[1] makes the `lift` and `liftTyped` methods of the `Lift`
> | class levity-polymorphic, which allows us to write proper `Lift` instances
> | for unlifted types. It would also allow GHC to remove the special logic
> | that currently supports lifting records with unlifted fields.
> | >>>
> | >>> The main downside is the potential for breakage since `lift @Foo` would
> | now fix the RuntimeRep parameter rather than the `a`. This is unfortunate,
> | but I doubt it will show up much. It also unfortunately requires making
> | both `lift` and `liftTyped` methods, when we had just decided to extract
> | `lift` from the class.
> | >>>
> | >>> I recommend accepting the proposal.
> | >>>
> | >>> Thanks!
> | >>> Eric
> | >>>
> | >>> [1]:
> | https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.co
> | m%2Fharpocrates%2Fghc-proposals%2Fblob%2Flevity-polymorphic-
> | lift%2Fproposals%2F0000-levity-polymorphic-
> | lift.rst&data=02%7C01%7Csimonpj%40microsoft.com%7C179fc89805fa4e1a3f600
> | 8d6a11adfa7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636873534124463833
> | &sdata=K9%2BIbsRdxplDnvRirFgrgzspyPjf3F1iZrRK5vE7q7c%3D&reserved=0
> | >>> _______________________________________________
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> | >>
> | >>
> | 
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