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

[Simon Peyton Jones]

| > 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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| >>> ghc-steering-committee at haskell.org
| >>>
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| >>
| >>
| 
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