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

Tue Mar 5 16:34:21 UTC 2019

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

OK, so just like (==) and (/=) in class Eq.  What's the time-bomb?  Does Eq suffer from it?

Simon

| -----Original Message-----
| From: Eric Seidel <eric at seidel.io>
| Sent: 05 March 2019 11:59
| To: Simon Peyton Jones <simonpj at microsoft.com>
| Cc: Richard Eisenberg <rae at cs.brynmawr.edu>; ghc-steering-
| committee at haskell.org
| Subject: Re: [ghc-steering-committee] Proposal #209: Levity polymorphic
| lift. Recommendation: accept
| 
| It’s not that lift has become levity-polymorphic, rather both lift and
| liftTyped are back in Lift with mutually recursive default implementations.
| 
| Sent from my iPhone
| 
| > 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
| > | >>> _______________________________________________
| > | >>> ghc-steering-committee mailing list
| > | >>> ghc-steering-committee at haskell.org
| > | >>>
| > |
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| > |
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| > |
| 08d6a11adfa7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C63687353412446383
| > |
| 3&sdata=37JiqdFQvu35db6WyYz6Q60jEgNQULRvhByqDUN%2FPjI%3D&reserved=0
| > | >>
| > | >>
| > |
| > | _______________________________________________
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| > |
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| 08d6a11adfa7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C63687353412446383
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| 3&sdata=37JiqdFQvu35db6WyYz6Q60jEgNQULRvhByqDUN%2FPjI%3D&reserved=0