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

Tue Mar 5 16:45:55 UTC 2019

| It's not a problem with the mutually recursive definitions per se, rather
| with the migration path.

Ah.  Now I understand, thanks.

S

| -----Original Message-----
| From: Eric Seidel <eric at seidel.io>
| Sent: 05 March 2019 16:42
| 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
| 
| The time-bomb is that there may exist empty Lift instances that are
| *currently safe* because there's a single lift method with a default
| implementation. Once you add the liftTyped method, and make the default
| implementations mutually recursive, these empty instances suddenly become
| unsafe.
| 
| Eq would have the same problem if we had first defined it solely in terms
| of (==), with a default implementation, and then later decided to add (/=)
| and make the defaults mutually recursive.
| 
| It's not a problem with the mutually recursive definitions per se, rather
| with the migration path.
| 
| On Tue, Mar 5, 2019, at 11:34, Simon Peyton Jones wrote:
| > | 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
| > | > | >>>
| > | > |
| > |
| https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmail.hask
| > | > | ell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-steering-
| > | > |
| > |
| committee&data=02%7C01%7Csimonpj%40microsoft.com%7C179fc89805fa4e1a3f60
| > | > |
| > |
| 08d6a11adfa7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C63687353412446383
| > | > |
| > |
| 3&sdata=37JiqdFQvu35db6WyYz6Q60jEgNQULRvhByqDUN%2FPjI%3D&reserved=0
| > | > | >>
| > | > | >>
| > | > |
| > | > | _______________________________________________
| > | > | ghc-steering-committee mailing list
| > | > | ghc-steering-committee at haskell.org
| > | > |
| > |
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| > | > | ell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-steering-
| > | > |
| > |
| committee&data=02%7C01%7Csimonpj%40microsoft.com%7C179fc89805fa4e1a3f60
| > | > |
| > |
| 08d6a11adfa7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C63687353412446383
| > | > |
| > |
| 3&sdata=37JiqdFQvu35db6WyYz6Q60jEgNQULRvhByqDUN%2FPjI%3D&reserved=0
| >
| >