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

[Eric Seidel]

I believe you're thinking of https://github.com/ghc-proposals/ghc-proposals/pull/175. 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://github.com/harpocrates/ghc-proposals/blob/levity-polymorphic-lift/proposals/0000-levity-polymorphic-lift.rst
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