[ghc-steering-committee] Quick look impredicativity (#274) — recommendation: accept

[Richard Eisenberg]

I'm in support.

Thanks for the nice summary.

Richard

> On Oct 14, 2019, at 7:53 AM, Spiwack, Arnaud <arnaud.spiwack at tweag.io> wrote:
> 
> Dear all,
> 
> As the title implies: I'm hereby recommending accepting the Quick look impredicativity proposal.
> 
> Summary:
> 
> The proposal modifies the `-XImpredicativeTypes` extension with a well defined semantics which consists in considering n-ary applications, rather than binary. And look, in such an application, for arguments which absolutely require impredicative instantiation (either universally quantified type applications, or arguments where a universal type is guarded by an invariant type constructor).
> 
> In order to have the expected behaviour for `($)` and `(.)`, the proposal also makes the left-hand argument of the arrow invariant with respect to instantiation (currently: contravariant). The proposal is to make this change effective, even if `-XRankNTypes` is turned on, but not `-XImpredicativeTypes`. The proposal also introduces an extension `-XContravariantFunctions` to restore the old behaviour, for compatibility.
> 
> Rational:
> 
> `-XImpredicativeTypes` is a useful extension which is currently in a rather sad state. I'm happy to see it stabilise to a clear semantics. The semantics in the proposal covers a ton of use cases. It strikes a very good balance between usefulness and predictability.
> 
> The use of n-ary application in the type system also has good theoretical roots: sequent calculi have n-ary applications. (in manyrespects, bidirectional type systems are also about n-ary application). So I don't consider it a wart at all.
> 
> The propsal's semantics, including the absence of contravariant functions, goes in the direction of _guessing less things_. This is a direction that many GHC features have taken lately. I think it's a very very good direction! Guessing can be damaging for predictability, but. more importantly, guessing has a tendency to compose badly with other typing features. (it is to be noted, too, that the implementation of contravariant functions. in GHC, eta-expands the function, which can casually change its semantics, if the function was bottom).
> 
> So, everything in this proposal does look good to me, except that, I think, I would like a deprecation route for the `-XContravariantFunctions` extension. Otherwise, we will be stuck with legacy code forever.
> 
> Best,
> Arnaud
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