[ghc-steering-committee] #425: invisible binders in type declarations; rec: accept

[Spiwack, Arnaud]

I'm generally in favour. But I'm not convinced that the secondary changes
(points 4–7) are worth it. They are certainly better place than we are
today, but are they worth breaking existing code for? Point 5–7 can
probably be replaced by warnings. I don't know about 4.

On Tue, Oct 19, 2021 at 11:20 PM Richard Eisenberg <rae at richarde.dev> wrote:

> Hi all,
>
> I am the shepherd for proposal #425,
> https://github.com/ghc-proposals/ghc-proposals/pull/425, proposing to add
> invisible binders in type declarations.
>
> The main payload of the proposal is to allow definitions like
>
> > data Proxy @k (a :: k) = Proxy
>
> instead of today's
>
> > data Proxy (a :: k) = Proxy
>
> which has no explicit binding site for k.
>
> This new syntax solves a number of smallish syntax conundra, as very well
> outlined in the proposal.
>
> In addition, the proposal includes two small unrelated tweaks to the
> syntax of type family instances; these are points (6) and (7) in the
> proposal. Both changes will break some obscure (but still realistic,
> knowing Haskell) programs, but both fixes are backward compatible.
>
> ---
>
> I recommend acceptance. The proposal is motivated nicely (do check out the
> examples) and solves a real problem. The new syntax fits in with other
> similar features. The small cleanups to existing syntax will lead to better
> error messages.
>
> The one drawback, as I see it, is that this proposal includes point (5),
> which is a breaking change to the way type synonyms work. Right now, we can
> say
>
> > type P = (Proxy :: k -> Type)
>
> and GHC will infer P :: forall k. k -> Type. Under this proposal, you
> would have to write
>
> > type P @k = (Proxy :: k -> Type)
>
> bringing k into scope explicitly. The fix is not backward compatible, and
> so I think this proposal should come with a migration strategy, where we
> warn about the former version for some releases before banning it.
> (Continuing to support it is possible, but it's very awkward to have a
> variable mentioned only in a synonym's right-hand side.)
>
> Sorry for the delay in producing this recommendation!
> Richard
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