scopedSort and kind variable left-biasing

[Simon Peyton Jones]

| Once we treat kind and type variables identically, I want to just rely on
| ScopedSort. That is, GHC should infer the order [j,a,k,b]. While I
| understand the aesthetic appeal of moving k before a, I think it's simpler
| and more uniform just to use ScopedSort. No exceptions!

I agree.  And then we can dispose entirely of the FreeKiTyVars stuff in RnTypes.

Vlad: doing so would fit very neatly into 
https://gitlab.haskell.org/ghc/ghc/merge_requests/361

Simon

| -----Original Message-----
| From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Richard
| Eisenberg
| Sent: 14 February 2019 19:04
| To: Ryan Scott <ryan.gl.scott at gmail.com>
| Cc: ghc-devs at haskell.org
| Subject: Re: scopedSort and kind variable left-biasing
| 
| 
| 
| > On Feb 14, 2019, at 10:34 AM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
| >
| >  the answer [j,a,k,b]! Strictly speaking, this answer meets the
| specification of ScopedSort,
| 
| I wish to point out that the specification of ScopedSort is very tight: it
| says exactly what we should get, given an input. This is important, because
| the behavior of ScopedSort is user-visible and must be stable. Another way
| of saying this: if we got [j,k,a,b], that would be wrong. Arguably, GHC's
| behavior w.r.t. type and kind variables is wrong because of its habit of
| putting kind variables first.
| 
| Once we treat kind and type variables identically, I want to just rely on
| ScopedSort. That is, GHC should infer the order [j,a,k,b]. While I
| understand the aesthetic appeal of moving k before a, I think it's simpler
| and more uniform just to use ScopedSort. No exceptions!
| 
| Richard
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